The study of perception is the attempt to understand those aspects of observations of the world of things and people that depend on the nature of the observer. Such understanding is obviously important to the physician, to the physiologist, and, it was once thought, to the philosopher concerned with the question of how we can be sure about the truth of our ideas. Despite these different interests, perceptual study remains predominantly psychological.

Research in perception requires the most sophisticated controls of the motivational, judgmental, and learning processes, i.e., the use of the techniques of experimental psychology. More important, the touchstone of perceptual research is perceptual experience: the fact that the prism bends light energy of different wave lengths by different amounts is, of course, a physical discovery; the fact that white-appearing light thus spreads out into the myriad colors of the spectrum is a matter of perception. Similarly, the fact that a sensory end organ, or receptor, has been found that is electrically responsive to a particular wave length remains of unknown import until its effects on experience (or on suitably selected discriminatory behaviors, which is an equivalent statement for most purposes) are demonstrated [seeHearing; Learning; Motivation; Senses; Skin Senses and Kinesthesis; Taste and Smell; Vision].

The structuralist theory . The oldest and most complete theory of perception, now known as structuralism, held that simple elementary experiences, or sensations, recur in various combinations to compose the world we perceive; sensations presumably result from the excitation of individual sensory cells, or receptor neurons, each contributing a characteristic signal, called a specific nerve energy, to the central nervous system. Whether acting alone or with a host of others, any receptor would produce the same sensation, although the memories of past experiences evoked by each different context in which the sensation is embedded would usually conceal from the untrained observer (but not from the observer trained to ignore those memories, to practice analytic introspection) the fact that the same elementary sensation has occurred.

Not only were the physical qualities of objects (their sizes, distances, shapes, etc.) to be explained in terms of these basic simple sensations but also our perceptions of people, of their expressions and intentions, of their social relationships.

This was much more than a theory of perception, of course; it was central to a comprehensive program for understanding man’s mind, since all ideas about the world must (it was held) be composed of just these sensations, plus memories of previous sensations, in various combinations. Structuralism has had to be abandoned as far as any such unitary program is concerned, and a variety of competitive schools have attempted to replace it (behaviorism, gestalt theory, and functionalism). Nevertheless, structuralism remains important. Most knowledge about perception has been gathered either within or against the structuralist program, which has provided the framework of problems around which present research continues and within which we have to evaluate more recent alternative approaches [see Titchener; Wundt].

In this article we shall first consider the elementary analytic sensations and then those properties of the perceived world that structuralism hoped to explain as compounds of sensations and to which the opposing schools hope to address themselves more directly. This survey will be restricted to visual perception, which has been most fully studied and which has produced most of the critical problem areas that characterize the disciplined study of perception.

Structural analysis . As we examine our observations of the world, we note separate sensory channels, or modalities; closing our eyes, for example, leaves our hearing relatively unaffected, and closing our ears leaves our sight relatively unaffected. Although not all of the senses are so immediately separable, this division gives us a first step in an analytic system.

Sound energy (pressure waves) at the ear stimulates specialized receptor cells in that organ, and their specific nerve energies (their effects on the brain) result in the sensations of hearing. Light energy stimulates the receptor cells in the retina of the eye, and sensations of sight result. This was the model of perceptual analysis self-consciously followed by the sensory physiologists and sensory psychologists at the turn of the century, and most of our perceptual problems derive from the failure to obtain elements of analysis within modalities even approximately as well defined as the differences between modalities.

We can do much more than observe whether it is dark or light, silent or noisy. We also see, hear, feel, etc., an infinitely variegated world of objects and happenings. What analytic methods can we apply to this problem? How does a seen apple differ from an orange, a hostile face from a friendly one?

Within each modality there are evident differences in quality and quantity that also seem to be independent of each other in the same way that the modalities are independent of each other. Light is not only off or on; it is red, blue, heliotrope, maize, and these qualities seem to recur again and again, in different combinations, in the world we see. It is extremely tempting, therefore, to assume that—as between modalities, so within modalities—there are elementary sensations within each sense, each of which can be aroused by the action of a particular stimulating energy on receptor cells specialized to receive it, and that these elementary sensations combine in various ways to compose the world we perceive. How are we to discover them and their physiological bases?

In theory, we might vary any conceivable aspect of stimulation to discover these elementary components. In historical practice, most inquiry has centered on a few physical dimensions, partially because they can be precisely manipulated with available apparatus but mainly because they seem to be sufficient to account for (or add up to) the world we perceive. Within vision, for example, any static scene at all, no matter how complex and exotic, can be duplicated by an array of spots (or points) of color, varying only in their wave-length composition and intensity, to which variables the simple experiences of hue and brightness seemed to correspond and to which simple receptor cells seem to be sensitive.

Color sensation and visual perception . Consider an object before our eyes. This is a distal stimulus; it affects our senses only by the energy, called the proximal stimulation, which is actually transmitted to the sense organ. This energy presents the eye with an optic array, and the eye’s optical system focuses some selected region of that array to form an image on the retina which is a mosaic of light-sensitive cells, connected by a network of lateral cells, whose output goes eventually to the optic cortex of the brain. (The selection depends on the eye’s direction, which in turn depends on the state of tension of a vast number of muscles.) Usually, the retinal image is considered to be the proximal stimulation, but, almost always, the optic array is analyzed in its place. The optic array can vary in only certain measurable ways that are known to affect vision. At each point (if we take points that are small enough—say, less than about one minute of visual arc), the array can effectively vary only in its energy at each wave length of the spectrum. That is, given any distal object at all, we can duplicate its effect on the nervous system of the observer by producing the same intensity pattern of wave lengths in the optic array. Such fidelity is unnecessary, however. For the normal eye, we also can duplicate any (or all) colors in the spectrum by any set of only three wave lengths that meets certain conditions. Here, then, is our starting point: we need only find elementary experiences corresponding to each of three colors at each point in a two-dimensional array.

Theories of color vision. The traditional Younghelmholtz theory of color vision is based on the assumption of three kinds of retinal receptors, or cones, each kind being maximally sensitive to one region of the spectrum (in addition, there are rods, which are only brightness-sensitive). When only one kind of cone is stimulated, the appropriate color sensation (red, green, or blue) is experienced. These hues were chosen because they are the purest-appearing colors that correspond to a set of three wave lengths, 650, 530, and 460 millimicrons (m¼), sufficient to mix all of the other colors of the spectrum. When more than one cone is stimulated, we perceive a mixture of the two; for example, 480 m¼ stimulates the blue cone and the green cone about equally, so that we see bluegreen; 460 m¼ stimulates the blue cone and 530 m¼, the green cone by about the same amount, so that with the combination 460 m¼ and 530 m¼ we see the same blue-green [seeHelmholtz; Vision, article onColor Vision and Color Blindness].

One way in which this traditional theory might be wrong is particularly instructive. It may have chosen the wrong elements. For example, at 580 m¼, the hypothesized red and green cones are stimulated, so that we should see a reddish green (!); we do not, of course: at 580 we see yellow. Thus, even though yellow seems to be just as pure and unitary as red or blue, it is supposedly not a sensation at all, but a mixture of two other sensations, so welded together by the many experiences we have had with that pair that we can no longer distinguish the underlying red and green sensations. This is but one example of what is encountered repeatedly in the structuralist system, namely, that the elements combine to produce something unexpected in terms of the parts and the system is saved only by declaring that some of the sensations (which are supposedly elementary observations) are unobservable.

In this case, however, an alternative set of elements may extricate us from the apparent paradox. Although three colors are indeed sufficient to explain the psychophysical facts of color mixing (i.e., what mix of wave lengths will match the appearance of any other wave length[s]), a much more complete perceptual account is given by the opponent-processes pairs proposed by Ewald Hering and recently developed in impressive detail by Hurvich and Jameson (1957). The elements here are three paired opponent processes, one yielding red or green, one yielding blue or yellow, one yielding black or white. With this analytic system, one can predict the appearance of any mixture simply from the appearances of the components, whereas discrepancies occur when we attempt to add up the elements in the traditional system. [See the biography ofHering.]

Contrast and constancy. With the first attempt to step beyond single small patches of color, far greater discrepancies appear; for example, if you keep two identical patches of light constant but change the illumination of the region surrounding one of them, the two patches no longer match. Exactly the same energy falls on each patch, and presumably the specific nerve energies activated in the retina remain unchanged; but the appearances now differ. This is the familiar illusion of induced color (contrast); several recent attempts to find quantitative laws to predict the effects of one colored region on another have met with reasonable success; thus, it may eventually be possible to predict the apparent color of each region of any optic array, but we certainly cannot do so yet.

There is another, not unrelated class of discrepancy called color constancy. Consider a piece of paper in the shade and a piece of coal in sunlight; both may present exactly the same amount of light energy to the eye, yet they do not appear equal. Now this seems to be an achievement, not an illusion (we see the world as it is, even though the proximal stimulus variable on which our sight is presumably based would mislead us), but it is nonetheless a discrepancy to be explained. Some of the preferred explanations are clearly false (e.g., the explanation that we see the colors as we know them to be must be false, since good color constancy obtains with unfamiliar objects); other explanations, such as those which subsume color constancy and contrast under the same formula (Helson 1964, pp. 260-297; Wallach 1948), are only partially complete.

This much is clear, however: the colors that we perceive by stimulating one small homogeneous region at a time are not observed unchanged when those regions are viewed in other contexts.

Spatial direction and motion . Assume that we had accounted for color. Next we must ask, How does the eye analyze space? Here is one simple model: If one rod or cone is stimulated, we would see one spot; if two or more are stimulated, a larger spot; if the two are separated by an unstimulated receptor, two separate spots. The smallest such separation that we can see—our visual acuity—would set the limits of the spatial differences we can see, that is, no two perceived objects could differ in size, shape, or position by less than this difference. Although this simple model of acuity still lies behind most casual thinking about perception, the actual discriminations that we can make are, optimally, even finer than can be accounted for in terms of the retinal image and the known sizes of the receptors.

We must also, of course, be able to tell where the points are in relation to one another—left, right, up, down. Does each stimulated retinal point produce not only an elementary sensation or impression of color but a sensation of place as well—a “local sign”? The structuralist answer was that the qualities by which each receptor in the mosaic contributes to our perceptions of space and form are not simple sensations, but are themselves composed of the memories of those eye movements that were involved in the movement of a particular distal stimulus from one retinal point to another. Thus, a straight line would differ in appearance from a curved one only because the straight line falls on a row of retinal receptors that have in the past been stimulated by a set of eye movements that carry the same retinal point along the retinal image of some object’s edge that is straight to the sense of touch. Since a single spot of light in an otherwise dark room has a very variable apparent locus, this theory cannot be tested by studying the local sign of each retinal point. In 1896, Wundt showed that after prolonged wearing of prismatic spectacles that produce curvature distortions in the images of straight lines, the distortions disappear and straight lines appear straight—until the spectacles are removed, after which straight lines appear curved in the opposite direction. This showed, Wundt argued, that the eye-movement components of the local signs had been relearned. After much confusion caused by the discovery that similar aftereffects could be produced purely intravisually (i.e., by the prolonged inspection of curved lines, even without eye movements), curvature aftereffects have recently been demonstrated that are in fact contingent upon movements made while wearing prismatic spectacles (indeed, that are contingent only on active movement by the observer, which is somewhat different from what Wundt had in mind). Many different kinds of inverting, distorting, and reversing spectacles have since been worn, for different periods of time, but no clear summary statement can yet be abstracted. In some kinds of distortion rapid perceptual change occurs, while in others it is not clear whether it is the visual world that changes or the motor habits (or the “body image”) of reaching and walking that accommodate themselves to a changed relationship between vision and action. It is certain, however, that visual direction depends as much on muscle systems as on retinal stimulation. We scan the world continually, and the eye movements that have been made must be taken into account in deciding the directions and motions we perceive. As the eye moves in its socket (or the head and body move in the world), a stationary distal object must produce a moving retinal image; yet we usually see the object’s position as constant, even though very different local signs must be involved. This direction constancy means that a visual local sign, if it is to be retained as a component of observation at all, would have to remain unconscious (an “unobserved observation”) while it is somehow coupled with information about movements of the eye (and position of the body). This kind of reliance on unconscious sensations, necessary if this analytic system is to be saved, will become unbearably cumbersome when we turn to more ambitious—but still rather primitive—syntheses of our elements of analysis into the objects of the perceived world.

Perception of shape . How does a circle differ from a square? Are any additional specific nerve energies needed to account for either one?

In structuralist syntheses the perception of some particular square consists of its immediate visual sensations of color, plus all of the tactual memories of direction—of straightness, rectangularity, etc.—that were initially discerned by the exploring movements of the hand and eye, plus memories of the whole host of different sensations of color and direction that would be produced as the square is displaced slightly laterally, or rotated from the vertical by the tilting of the head, or as its retinal image increases or decreases in size as we move toward or away from the distal object. (All of these must be part of the same perceptual structure since we do not even recognize their changes in stimulation when they occur.) A “shape” is, thus, a submerging of a vast number of now unnoticeable sensations, by the effects of a great deal of past experience, into a single named perception.

Given this principle, there is no need to elaborate it by considering more complex shapes, such as letters, people, or even the perception of motion. All of these would merely be further examples in which the elementary observations of color and direction or position are smoothed together and pieced out into an apparently unitary structure, so strongly cemented by the memories of previous experiences that even the most practiced observer cannot discern the sensations that presumably evoked those memories. (If we try to expand this analysis to include any of the geometrical illusions, in which—analogous to the illusions of color—the shape that is seen is discrepant from the shapes actually present in the retinal image, the explanation merely becomes much more involved but remains unsatisfactory; the illusions are still without adequate explanation.)

The gestalt approach. Gestalt criticism was most effective in questioning whether shapes really appear in the way structuralists claimed.

Consider the figure-ground phenomenon, in the famous vase-and-faces Rubin figure in Figure 1.

At any region of the contour, one can see the vase or one can see a face, but not both simultaneously. One can easily construct other puzzle-pictures in which even a long search may not reveal the “hidden” shape. Since the sensations must be the same on both sides of the contour—in figure and in ground—the structuralist explanation would have to be that one sees whatever shape is most familiar. Against this argument, gestalt psychologists demonstrated that thoroughly unfamiliar shapes can absolutely conceal such long familiar shapes as letters and numbers. For example, consider the perfectly good 4’s concealed in the scribble in Figure 2. Not familiarity but the over-all

configuration determines which way a contour will face, what function a part will play. The whole is not the sum of its parts, and we cannot say what shape we will perceive by adding up the experiences we have with each individual point.

“The whole is not the sum of its parts.” A catchy paradox; if taken seriously, it would mean an end of analysis and of science. The gestalt psychologists, of course, offered to search for other parts—other units of analysis. Implicitly, the “laws of organization” are such units: rules by which we can predict what will be figure and what will be ground in a given stimulus pattern. For such a major challenge, these “laws” have, until recently, received little direct experimental examination; hence it is still too early to evaluate the more positive gestalt proposals. The effects of gestalt criticisms, however, were considerable. The central support of the structuralist enterprise, which, as we have seen, was badly weakened anyway, was destroyed. We simply do not see the “underlying” sensations as we should be able to do. However, in rebuttal, we should note that the properties of such figures (hard, shaped, with an edge that is in front of, and not shared by, the ground) are suspiciously like those of real objects, from which they might have been learned. And the laws of organization may themselves have been learned. When the familiar shape, the 4, is concealed in the presumably unfamiliar scribble, it may simply be because the observer’s expectations about contours (that a contour that proceeds without interruption is generated by a single object or surface, i.e., the “law of good continuation”) are even more deeply ingrained than are his expectations about the figure 4 [seeGestalt Theory].

We cannot now evaluate this possibility directly, because testing it would depend on a better understanding of the process of perceptual learning than we have and on making the kind of ecological surveys, as Egon Brunswik (1947) has proposed, that would tell us what the environment is likely to have taught us perceptually. However, the very possibility that the laws of organization are the result of learning opens new vistas. Laws do not have to be innate in order to be widespread; they can be the result of regularities in the environment in which we have developed and be nonetheless usable for that reason [seeBrunswik].

This possibility offers a different line along which we might take up again the structuralist program, if not its set of elements.

Cell assemblies and phase sequences. D. O. Hebb proposed (1949) that there are in some fashion “specific nerve energies” that correspond to the gestalt phenomena; he suggested that corners, contours, and edges have their neural counterparts in organized networks of cellular connections in the brain, called cell assemblies, which have come to act as single functional units as a result of perceptual learning. From these cell assemblies (which would consist of counterparts of frequently encountered fragments of shapes, etc.) still larger temporally extended units, called phase sequences, might be built up—for example, the succession of cell assemblies that would be produced by repeated sweeps of the eye over the same pattern (say, in picking out the corners of a square). This theory makes explicit provision for obtaining different responses to the same retinal stimulation. Which cell assemblies would fire and what we should see are functions not only of which receptors are stimulated but also of what other cell assemblies are simultaneously firing (i.e., the effect of the whole on the part) and of what other cell assemblies have just been firing. This last point provides for the effects stemming from the observer’s attention and expectations, an immensely important component of the perceptual process, which has been missing from our previous discussions. As a new structural theory, Hebb’s proposal is still in process of testing and revision, and although it permits us to expect that elementary units to analyze shape and form might be found, it does not tell us how we should start to find them.

Classifying shapes. One obvious starting point is to classify shapes and forms in accordance with their apparent similarities and differences, that is, to obtain the dimensions of variation of shapes. This task is more difficult with shape than with colors simply because shapes can be sorted or classified in so many different ways. Several systems for classification have been proposed, including procedures for the random generation of “nonsense shapes.” While these procedures may help us standardize our discussions, it is not yet clear what they do as a choice of subject matter (that is, to what extent they are representative of the kinds of shapes about which it is important to be able to generalize).

Alternatively, we can start from ways in which form and shape are already known to be important and try to develop functional elements with these important ways in mind. What are the elementary distinctive shapes involved in reading (i.e., what features do children, learning to read, single out when discriminating letterlike shapes from each other)? Complexity is an important formal variable in any attempt to systematize the gestalt “laws of organization.” What features determine subjects’judgments of the complexities of shapes? More directly, the gestalt thesis is that whenever alternative organizations can be perceived, we see whichever is simpler, and where patterns are less complex in two dimensions than in three, we see them as flat.

It is too soon for a general evaluation of attempts to find elements and combining laws for the study of form perception, but it is quite clear that attempts are worthwhile and that they have had some limited success.

Perception of distance . To gestalt psychology there is no real difference between shape perception and space perception: both express the same organizational processes, and neither necessarily depends on learning. To structuralism, distance perception must be learned, since each individual retinal receptor cell can only indicate by its specific nerve energy whether or not it is stimulated, not whether the energy stimulating it comes from near or far. There are, it is true, patterns of stimulation that differ with near and far objects, and many of these had already been discovered by artists, but if points are the only “basic” sensations, these patterns would themselves have to be the results of learning and could evoke depth perceptions only by their previous associations with walking, reaching, and touching.

This argument becomes far less critical if we abandon point sensations as the only elementary experiences. Once we believe that the individual’s early personal history can build up neural networks that respond to patterns (Hebb’s cell assemblies), we can also believe that millions of years of evolution may have selected individuals born with these (or equivalent) connections “prewired.”

What about this issue as a question of fact? On the one hand, some animal species will, with no prior visual experience at all, avoid the edge of a “visual cliff”—i.e., that side of a sheet of plate glass under which there is an abrupt drop, to which the only cues are visual ones. On the other hand, cats, at least, seem unable to respond to such visual depth cues if they have received visual stimulation but have been deprived since birth of active visuomotor behavior. Recent human research, necessarily much less direct, has centered mainly on the visual distance cues and has been less conclusive. Clear-cut consequences of either alternative are not easy to come by. Any given distance cue might be learned early and remain irreversible, or it might be innate and yet subject to relearning (which is why the wearing of distorting spectacles—discussed in the section on direction—cannot tell us anything at all about the ontogenetic bases of space perception). The problem is no longer central to all of psychology, as it once was, but it does remain a technical question, vital to the study of growth and development.

Setting aside for the moment the age-old preoccupation with whether distance perception is learned, what stimuli can we find that might account for our perceptions of distance? Some of the distance cues have been known for centuries, but there are probably many more still to be discovered.

In the last decade, Gibson (1950; 1959) has shown that there are static and dynamic pattern variables (gradients, to be precise) that are at least potentially informative not only about the distances of objects and surfaces but about their sizes and proportions as well. Surfaces at some slant in relation to, the observer produce a constant rate of change of texture density in the retinal image, and if the eye is sensitive to such gradients (innately or by learning), these would indicate the relative distances of two objects by the site of their intersections with the ground plane’s gradient and would indicate their relative sizes by the number of texture-gradient units their images subtend, etc. If the observer locomotes in his environment as well, the dynamic pattern of textural gradients is even more potentially informative.

Although we do not yet know the extent to which this particular set of “higher order” variables can be used by the perceptual system, they reveal so much information in the proximal stimulation about distal space that the question of what in space perception is learned becomes a very different kind of inquiry. Moreover, these variables suggest a new set of units with which to analyze the world of perceived space—units of surfaces and their slants, of their distances and angles toward each other and toward the observer.

Whether or not this promise is fulfilled, however, distance perception is no longer the culminating problem for the study of perception (though there remains a residual aura about this class of research that it is in some sense “basic”), and we must not expect that our perceptions of social events and personal qualities, for example, are built up out of perceived surfaces any more than perceived surfaces can be built up out of sensed spots of color.

Perception of people and social events . Much research on what is called social perception has studied the effects of the observer’s needs and expectations on his judgments of physical variables, for example, the effects of economic class on children’s estimates of coin size, or the effects of emotional connotations of words on the amount of light needed to recognize them. One goal of such research has been to demonstrate that unconscious perception, or subception, can occur; even if this goal could be achieved—and it is doubtful that it can be in any conclusive fashion—we have seen in our previous discussion that the concept is not a novel one in the history of perceptual theorizing. The asserted findings of such effects are few and still contested after a decade of controversy; even were they large and reliable, however, there is no clear path by which we could apply such findings to the study of the perception of people and of social events. This latter inquiry, which is still in its infancy, will almost certainly require new analytic units and experimental variables different from those of physical size or light intensity.

A primary problem here is that of defining the areas of research. We do not yet know how to classify the problems most fruitfully or what variables are important to those problems. A start at classifying problems might be in terms of the perception of other persons’ permanent qualities (e.g., character traits), their temporary states (e.g., emotions, desires), and their interpersonal relations. A beginning at an introspective classification system has been undertaken by Heider (1958), and a few studies have been made in which the physical proportions of faces were varied (or in which the physical qualities of voices were manipulated) to discover the effects of such variations on perceived social traits. Schlosberg (1952) has reduced the facial expression of emotions, as portrayed in photographs, to the different combinations that can be achieved with two underlying dimensions (pleasantness-unpleasantness, attention-rejection). However, the manipulation and measurement of facial or bodily configurations are almost impossibly difficult if the research is to have any relevance to nontrivial samples of social events.

An ingenious and promising solution to this general problem is illustrated in several studies by Secord (1958) and his colleagues. If two different groups of subjects agree in their judgments about a set of people (the social objects), there must be certain stimulus features of the social objects to which both groups of judges are responding in the same way, and it is the task of the student of social perception to discover what those stimulus features may be. (This is, of course, precisely the same problem encountered in starting research in any area of perception.) Still another group of subjects can help us in this task by serving as a set of human measuring instruments. All we want from this last group is their judgment of such features as grooming, mouth curvature, complexion, and so forth, for each social object. With these measures (for which we need not know the physical bases), we can then try to discover which features or combinations of features are responsible for the social properties that were reported by the first set of subjects—for example, glasses impart apparent dependability, industriousness, intelligence; relaxed lips enhance a woman’s apparent sexuality, etc. Eventually, it is to be hoped, a set of stable stimulus elements will be found with which to analyze more complex social properties.

Why should we try to find such analytic units? Units of analysis are needed in social perception for the same reasons that they are needed in other fields of perceptual inquiry: this knowledge may permit us to predict more economically and more heuristically what our subjects will do—including that particularly important class of behaviors, what they say they observe. It is important that we keep our purposes clear in undertaking social-perceptual inquiry, because in this problem area we do not even have the heritage of a previous goal and purpose (i.e., the structuralist framework) as landmarks by which to set our course and gauge our progress.

Julian E. Hochberg

[See alsoSenses. Other relevant material may be found inGestalt Theory; Learning, article onDiscrimination Learning.]

bibliography

For an introduction to the attempt to analyze perception into sensory elements, Titchener 1896 isprobably unsurpassed; a more recent general introductory survey of the status of this attempt and of its successors appears in Hochberg 1964.The problem of spatial direction is enjoying a resurgence of interest; three anchors in the flood of research are Kohler 1962,a popular article; Mikaelian & Held 1964,a research paper; and Harris 1965,a critical, integrated literature review. The study of shape perception has produced a vast and expanding literature, much of which is devoted to attempts to formulate the problem so that the results of various exercises with digital computers will be of evident relevance. The best gestalt treatments are those in Koffka 1935and Metzger 1935-1942.Empiricist rebuttals are to be found in Hebb 1949and Brunswik 1947.Both classifications and generation procedures, as well as analyses of form based on measures of other functions to which form is important, are briefly reviewed in the last chapter of Attneave 1959and still more briefly in Hochberg 1964.The perception of distance is best reviewed in Gibson 1950;an analysis of the entanglement of this problem with the more general controversy between the nativistic and the empiricist views of man, is given in Hochberg 1962,and a review of the literature is in Epstein 1964.Modern research in the area of social perception, in the restricted sense in which the term is used here, starts with a seminal paper, MacLeod 1947. Heider 1958and Secord 1958form the background for most of the above discussion.

Attneave, Fred 1959 Applications of Information Theory to Psychology: A Summary of Basic Concepts, Methods, and Results. New York: Holt.

Brunswik, Egon (1947) 1956 Perception and the Representative Design of Psychological Experiments. 2d ed., rev. & enl. Berkeley: Univ. of California Press.

Epstein, William 1964 Experimental Investigations of the Genesis of Visual Space Perception. Psychological Bulletin 61:115-128.

Gibson, James J. 1950 The Perception of the Visual World. Boston: Houghton Mifflin.

Gibson, James J. 1959 Perception as a Function of Stimulation. Volume 1, pages 456-501 in Sigmund Koch (editor), Psychology: A Study of a Science. New York: McGraw-Hill.

Harris, Charles S. 1965 Perceptual Adaptation to Inverted, Reversed, and Displaced Vision. Psychological Review 72:419-444.

Hebb, Donald O. 1949 The Organization of Behavior: A Neuropsychological Theory. New York: Wiley.

Heider, Fritz 1958 The Psychology of Interpersonal Relations. New York: Wiley.

Helson, Harry 1964 Adaptation-level Theory: An Experimental and Systematic Approach to Behavior. New York: Harper.

Hochberg, Julian E. 1962 Nativism and Empiricism in Perception. Pages 255-330 in Leo J. Postman (editor), Psychology in the Making: Histories of Selected Research Problems. New York: Knopf.

Hochberg, Julian E. 1964 Perception. Englewood Cliffs, N.J.: Prentice-Hall.

Hurvich, Leo M.; and Jameson, Dorothea 1957 An Opponent-process Theory of Color Vision. Psychological Review 64:384-404.

Koffka, Kurt 1935 Principles of Gestalt Psychology. New York: Harcourt.

Kohler, Ivo 1962 Experiments With Goggles. Scientific American 206, no. 5:62-72.

Macleod, Robert B. 1947 The Phenomenological Approach to Social Psychology. Psychological Review 54: 193-210.

Metzger, Wolfgang (1935-1942) 1953 Gesetze des Sehens. 2d ed., enl. Frankfurt am Main (Germany): Kramer.

Mikaelian, Harutune; and Held, Richard 1964 Two Types of Adaptation to an Optically-rotated Visual Field. American Journal of Psychology 77:257-263.

Schlosberg, Harold 1952 The Description of Facial Expressions in Terms of Two Dimensions. Journal of Experimental Psychology 44:229-237.

Secord, Paul F. 1958 Facial Features and Inference Processes in Interpersonal Perception. Pages 300-315 in Renato Tagiuri and Luigi Petrullo (editors), Person Perception and Interpersonal Behavior. Stanford (Calif.) Univ. Press.

Titchener, Edward B. (1896)1902 An Outline of Psychology. New ed., enl. New York: Macmillan.

Wallach, Hans 1948 Brightness Constancy and the Nature of Achromatic Colors. Journal of Experimental Psychology 38:310-324.

Wundt, Wilhelm (1896) 1907 Outlines of Psychology. Leipzig: Engelmann. → Translated from the seventh, revised German edition.

II PERCEPTUAL DEVELOPMENT

The problem of how man develops his knowledge of the world around him in the course of his lifespan is one of the oldest in the history of psychology and at the same time one of the newest for modern experimental psychology. Historically, the problem has received attention from philosophers beginning with the Greeks and became the focus of the controversial question of nativism-empiricism in the seventeenth and eighteenth centuries. The British empiricists, beginning with John Locke and David Hume, espoused the notion that man gets his knowledge of objects and space in the world around him through his senses, in contrast to the nativistic assumption of innate, God-given, prototypical ideas. The concept of the mind as a ’Iblank slate” that is written upon by experience was set forth so persuasively by these empiricists that it dominated Western thought for centuries. George Berkeley carried the arguments into a more psychological realm by considering some specific problems, especially the perception of visual space and the third dimension, and proposed that visual perceptions of distance are mediated by and dependent on habitual connections with touch—an idea that persists yet. The principle of association, accounting for the linking together of sensory elements of experience, was invoked by David Hartley, and frequency as a primary principle of such associations was proposed by James Mill. Thus the concept of associative frequency, assumed by many psychologists to be the fundamental principle of learning, was born in an attempt to account for the development of man’s complex perceptions of objects, their sizes, their shapes, and their distances [seeBerkeley; Hartley; Hume; Locke].

With the nineteenth century came the first experimental work on the physiology of the senses, work which led to further elaboration of the theory of perceptual learning. Hermann von Helmholtz, the genius of this period, carried on the empiricist tradition, supplementing it with the notion of “unconscious inference.” He thought that certain assumptions were built through repeated sensory experience and were drawn on, albeit unconsciously, to interpret the momentary sensory input [seeHelmholtz].

There were proponents of nativism as well during these centuries: Kant its principal philosophical advocate and Ewald Hering its principal physiological advocate. Nativism had little impact on scientific psychology in the United States and England compared with the effects of the empiricists’ theories. But it did foreshadow the gestalt psychology of the early twentieth century. The gestalt psychologists’ emphasis on characteristics of the “whole”—patterns and relations—that cannot be explained as summations of discrete sensory experiences (for example, a melody retaining its identity despite transposition to another key) was clearly at odds with the notion that complex perceptions are constructed by association of elementary sensations and ideas. So was the gestalt notion of PrdgnanZy of forces tending toward a “best” structure (for example, the soap bubble), both in the physical world and in man’s neural processes. Furthermore, isomorphism between neural process and perception was assumed. When self-regulating processes of organization resulting in one “best” structure are postulated, associative learning can have no role in perception. In 1921, Koffka published Growth of the Mind, a gestalt psychologist’s view of how cognition develops. He conceived of the developmental process in general as one of articulation and differentiation, rather than as a concretion of experience, and of perceptual development as the process by which a view of the world gradually emerges from early and inarticulate experiences (1921, p. 280 in 1924 edition). For instance, with respect to color, he suggested that first there is a distinction made only between “colour and non-colour”; after this stage, colors become distinguished as “warm” and “cold”; then, within the warm and cold groups, differentiations are made of the four principal colors, red, yellow, green, and blue (1921, p. 291 in 1924 edition). Similar views of perceptual development characterize the work of Werner (1926). “Creative learning” and maturation, as opposed to associative frequency, are the key principles [seeGestalt Theory; Hering; Kant; Koffka].

During the same period, interest in perception in the United States was almost eclipsed by the victory of behaviorism. However, the functionalists, especially Carr (1935) and his students at the University of Chicago, kept some interest in space perception alive. As theorists, they stayed within the empiricist tradition, but, influenced by the work of George M. Stratton, they introduced a strong emphasis on “localizing” movements of the eyes, head, hands, body. These movements become directly associated with specific local signs in each sense department in temporal contiguity with them (Carr 1935, p. 28 ff.). Stratton’s early experiment of wearing an inverting lens system (a lenticular pseudoscope producing a 180° rotation of the visual field) and, in the manner of a learning experiment, studying the ensuing changes was an important impetus to experimental research (1897).

During the 1930s and 1940s the collection of developmental data, with an emphasis on age norms, was also a strong trend (Gesell et al. 1949). The study of “sensorimotor development” (for example, the time at which an infant first follows a moving light, turns his head to a sound, coordinates his eye movements) replaced “perceptual development” as an area of concern at this time, as a result of the influence of the stimulus-response(S-R) concept in psychology.

By the end of World War n a number of trends combined to produce the re-emergence of perceptual development as a full-fledged experimental problem, gradually shaking off its roots in philosophy. During the war psychologists found that perceptual skills could be improved by training; and comparative psychology and ethology discovered once more the importance of the analysis of the sensory aspects of “innate” behavior [seeEthology]. Lashley, in a 1938 address called “The Experimental Analysis of Instinctive Behavior,” had already strongly urged an emphatic and systematic analysis of real stimulus attributes in studies of instinct, reflexes, and learning, claiming that psychological theories would remain sheer nonsense if the stimulus continues to be defined only as what “the experimenter puts in front of the animal” (Lashley [1938] 1960, p. 380). He spoke of the “innate components of sensory organization” and suggested experimental methods for the analysis of the adequate stimulus of innate behavior. Two of these methods, “sense privation” (now generally termed sensory deprivation) and successive elimination of properties of the stimulus object, are responsible for the design of much modern research.

Methods of studying perceptual development

In present-day psychology, the nativism-empiricism, or nature-nurture, controversy has been rephrased. The questions asked—how does perception develop? to what extent is it trainable? and to what extent is it dependent on innate factors?—are ones that can be answered scientifically. Evidence from ethology of “species-specific innate releasers” has convinced the most die-hard empiricists that certain stimulus-to-perception correlations are built in. And laboratory evidence of the improvement of perceptual skills through training has made dogmatic acceptance of a nativistic bias equally impossible. A number of methods are being pursued in an intensive effort to uncover the laws of perceptual development.

Sequential studies

Longitudinal observational study, with a comparison of different age levels and a search for a definable sequence (employed by Gesell for sensorimotor development) has been utilized in a long-term program of research on cognitive development by Piaget and his co-workers. Piaget’s observations on perceptual development are summarized in Les mecanismes perceptifs(1961). His theory of perceptual development emphasizes constructive activity on the part of the child. There are perceptual “mechanisms” and at a later stage intellectual processes as well, he believes, operating on the sensory input and in a sense constructing the perceptual world. The breadth of Piaget’s work is enormous, but one example, the development of form perception by touch, must suffice (Piaget & Inhelder 1948). Familiar objects or cardboard cutouts of geometrical shapes were hidden from the child’s visual field but presented behind a screen to the child’s touch. Exploratory hand movements were observed, and the child was asked to identify or describe the object. Developmental progress appeared to occur in stages. At the begining of Stage i (up to four years), the child identified only familiar objects, and tactile exploration was gross and relatively passive. Later in Stage i, geometrical shapes that differed topologically (properties such as closed versus open) were distinguished. In Stage n (four to six years), the child progressed to crude differentiation of linear from curvilinear shapes and then to progressive differentiation according to angles and dimensions. In Stage in (six years and over) there was ability to distinguish between complex forms, with methodical exploration. Shifts from one stage to the next were accompanied by a more active, elaborate, and planned tactile search that permitted “construction” of a whole, complex contour. This motor activity Piaget believes to be essential in the final abstraction of shape itself.

Piaget’s laboratory has produced much experimental work on illusions and perceptual constancy, usually employing samples of subjects at different age levels. Perceptual constancy has often been explained as the result of a learning process. To take size constancy as an example, the child presumably learns (through moving around his environment) that objects at a distance are not smaller than near ones, and he comes to infer their real size from cues to their distance and from familiarity. Experiments comparing size constancy in children and adults performed in Piaget’s laboratory (and others) generally reveal developmental differences, but their interpretation is ambiguous. Adults are often less accurate than children; adult errors tend toward overcompensation and suggest the operation of attitudinal factors rather than growth in perceptual processes as such [seeDevelopmental Psychology, article ona Theory of Development].

Comparative studies

Ontogenetic studies such as Piaget’s have demonstrated sequential changes in perceptual development, but the relative contributions of maturation and experience are difficult to assess. Two other comparative methods have been used as well: interspecies and cross-cultural comparisons.

Interspecies comparisons. Comparisons of species using the very young animal have revealed wide interspecies differences, due either to the maturational stage of the neonate or to differences in structure—in either case the emphasis being on inborn rather than experiential factors. Walk and Gibson (1961) made comparisons of young animals’ responses to a simulated cliff (constructed to provide only visual information about the depth of the drop-off) as soon as the young of each species were capable of locomotion. All the species tested exhibited a preference for the shallow over the deep side of the cliff as soon as tests were possible. But some animals (for example, chicks and ungulates) could be tested at birth, while others (such as human infants) were too immature for testing until considerably later. Fantz (1961) has studied young animals of several species with a selective-attention method and revealed the presence of pattern discrimination in young primates far earlier than had been supposed by most empiricists.

Cross-cultural comparisons. The other comparative method is the cross-cultural study. Comparisons of responses of different ethnic groups to illusions (for example, those done by Allport & Pettigrew 1957) have shown some differences, inferred by the authors of the studies to be the result of varying experiences, ecologies, and education. Whorfs hypothesis that man’s perceptual system is structured in part by his language is well known to social scientists. But the limits of cultural influence must not be glossed over. Lenneberg (1961) found that young Zuni subjects did show differences in naming colors that reflected the linguistic peculiarities of their color nomenclature. But when psychophysical measures of differential limens (thresholds) were taken from the same population, there was no corresponding shift; indeed, the limens were the same as for a native English-speaking group.

Laboratory studies

Interesting as the comparative studies may be, it is clear that manipulative laboratory study is necessary for the ultimate understanding of how perception develops.

Deprivation studies. Lashley’s method of “sense privation” is one way of testing experimentally the hypothesis that normal perceptual development is a function of accumulated sensory experience. Elimination of visual stimulation from birth onward should, from an empiricist’s standpoint, prevent the development of adequate perception of space, shapes, and objects. However, the findings are equivocal in cases of human adults whose congenital cataracts have been surgically removed. Visual perception is not normal in these cases (obviously, visually perceived objects cannot be named correctly), but factors other than absence of visual stimulation could be responsible, for example, inferior accommodation, an emotional upset, or interference from previous habits. Lorenz (1965) has commented on the limitations of the deprivation experiment.

It is possible, though, that visual apparatus must mature in an environment of patterned light for normal pattern vision to develop. Chimpanzees reared in darkness or with only unpatterned light are incapable of normal visual discrimination of pattern when tested after seven or more months of such treatment (Riesen 1958). Cats reared in darkness likewise exhibit disturbed visual development. On the other hand, rats reared in darkness behave like their light-reared controls after being taken into the light. Lashley and Russell (1934) found that in dark-reared rats the force of jump from a platform to a target was related to the length of the distance to be covered; and Walk and Gibson (1961) found that they avoided a visual cliff as well as the normal animal. It would thus appear that species differences exist even among the mammals, since the more visually dominant primates and cats require some patterned light (and possibly active movement within the lighted environment as well) for normal visual development, whereas the rat does not. But the mere statement that the rat’s spatial vision is “inborn” is insufficient. What are the adequate stimuli to which it responds so immediately? Walk and Gibson used Lashley’s method of successive elimination of properties of the stimulus and found that the darkreared rat discriminated depth differences with certain isolated stimulus properties (for example, motion parallax) but not with all. Isolated texture difference in the cliff situation yielded a 100 per cent preferential response in the light-reared rat, but in the dark-reared animal a chance response was found to texture differences. After living in a lighted environment for a week the deprived animals responded like the normal animals. This experiment makes it evident that innate and experiential factors combine in the process of perceptual development. Location of the factors and determination of their interaction has only begun.

Enhancement studies. An alternative to the method of deprivation is that of rearing the experimental animal with enhanced opportunities for stimulation. The hypothesis tested is that the animal will later exhibit superior perceptual performance. Gibson and her associates (1959) used this method in studying form discrimination in rats. Shapes (circles and triangles) were cut out of metal and hung on the walls of the experimental animal’s living cage. After three months of such daily exposure, the animals were presented with a triangle-circle discrimination task. Discrimination learning in these experimental animals appeared at first to be facilitated in comparison with control groups, but further experiments suggested that selective attention rather than literal “visual learning” of form was responsible. Transfer of selective attention from a simple situation to a more complex one may be one of the most powerful mechanisms of perceptual development.

Adaptation to distorted environments. Stratton’s early experiments with inverting optical systems have been carried further, especially at the University of Innsbruck (Kohler 1951). The hypothesis underlying these experiments is that progressive adaptation to the distorted world thereby produced is analogous to the original development of space perception in the infant. This reasoning is questionable, but the experiments are interesting because they demonstrate great plasticity in the human subject in compensating for a transformed stimulus array. The transformation can be an extremely complex one. Not only have spatial transformations (up-down or right-left reversals, prismatic shifts) been employed but spectacles with different colors of glass in different halves (upper versus lower, right versus left) have also been used. Even in the latter case, the subjects eventually adapt, and most astonishingly show, by the aftereffects when the spectacles are removed, that adaptation has been specifically conditioned to eye position. It is essential to remember that all these transformations, however complex, are still in systematic correspondence with the original spatial order. Thus, one cannot infer that because adaptation to the new order is possible the infant in the beginning creates spatial order out of a random or chaotic fusillade of stimulation.

The plasticity of the human species in these experiments is in remarkable contrast to that of the chicken. Hess (1956) placed prisms that displaced the visual field 7° to the right or left on chicks and observed pecking accuracy. Despite four days of practice, not a single chick learned to correct for the displacement. This species difference again argues for some innate basis of spatial visual development, with greater plasticity in (at least) the human species.

Trainability of perceptual skills. The plasticity of the human observer is convincingly demonstrated also by his trainability in numerous perceptual tasks. Experiments have demonstrated the effects of practice on judgments of pitch, smell, hue, distance, and weight (see Gibson 1953). Probably the first learning curve in experimental psychology was obtained by W. F. Volkmann, who found that the two-point limen on the skin becomes progressively lower with practice. An example of educated pitch discrimination is found in an experiment of Heinz Werner’s with “micromelodies.” Observers were trained in judging fine differences of pitch until they could eventually hear (and transpose) melodies embodying pitch differences so tiny as to have been indistinguishable previously. Both these experiments demonstrate differentiation of perception, the development of greater precision of judgment within a stimulus dimension. A rather different case is learning to respond differentially to stimulus differences never attended to before. It has been shown that some animals (for example, bats) and many blind persons get information about obstacles in the space around them, and accordingly steer themselves, from echoes reflected from objects at different locations. The bat emits a high-pitched noise, the blind man taps a cane, thus enhancing the opportunities for possible differential echoes. That this is a learned skill in man has been demonstrated in experiments with blindfolded subjects, who progressed from random performance to virtuosity after training.

Modern theories of perceptual development

It should be clear that for modern psychological theory the problem is no longer whether perception is innate or learned, but rather how it develops.“Enrichment” theories and “differentiation” theories are two general approaches to perceptual development (Gibson & Gibson 1955).

Enrichment theories

Enrichment theories assume that perception begins with bare sensory input, and that this is supplemented somehow by past experience. The supplementation may be by way of an associated context of images and ideas, or it may be by inference from “hypotheses” built up through experience. The latter position, deriving from Helmholtz, has a number of modern proponents.

Egon Brunswik’s probabilistic theory held that experiences during development reflect the environmental ecology and result in a network of hypotheses used by the observer to make a best guess about the probable distal reality of a present proximal stimulus. A number of demonstrations reflecting this general orientation were designed by Adelbert Ames. They involve stimulus arrays presented under nonoptimal viewing conditions and show, presumably, the interpretative effect of the observer’s accumulated assumptions.

Another version of the enrichment hypothesis is “schema” theory, which likens the perceptual process to categorizing. Schemata are cumulatively built up through experience; new sensory experiences are matched with them, thus explaining recognition and identification of objects by way of past experiences. The matching process has been thought of as a successive trying-out of hypotheses. The schema presumably influences the perceptual process in addition to serving in identification.

A behavior theory (as opposed to a cognitive theory) of enrichment of the sensory process also has its adherents. Improvement in discrimination is explained as the learning of differential responses; these in turn, by way of response-produced stimulation, add distinctiveness to the sensations produced by the original stimuli. The hypothesis is generally referred to as “acquired distinctiveness of cues.” The opposite process, acquired equivalence, should occur when common responses are learned.

Differentiation theories

Differentiation theories of perceptual development contrast with enrichment theories by assuming that the environment, by way of the stimulus array, provides a wealth of potential information, and that development is a process of “getting” more and more of this information. It is not a matter of adding something to a sensory process but, rather, of responding more selectively to the variables and invariants of stimulation. It is a matter of learning to attend to distinctive features of the stimulation, as opposed to adding distinctive responses. Development is characterized by reduction of primary generalization (unselective response to a wide range of similar stimuli) and increased refinement and precision of discrimination. Emphasis is thus on learning discriminations, rather than equivalences (as in a schema theory). The differentiation process may be one of progressive splitting of large dimensions, or it may be one of training the attention to critical dimensions, or it may be both. Any increase in “equivalence” responses, such as those produced by perceptual constancies, is explained as learning to respond to higher-order invariants in stimulation, rather than as the learning of common mediating responses, or interpretations.

Models of perceptual development

Neurological models of perceptual learning (see, for example, Hebb 1949) are becoming fairly numerous, and so are computer models (Uhr 1963). It is to be expected that these models will influence psychological theories (and vice versa) and that achievement of a satisfactory merger of all three will lead to faster progress in our knowledge of how perception develops.

Eleanor J. Gibson

[See alsoSensory and Motor Development. Other relevant material may be found inDevelopmental Psychology; Hearing; Skin Senses and Kinesthesis; Vision.]

bibliography

Allport, Gordon W.; and Pettigrew, Thomas F. 1957 Cultural Influence on the Perception of Movement: The Trapezoidal Illusion Among the Zulus. Journal of Abnormal and Social Psychology 55:104-113.

Carr, Harvey A. 1935 An Introduction to Space Perception. New York: Longmans.

Fantz, Robert L. 1961 The Origin of Form Perception. Scientific American 204, May: 66-72.

Gesell, Arnold; ILG, F.; and BULLIS, G. 1949 Vision: Its Development in Infant and Child. New York: Hoeber.

Gibson, Eleanor J. 1953 Improvement in Perceptual Judgments as a Function of Controlled Practice or Training. Psychological Bulletin 50:401-431.

Gibson, Eleanor J.; Walk, R. D.; and Tighe, T. J. 1959 Enhancement and Deprivation of Visual Stimulation During Rearing as Factors in Visual Discrimination Learning. Journal of Comparative and Physiological Psychology 52:74-81.

Gibson, James J.; and Gibson, Eleanor J. 1955 Perceptual Learning: Differentiation or Enrichment. Psychological Review 62:32-41.

Hebb, Donald O. 1949 The Organization of Behavior: A Neuropsychological Theory. New York: Wiley.

Hess, Eckhard H. 1956 Space Perception in the Chick. Scientific American 195, July: 71-80.

Hochberg, Julian E. 1962 Nativism and Empiricism in Perception. Pages 255-330 in Leo J. Postman (editor), Psychology in the Making: Histories of Selected Research Problems. New York: Knopf.

Koffka, Kurt (1921) 1928 The Growth of the Mind: An Introduction to Child-psychology. 2d ed., rev. New York: Harcourt. → First published as Die Grundlagen der psychischen Entwicklung: Eine Einfiihrung in die Kinderpsychologie.

Kohler, Ivo (1951) 1964 The Formation and Transformation of the Perceptual World. New York: International Universities Press. → First published as Uber Aufbau und Wandlungen der Wahrnehmungswelt.

Lashley, Karl S. (1938) 1960 The Experimental Analysis of Instinctive Behavior. Pages 372-392 in Karl S. Lashley, The Neuropsychology of Lashley: Selected Papers. Edited by Frank A. Beach et al. New York: McGraw-Hill. → First published in Volume 45 of Psychological Review.

Lashley, Karl S.; and Russell, J. T. 1934 The Mechanism of Vision: 11. A Preliminary Test of Innate Organization. Journal of Genetic Psychology 45:136144. → This is No. 11 in a series of 13 articles on the “mechanism of vision” that appeared in various psychological journals between 1930 and 1937.

Lenneberg, Eric H. 1961 Color Naming, Color Recognition, Color Discrimination: A Re-appraisal. Perceptual and Motor Skills 12:375-382.

Lorenz, Konrad Z. 1965 Evolution and Modification of Behavior. Univ. of Chicago Press.

Piaget, Jean 1961 Les mecanismes perceptifs: Modeles probabilistes, analyse genetique, relations avec Vintelligence. Paris: Presses Universitaires de France.

Piaget, Jean; and Inhelder, Barbel (1948) 1956 The Child’s Conception of Space. London: Routledge. → First published in French..

Riesen, Austin H. 1958 Plasticity of Behavior: Psychological Aspects. Pages 425-450 in Symposium on Interdisciplinary Research, University of Wisconsin, 1955, Biological and Biochemical Bases of Behavior. Edited by Harry F. Harlow and Clinton N. Woolsey. Madison: Univ. of Wisconsin Press.

Stratton, George M. 1897 Vision Without Inversion of the Retinal Image. Psychological Review 4:341360, 463-481.

Uhr, Leonard 1963 “Pattern Recognition” Computers as Models for Form Perception. Psychological Bulletin 60:40-73.

Walk, Richard D.; and Gibson, Eleanor J. 1961 A Comparative and Analytical Study of Visual Depth Perception. Psychological Monographs 75, no. 15:1-44.

Werner, Heinz (1926) 1957 Comparative Psychology of Mental Development. Rev. ed. New York: International Universities Press. → First translated from the German in 1940. The revised edition was first published in 1948.

III DEPTH PERCEPTION

The term “depth perception” may mean both the perception of the distance of an object and the perception of its solidity (the slant or curvature of its surfaces). For reasons to be explained, these properties taken together are known as the third dimension. The term may also mean “space perception,” on the theory that perception of an array of objects at various distances from the observer and from one another implies the apprehension of abstract space. It is thus related to still other kinds of perception, such as direction from the observer and orientation relative to gravity. The term is obviously connected with object perception and with constancy of perception.

The phenomena of depth perception are highly various and have been described in different ways for centuries, cutting across the disciplines of geometry, physics, philosophy, and psychology as well as the arts of painting, sculpture, and architecture. Discussion has been frequently plagued by confusion of the concrete with the abstract and of phenomenal properties with physical variables. The relation of the visual to the tactual perception of depth has never been clear.

That many kinds of visual depth perception do exist is indicated by the behavior of animals, notably their avoidance of obstacles or precarious places and their choice of the nearest goal or the shortest path to a goal. The precision flying of birds and insects certainly suggests that they perceive space in some meaning of the word. This type of behavior is studied more and more as experimental psychology becomes less anthropomorphic. But the whole topic of depth perception is still centered on the classical problem of how an individual can perceive in the visual third dimension when the sensations on which the perception must be based are in only two dimensions. It should be noted that the problem has been thought to be visual, not auditory or tactual, and that it arises from human introspection.

The problem of visual depth perception

The puzzle of three-dimensional perception arises from the following very old assumptions about vision, depth, and perception:

(1) The eye works like a camera, and the retinal image is comparable to a flat picture. The information delivered by the visual sense is therefore bidimensional.

(2) When depth is perceived, a plane form is converted into a solid form in the same way that plane geometry is related to solid geometry. When distance is perceived, a third dimension is added to the two dimensions of the frontal plane just as the third axis is added to the two axes of Cartesian coordinates.

(3) Perception is a mental construction imposed upon the data of the senses; the nature of this construction is to be discovered.

These assumptions seemed unquestionable as early as the eighteenth century. In the first case, the gross anatomy of the eye was known by then; the projection of an image by a lens had been studied; and the inverted image on the retina of an excised eye had been observed, along with the image on the rear wall of a camera obscura. Newton could speak with confidence of “the picture painted on the fund of the eye.” Second, the power of analytic geometry in supplementing plane geometry had been demonstrated, and the geometerpainters were daily using the laws of perspective projection on a picture plane to create astonishing illusions of reality. Since a space of three dimensions could be converted into two, the natural question was how a retinal picture might be reconverted into three dimensions by the mind. Third, men had been made conscious of their sensations and their dependence on the corresponding sense organs by the discovery of curious phenomena like the blind spot of the eye. The doctrine of sensation as a necessary precondition for perception, or even knowledge, had become established. The question was how perceptions could arise from these bare impressions, or, for vision, how the experience of objects in space could arise from the data found in a picture, e.g., colored spots, lines, or plane forms.

Attempted solutions of the problem

For two centuries some of the greatest minds in Western civilization struggled with the problem of depth perception. It was the chief battleground for competing theories of human nature, such as nativism versus empiricism and rationalism versus sensationalism. Many solutions have been offered, but efforts to solve the problem continue into the present. The history of experimental psychology is permeated by it, as Boring (1942) has shown. The interest has been so great that diluted versions of certain explanations have become popular knowledge. Two of the most common beliefs are that depth perception depends on binocular vision and that it is learned (i.e., it is an acquired skill). The actual explanation, in contrast to popular belief, is not at all simple. The main efforts over the centuries are outlined below.

Binocular vision. According to the binocularvision hypothesis, tridimensional vision, now often called stereovision, comes from the ability to use two eyes, either one alone being insufficient. The idea goes back at least to Berkeley (1709). The first form of the hypothesis asserted that the mind (or the brain) computes the depth of a fixated object by “feeling” the directions of the two eyes and then triangulating and also by “feeling” the amount of focusing necessary for each lens. There is still argument about whether the eye muscles can register and report such convergence and accommodation. A later, more sophisticated addition to the binocular hypothesis declared that the disparity of the two retinal pictures was transmitted to the brain and converted into perceived depth. The validity of this lawful geometrical incongruence, the difference in perspective due to the differing parallax of the eyes, was impressively shown by the discovery of the stereoscope (see Boring 1942, p. 282) and was popularized in the late nineteenth century by the manufacture of parlor stereoscopes.

Binocular disparity does provide information about depth if the ocular equipment can register it. But not all animals have eyes that can do so. Moreover, questions of how two separate pictures could be transmitted to the brain and how they could be compared by anything short of a man in the brain have never been answered. In general, the hypothesis is not sufficient to explain depth perception, for it does not take into account the animals that have lateral eyes instead of the forward-pointing eyes with compulsory conjugation of eye movement of humans, or the men who have only one eye or who, having two eyes, are unable to see any special sort of depth in a stereoscope. [SeeVision, article onEye Movements.]

Past experiences. A second general hypothesis states that depth perception consists of learning to perceive the flat retinal images as objects. The original sensations are converted into perceptions by an accumulation of experiences with the outer world. This has been by far the most appealing explanation over the centuries. One formula suggests that a given sensation calls up from memory all the other experiences—such as approaching, touching, and handling the object—with which it has been regularly associated. Vision gets its meaning from touch. Experiments with artificial inversion or displacement of the visual field are tests of this hypothesis. With upside-down vision do we eventually learn to see things where we feel them to be? The answer is no, despite a popular understanding to the contrary. A more defensible formula suggests that certain properties of retinal images are signs, indicators, or criteria of depth in the corresponding objects and that the child learns to interpret these signs. This notion was elaborated by Helmholtz in 1867 in the theory of unconscious inference, which asserts that the original act of interpretation drops out of consciousness in the adult, who therefore believes he sees the depth of the world immediately. [See the biographies ofHelmholtzandLotze.]

The usual list of the clues for depth (or “cues,” to play down the rational implication) consists of linear perspective, the known size of familiar objects, the heights of the form in the visual field, the covering of one object by another, aerial perspective, and motion parallax. All of these except the last can be represented by a painter and are supposed to occur in the retinal picture. The cues are difficult to isolate experimentally or to specify exactly, and no listing can be considered exhaustive. And the so-called cue of motion parallax is not so much a motion of an object as a complete transformation of the entire field of view of an observer during locomotion (Gibson 1950).

Intuition. Another hypothesis holds that the sensations are converted into spatial perceptions by the intrinsic capacities of the mind. The simple doctrine of innate ideas—of the soul that enters the body at birth—lost ground during the Enlightenment, but to some thinkers the abstract ideas of space, motion, and form could not be derived from sensations. A capacity for other-than-sensory intuition was demanded, and a highly sophisticated form of nativism evolved in the writings of Kant. He asserted that depth is perceived because the very nature of perception requires a preconception of tridimensional space.

In general, scientific psychologists have been unsympathetic to any appeal to innate capacities and continue to search for an explanation in terms of learning whenever possible. Experimental evidence keeps suggesting, however, that animals and children do not experience sheer sensation even when they have had no opportunity to learn the cues for depth, to associate vision with other experience, or to acquire visual conditioned responses. This is perforce interpreted as evidence for nativism. The issue persists therefore as a dilemma: Empiricism will not explain all the facts, yet nativism is unwelcome.

Gestalt theory. A fourth general hypothesis, proposed in the early 1930s, purports to provide an alternative to either nativism or empiricism. It suggests that depth perception depends on a spontaneous process in the brain called “sensory organization.” Bits of color are transmitted to the brain and converted into phenomenal objects, but not by associative connections and not by intuition. Instead, it was suggested that there exist field forces that yield some electrophysiological entitylike the object. Since the brain is tridimensional, the neural substrate of experience is also tridimensional, and the experience is tridimensional.

It was hoped that the laws of visual sensory organization could be discovered by presenting spots, lines, angles, and curves to an eye and simply noting that these pictorial elements, depending on their relations, tended to yield perceptions of objects in space. They did in fact do so, and the study of this seeming process of unit formation was a considerable advance over the classical listing of the cues for depth. These investigations (Koffka 1935) have not yet developed into a scientific enterprise, but they did establish the fact that the components of a picture need not be representational in order to arouse spatial experiences. They suggested, or now suggest, that the physical structure of light entering an eye may carry a much richer load of information about the environment than had previously been supposed. But they did not serve to display the hoped-for laws ofneural organization. Moreover, the independent neurological evidence for field forces is dubious.

The expectation that a field theory of brain activity would resolve the dilemma of nativismempiricism has thus not been realized. Most psychologists consider it a new form of nativism, on the grounds that if we must assume a brain that automatically produces fields that are like objects, we might as well assume an intuition of objects in the first place. There have been attempts to show how a brain might automatically produce forms by the residues of eye movements that have repeatedly traced the contours (e.g., Hebb 1949), but these attempts are motivated by empiricism and, in any case, do not account for depth perception. [SeeGestalt Theory; Nervous System, article onStructure and Function of the Brain; and the biography ofKoffka.]

Summary. None of these four efforts at solution of the problem of depth perception has been successful. To summarize, binocular vision is only a partial explanation; past experience and intuition pose dilemmas; and gestalt theory does not really provide an alternative.

Contemporary trends of thought

Since the 1930s there has been a shift of interest away from the classical problem of tridimensional vision and toward another problem that is directly related to it but is a little closer to life: the puzzle of the perceived constancy of the size and shape of an object despite the variation in the size and shape of the retinal image. If sensations of retinal size and shape are prerequisite to perceptions of objective size and shape, then a perception of distance must enter into any perception of size and shape. The classical problem is not avoided by this formula; one puzzle is only substituted for another. However, the emphasis on objects instead of abstract space points to the possibility that the essence of the problem may lie in the perception of environmental surfaces and their layout. This is the writer’s belief.

Another recent trend is the avoidance of categorizing perceptions as either innate or learned and the study of the development of environment perception as it actually occurs in children or animals. The contribution of learning to development can be assessed by observations of the development of spatial perception when there is no opportunity to learn. The results vary according to the spatial situation and the species of animal, but recent results give little encouragement to theories advocating that organisms learn cues for depth or the association of vision with touch.

Still another tendency is a growing recognition that the perception of space cannot be considered apart from the perception of time. In fact, it is possible that the first and simplest perceptions do not involve either abstract space or time but are instead direct detections of motion, change, and transformation in the world. If it were true that transformations were themselves stimuli, the formulation of an entirely new theory of stimulation would have to take place, as well as a re-examination of the senses, and a rejection of the theory that perception is based on the classical data of sense. Perhaps the difficulty of the old theories lies in their conceptions of the nature of sensations and their assumption that sense organs could do no more than provide them. [See Time, article on Psychological Aspects.]

Is the problem one of depth perception?

The writer believes that the problem of depth perception has been insoluble because the assumptions that gave rise to it can be challenged. First, it is possible that the information delivered by vision is not bidimensional, since the retinal image is not really analogous to a picture and since the ocular equipment does not actually work like a camera; the information obtained by vision is multidimensional. Second, it is likely that what animals and men perceive is not depth (the third dimension) but is the layout of environmental surfaces and their own movements relative to this arrangement. The information for this perception is mathematically and optically complex, but the resulting experience is simple. Third, it is possible that perception is not a mental construction imposed upon the data of the senses but a direct registration of the information available in ambient light, in the direction of gravity, and in the pressure of the surface of support. The visual sensations are symptoms of perception, but they are not entailed in perception. If so, the need to explain how they might be converted into perceptions disappears. The problem of depth perception vanishes. What emerges instead is a new problem, the problem of perception as such.

For this new problem, to be sure, one has to assume that there is information in reflected light about the layout of surfaces in the world, not just energy differences of intensity and frequency. One needs a new kind of optics concerned with gradients and transitions instead of the old optics concerned only with points. Similarly, one has to assume that the pull of gravity on the weights inside the inner ear provides information about the direction of gravity, not sensations from the receptors, and that the pressures of the earth on the skin specify where the earth is relative to the body, not just where the pressures are relative to the skin. In all these cases the information is invariant with a change of position of the observer, whereas, of course, the sensations vary with every movement of the eye or the head or the body.

When the stimulus information is reduced or impoverished, as it often is in experiments on sensation, the remaining perceptual process can be fairly described as one of mental construction, inferring, or guessing. But when multiple redundant information is available, the perceptual process can be conceived as one of detecting the invariant information in the stimulus flux. If the observer’s perceptual system is not already attuned to it, he can usually learn to extract what is invariant by exploration. Or his attention may be educated to the regularities that specify the subtle features of the world by training or teaching. Exploratory looking, listening, feeling, smelling, and tasting are characteristic of the child, and the development of these modes of attention continues in the adult. But this theory of the development of perceptual systems contrasts with the doctrine that modes of sensation are the starting point of perception in the newborn child and that learning consists of mental operations on these raw data.

Theories of the conversion of sensations into perceptions seem unable to resolve the issue of whether this conversion is innate or learned. Hence the continuing controversy over evidence showing that infants can see depth and evidence showing they cannot. A theory of the development of information pickup would escape this dilemma, for flat pictorial visual sensations do not enter into it. The assumption would be that infants see the main features of the surrounding environment (differing somewhat from one species of animal to another) and that by maturation and learning the finer features of the world come to be discriminated. The change is not from flat vision to spatial vision but from vague to specific perception.

So radical a revision of the concept of perception implies an unwelcome reconstruction of other psychological theories. Few psychologists would be so skeptical, although most would agree that the problem of depth perception has been misformulated in one way or another. The concept of a retinal picture is so simple and convincing, however, and is so firmly asserted by all the physicians who treat our eyes and all the designers of instruments to make optical pictures that it will die hard. Even physiologists who understand that the ocular system found in many animals—bees, for example—does not provide a retinal image, although it registers some of the information in ambient light, continue to assume that a picture must be formed if an animal is to perceive visually.

A solution to the problem of the perception of the environment by the use of the eyes will probably depend on new theories in optics concerning the structure of ambient light instead of images, new studies of the ocular equipment of insects, animals, and men concerned with the system as a whole instead of the retina merely, and a new theory of the perceptual process starting from a position of epistemological realism instead of subjectivism.

James J. Gibson

[Other relevant material may be found in Gestalt Theory; Senses; Sensory and Motor Development; Skin Senses and Kinethesis; Vision.]

bibliography

The facts and phenomena of spatial perception are voluminous, and no modern compendium is available except Gibson 1950. Textbook treatments tend to repeat one another. Most popular accounts of depth perception are untrustworthy. The problem of depth perception and its history are given in Boring 1942. The past experience theory is still best represented in Helmholtz 1867. The gestalt theory is most fully described in Koffka 1935. The complexities of human binocular vision are described in the textbooks of ophthalmology, e.g., Linksz 1952. For an account of what pictures and paintings can do, with an effort to comprehend the muddle of psychological theory, see Gombrich 1960. The position expressed at the end of the above article is further elaborated in Gibson 1950; 1959; 1966.

Berkeley, George 1709 An Essay Towards a New Theory of Vision. Dublin: Pepyat.

Boring, Edwin G. 1942 Sensation and Perception in the History of Experimental Psychology. New York: Appleton.

Gibson, James J. 1950 The Perception of the Visual World. Boston: Houghton Mifflin.

Gibson, James J. 1959 Perception as a Function of Stimulation. Volume 1, pages 456-501 in Sigmund Koch (editor), Psychology: A Study of a Science. New York: McGraw-Hill.

Gibson, James J. 1966 The Senses Considered as Perceptual Systems. Boston: Houghton Mifflin.

Gombrich, Ernst (1960) 1961 Art and Illusion: A Study in the Psychology of Pictorial Representation. 2d ed., rev. London: Phaidon; New York: Pantheon.

Hebb, Donald O. 1949 The Organization of Behavior: A Neuropsychological Theory. New York: Wiley.

Helmholtz, Hermann VON (1867) 1925 Helmholtz’s Treatise on Physiological Optics. Edited by James P. C. Southall. Volume 3: The Perceptions of Vision. New York: Optical Society of America. → First published in German.

Koffka, Kurt 1935 Principles of Gestalt Psychology. New York: Harcourt.

Linksz, Arthur 1952 Physiology of the Eye. Volume 2: Vision. New York: Grune.

IV PERCEPTUAL CONSTANCY

It is a well-known fact of everyday experience that such characteristics of objects as their color, form, and size tend to remain invariant under changing conditions of stimulation. Thus a white sheet of paper looks white in bright sunlight and in deep shadow, a coin still appears round when it is turned out of the frontal-parallel plane and its image on the retina is elliptical, and a man appears to be about the same height at twenty feet as he does at ten feet although the retinal image is reduced to one-half in linear dimensions in the former case. The constancy of object properties is not limited to visual perceptions but is found also in other sense modalities; for example, an orchestra sounds equally loud in all parts of an auditorium that has good acoustic properties, and different temperatures feel “right” or comfortable within wide limits. If the brightness of objects were determined solely by the amount of light they send to the eyes, then on an overcast day a piece of chalk would appear as dark as a lump of coal on a sunny day; and in the course of one day the same objects would take on all lightnesses between white and black. It is evident that objects as perceived tend to remain fairly stable in contrast to the changing conditions of stimulation. The fundamental nature of visual constancy was noted by Helmholtz (1867) and Hering (1872-1875), who saw that it posed important problems for physiological optics. We now know that perceptual constancy has even greater importance in that it is part of the broader problem of biological adjustment and survival.

Invariance of object properties . That perceptual constancy has its origins in basic mechanisms and is not wholly a product of memory or intellectual inferences about the way things ought to look is proved by experiments with very young children and adults. The results with children of different ages, while conflicting, show that they do not perceive the colors, shapes, and sizes of objects merely in accordance with the images projected on the retina but rather in accordance with the invariant properties of the objects (Woodworth [1938] 1960, chapters 14-16). Experiments with animals below man on the phylogenetic scale are decisive. Fish taught to get food in troughs painted a certain color continued to choose the same troughs even when the intensity and color of the illumination were changed radically (Burkamp 1923). Hens taught to discriminate white rice grains from yellow in daylight illumination continued to pick the white grains in strongly colored yellow light (Katz & Revesz 1921). Finally, chimpanzees that chose food from containers having high or low reflectance (white or black) under normal conditions of illumination also chose the same containers when the amount of light reflected from the containers with high reflectance was actually less than that from the containers with low reflectance (Kbhler 1915).

Psychological principles . Much mystery has surrounded the ability of organisms to respond to invariant properties of objects, but a consideration of certain facts enables us to bring phenomena of perceptual constancy within the framework of well-known physiological and psychological processes, thus relating them to the wider realm of biological adaptations. These facts may be summarized in a number of principles having general applicability (Helson 1964).

Organisms adjust to level of stimulation. The first of these principles is that organisms adjust their level of response to the level of stimulation. If the average level of stimulation is high, as in bright sunlight, the eyes rapidly adapt to the high energies reaching the retina and thereby reduce the net effectiveness of the bright light. Conversely, if the level of illumination is low, adaptation quickly makes the eyes more sensitive to incoming stimulation through regeneration of photopigments and also by the much faster action of less understood neural amplification mechanisms, thereby increasing visual effectiveness. The role of adaptation in constancy, while recognized, was not sufficiently appreciated by nineteenth-century workers in visual science because they thought only in terms of what Walls (1960) has called the “cold molasses” kinetics of photopigment concentration, which could only account for the slow, intensive sorts of adaptation. Modern work (e.g., Schouten & Ornstein 1939) has demonstrated that there is a rapid, almost instantaneous, adjustment of visual processes to take care of sudden changes in stimulation and that there are mechanisms in other sense modalities for rapid adjustments to changed stimulation to bring the organism into equilibrium with the environment.

Organisms respond to ratios of stimulation. The second principle at work to preserve constant properties of objects is that organisms respond to ratios of stimulation as well as to absolute amounts of energy. When the general illumination is raised or lowered, the relative amounts of light coming from different objects to the eyes remain the same: white objects still reflect about thirty times as much light as black objects, and hence the former look white and the latter look black in both bright and dim illumination. Contrast effects, whether simultaneous or successive, which depend upon ratios rather than absolute amounts of stimulation, are largely responsible for the stability of the visual world.

Constancy rarely perfect. A third, often overlooked, principle operative in the perception of object properties is paradoxical from the point of view of the concept of constancy. Constancy is hardly ever perfect; it is almost always approximate and partial. It is more proper to speak in terms of “approximation” to constancy than in terms that imply unchangeable perceptions in the face of all changes in physical input to sense organs. Let us suppose for a moment that constancy were perfect. Then we would not recognize differences between objects under bright and dim illumination; objects at a distance would appear as large as those close by; and we would feel no different on a cold day from the way we feel on a warm day. There is biological utility in the perception of changes in general illumination and in size and form of objects and in the discrimination between far and near sources of sound. Actually there is never perfect constancy but only what Thouless (1931) called perceptual “regression to the real” object. To be sure, a white sheet of paper looks white in low illumination, but the white is dimmer than in bright light and hence gives an indication of the lower amount of light it sends to the eyes; similarly, a man at twenty feet is perceived to be about as tall as he is perceived to be at ten feet, but he is seen in a perspective that places him at a greater distance and so is a somewhat different perceptual object. Thus we have constancy with change, the one giving information about invariant object properties, the other giving information about changes in the relations of objects to the organism. Technically the facts reduce to this: while some dimensions of perception remain constant with changing stimulation, others do not, with the result that we are able to recognize objects as the same in altered environments.

Constancy and homeostasis . Perceptual constancy has a parallel in the concept of homeostasis (Cannon 1932), according to which physiological mechanisms act to preserve “normal” values of certain critical constants such as 98.6° F. body temperature, pH of 7.40 acid-base equilibrium of the blood, normal blood-sugar levels, etc. The range over which objects are perceived as normal, or the same, is, however, much greater than the range over which physiological constants may change while maintaining a normal state of health. However, it should be remembered that much of the constancy found in behavior depends upon the action of the physiological homeostatic mechanisms, for example, the ability to withstand fairly large changes in external temperature and the reduction in oxygen supply at high elevations.

Factors affecting constancy . While the discussion of constancy to this point has made it seem like a fairly simple, univocal phenomenon, it is by no means so when all the facts are considered. Constancy may be reduced and even made to disappear both by altering the field conditions under which objects are seen and by instructions to observers to adopt various attitudes in judging attributes of objects. Thus if one looks at objects through a long, narrow, black tube, their color, size, and shape are seen in accordance with the properties of the retinal image and there is little or no constancy. If observers are asked to judge actual physical size as against apparent size, then the perceived size increases with the distance of the object from the observer (superconstancy or overcompensation). If, on the other hand, observers are asked to judge analytically, or in accordance with “apparent size in perspective,” then perceived size decreases with the distance of the object from the observer (Carlson 1960). For some purposes one attitude is better than another: the painter must view a scene analytically and depict objects neither as they are physically, nor as they are perceived naturally, nor entirely as they are seen analytically, but somewhere between these appearances in order that his picture may have some likeness to the object as perceived.

Personality and social factors. Studies purporting to show correlations between degree of perceptual constancy and intelligence or self-esteem (Coopersmith 1964) and psychotic states (Weckowicz 1964) may only be measuring the degree to which observers can adopt an objective or analytical attitude as against the natural way of looking at things, and hence should not be interpreted as direct correlations between the mode of perception and complex personality traits or states. Similarly, cross-comparisons, with respect to the degree of constancy, between children and adults, subhuman and human subjects, and subcultures and our culture should be interpreted with caution in view of the large part played by field conditions and attitudes on perceptual constancy. The complex nature of objects and the many factors determining how they will be perceived, that is, whether constancy, superconstancy, or no constancy is found, attest to the many resources organisms have at their command for responding adequately to objects under the changing conditions of the external world.

Harry Helson

[Other relevant material may be found in Homeostasis.]

bibliography

Burkamp, W. 1923 Versuche über das Farbenwiederkennen der Fische. Zeitschrift fiir Psychologie und Physiologie der Sinnesorgane 2 Abteilung: Zeitschrift fur Sinnesphysiologie 55:133-170.

Cannon, Walter B. (1932) 1963 The Wisdom of the Body. Rev. & enl. ed. New York: Norton.

Carlson, V. R. 1960 Overestimation in Size-constancy Judgments. American Journal of Psychology 73:199-213. → Good discussion of the effects of attitude on perceptual constancy.

Coopersmith, Stanley 1964 Relationship Between Self-esteem and Sensory (Perceptual) Constancy.Journal of Abnormal and Social Psychology 68:217-222.

Helmholtz, Hermann Von (1867) 1924-1925 Helmholtz’s Treatise on Physiological Optics. Edited by James P. C. Southall. 3 vols. New York: The Optical Society of America. → Translated from the third German edition.

Helson, Harry 1943 Some Factors and Implications of Color Constancy. Journal of the Optical Society of America 33:555-567.

Helson, Harry 1964 Adaptation-level Theory: An Experimental and Systematic Approach to Behavior. New York: Harper.

Hering, Ewald (1872-1875) 1964 Outlines of a Theory of the Light Sense. Cambridge, Mass.: Harvard Univ. Press. → First published in German as Zur Lehre vom Lichtsinne. A revised and enlarged edition was published in 1920 as Grundziige der Lehre vom Lichtsinn.

Katz, David (1911) 1935 The World of Colour. London: Routledge. → First published in German.

Katz, David; and Revesz, G. 1921 Experimentelle Studien zur vergleichenden Psychologic Zeitschrift fur angewandte Psychologie 18:307-320.

Koffka, Kurt 1935 Principles of Gestalt Psychology. New York: Harcourt. → Gives the gestalt interpretations of the perceptual constancies.

KÆhler, Wolfgang 1915 Aus der Anthropoidenstation auf Teneriffa: 2. Optische Untersuchungen am Schimpansen und am Haushuhn. Akademie der Wissenschaften, Berlin, Physikalisch-Mathematische Klasse,Abhandlungen [1915]: no. 3.

Locke, Norman M. 1935 Color Constancy in the Rhesus Monkey and in Man. Archives of Psychology 28, no. 5.

Schouten, J. F.; and Ornstein, L. S. 1939 Measurements on Direct and Indirect Adaptation by Means of a Binocular Method. Journal of the Optical Society of America 29:168-182.

Thouless, Robert H. 1931 Phenomenal Regression to the “Real” Object. British Journal of Psychology 21: 339-359; 22:1-30.

Walls, Gordon L. 1960 “Land! Land!” Psychological Bulletin 57:29-48.

Weckowicz, T. E. 1964 Shape Constancv in Schizophrenic Patients. Journal of Abnormal and Social Psychology 68:177-183. → Contains a brief review of previous investigations of size, distance, and shape constancy in schizophrenic subjects.

Woodworth, Robert S. (1938) 1960 Experimental Psychology. Rev. ed. by Robert S. Woodworth and Harold Schlosberg. New York: Holt.

V ILLUSIONS AND AFTEREFFECTS

Illusions

According to a dictionary, illusion is denned as “a perception of a thing which misrepresents it or gives it qualities not present in reality.” This is a common-sense definition, not reflecting the insights of scientists who study perception and insufficient for the purposes of this discussion.

The main implication of the term “illusion” centers on what is real and what is mistaken perception. The question of reality has engaged the best minds among philosophers and scientists for centuries, and for them reality turns out to be something different from the common-sense belief; but the ordinary man is well satisfied with his own conception of what is real. The traditional common-sense belief is that objects and their properties exist independently of perception and thus lie outside of perception and that these objects constitute absolute reality. It is believed that perceiving consists in becoming aware of these objects and their properties. Present-day science, on the other hand, holds that there are two “worlds” the world of the physicist and chemist, which is the energistic universe of molecules, atoms, and subatomic particles, and the perceptive and cognitive world that the human organism constructs, as a result of various forms of impingement, from the physicochemical world (Bartley 1950; 1958). The two worlds do not consist of the same kinds of items at all. Perception is the experience of the world of objects and things. Perception is induced by energy impinging upon sense organs but does not tell us anything directly about the physicochemical world. Perception must enable the organism to operate in this world. This operation is not on an absolute basis but on a probabilistic basis. Various sense modalities work together to this end. The probability of the perceiver’s effective action is greatly increased by the combined actions of several senses.

Illusions, then, are instances in which direct perception does not promote effective motor relations with the physical world or in which two sense modalities do not provide consistent information. The trouble is not one of perception misrepresenting something that it should mirror, represent, or copy, as is ordinarily supposed. In illusion there is no failure or breakdown in the lawfulness of nature, as is implied in the commonsense definition.

The classical example of two sense modalities conveying inconsistent information is provided by the wearing of glasses that invert the images projected on the retina (Snyder & Pronko 1952). The immediate result is that what is up visually is in effect down motorically. Obviously in this case vision is no guide to motor action, and great confusion and distress arise. However, with practice the motor and visual functions become harmonized, with the result that the direction in which the arm reaches for an object tallies with the visual appearance of the object’s position. Both the initial disharmony and the final harmony occur according to natural law.

The perceived location of objects need not bear any relation to where these objects may be reached by a motor act, such as extending an arm. The laws of optics determine the direction of rays of photic radiation (“light”) reaching the eye from objects. The only directional determinant given the viewer is the direction in which the rays enter the eye. Lenses, mirrors, and prisms can be interposed in various ways between photic source and eye so as to change the direction of the rays entering the eye, and as a consequence objects are seen in a new position (Bartley 1950). Every apparent position is as lawful as every other, although some positions will be unexpected and called illusory.

Kinds of visual illusions

It was shown in 1885 that intermittent lines are seen as longer than continuous ones. A short time later it was reported that vertical lines tend to be seen as longer than horizontal ones.

One of the earliest illusions involving angles was the Poggendorf illusion, reported in 1860. This was formed by a vertical rectangle or column with a diagonal line lying behind it. The two visible portions of the line do not seem to be segments of

the same line. The Zollner illusion was reported at the same time. It is depicted in Figure 1. The long diagonal lines are metrically parallel but appear to be nonparallel. The short cross-hatch lines appear to be either horizontal or vertical lines in planes that represent the treads and risers of a stairway. The whole figure seems to represent a three-dimensional situation, but it is not consistently drawn for that purpose. Therefore, the figure disintegrates into several portions, each of which does represent internally consistent elements of a three-dimensional situation. Consequently, the long diagonal lines do not seem to be parallel. In an over-all three-dimensional situation they do not represent parallel elements. Some pairs converge to a far vanishing point, and some to a near vanishing point.

Figure 2 represents another illusion, one of columns. At the left the three rectangles are parts of a frankly three-dimensional depiction. At the right they are items in a homogeneous frontal plane ground. In the three-dimensional situation, the illusion of the right-hand column being longer than those to its left emerges. In the unstructured situation, the columns look equal in size, which they should be if lying in the same frontal plane. However, for those observers who may see the three columns as if in a three-dimensional situation, some illusory effect results. The number of specific geometrical patterns in which unexpected (illusory) results are perceived is virtually endless.

Theories of illusions

A number of theories have been advanced to explain (account for) illusory effects. These have been dealt with by Boring (1942) and by Woodworth ([1938] 1960). None of the theories appears to be adequate in all respects, and many of them are even far-fetched. One of the more recent attempts to explain illusions involves the concepts of size and shape constancy (see Teuber 1960). It does not seem to be particularly convincing.

In addition to the constancy theory there are five major theories attempting to account for illusions: (1) the eye-movement theory; (2) the empathy theory; (3) the good-figure (Prdgnanz) theory; (4) the confusion theory; and (5) the perspective theory.

The eye-movement theory relies on something the observer does overtly, i.e., moving his eyes along the lines of a figure. Movement in one direction is assumed to be more strenuous than in the other; therefore, lines lying in such a direction will be overestimated. The empathy theory relies on the elicitation of an aesthetic or emotional bias in the observer. The good-figure theory assumes that

figures may either suggest or fully express some characteristic and that the observer tends to see the figure in a way that makes for the full expression of this characteristic. The confusion theory suggests that the observer is unable to abstract fully the item in question from the presentation as a whole, hence some of the properties of the whole cling to the item itself. THe perspective theory assumes that line drawings are not simply seen as items in a single plane (the frontal plane) but suggest three-dimensional scenes, with the law of perspective operating. [SeeGestalt Theory; Sympathy and Empathy; Vision, article onEye Movements.]

In all cases, the theories assume something about the way the observer operates and, if the assumptions are correct, do provide at least partial explanations of why visual presentations are seen in the ways called illusory.

It must be seen that for the result to be called an illusion, a comparison of some sort has to be implied. The present appearance of the specific item is compared with that on other occasions. Some sets of circumstances are taken to be veridical (truth telling), and others are taken to be incorrect or deceptive. It would require considerable space to discuss this matter fully, but what seems to be needed to account for the way items appear is a principle applicable as nearly universally as possible. One needs to move as far as possible away from explanations that are as numerous and variable as the examples (illusions) to be accounted for.

It would seem that to come to grips with the essential features of the unexpected, the illusory, in vision, one might resort to the use of Gibson’s texture-gradient concept of the visual field (1950). This concept has already provided an over-all, unitary understanding of the macroscopic visual field and would therefore seem to hold possibilities for the smaller portions of the field that are usually dealt with in illusion. This concept might preclude making explanations as various as the illusions to be explained.

The texture-gradient theory of space perception is, in a way, an elaboration of the perspective theory already mentioned, but it has gone so far beyond the earlier, abortive form that it merits special attention. Implicit in the theory is the idea that the perceiver tends to see things as threedimensional presentations. The manner and the degree to which this occurs depend on the particular presentation. Some presentations are geometrical figures with very few details and, of course, can be seen either as plane figures or as components of three-dimensional scenes. Some of these presentations are not consistent with actual three-dimensional scenes and cannot be so seen as totals. The figures tend to break into components, each of which can be seen as a component of a different three-dimensional depiction. Various inconsistencies result, and the illusory effects emerge from this.

Actually, the term “illusion” loses much of its traditional meaning once the operational factors underlying perception are understood.

Color illusions and nonvisual illusions

The principles involved in geometrical illusions in vision also pertain to such visual qualities as color. Color is not inherent in the physicochemical world. It is what the individual sees. It is not absolute and is not inherent in the wave length of the photic impingement (stimulus). Factors other than wave length, such as intermittence of the stimulus, also affect the color seen by the observer. Shifting rates of intermittence, while wave length remains constant, result in the sensation of different hues, saturations, and brightnesses. The entire range of connections between stimulus input and resulting color perceptions is so varied as to preclude any meaningful discussion about true colors and illusory ones. The only way one can proceed at all is to specify the spectrum source (such as color), the temporal features of stimulus presentation, parts of the retina involved, intensity of photic input, residuals of past experience, and the adaptive state of the visual system. Thus, expectations about what color will be seen are totally operationally defined, and although some results are thought of as unusual and possibly illusory, they are all the lawful products of the factors involved. [SeeVision, article onColor Vision and Color Blindness.]

Not all illusions are visual. Some are auditory. Some are haptic, involving the sense of touch. In fact, haptic parallels with most of the visual illusions are already known. They are not always recognized for what they are but are often dismissed as transfers from visual experience rather than inherent operations of the sense modalities involved.

Figural aftereffects

Figural aftereffects are but one class of manifestation of the fact that sensory inputs change the state of the responding system. Some figural aftereffects are quite ably demonstrated by repeating a given presentation of a stimulus and comparing the results. In vision, this is done by applying an image to the retina for a brief time and then applying it again, while concurrently applying the same pattern to a previously unstimulated portion of the retina for comparison. The two identical images will not produce identical perceptions.

Physiological residue from previous response is not at all unusual. In fact it is the rule. It so happens, however, that some kinds of sensory results stemming from residual physiological effects have proved of more interest than others. Those classed as figural aftereffects provide one of the best examples. Visual afterimages provide another.

Not all figural aftereffects are visual. They exist in other modalities, such as the tactual-kinesthetic complex. One of the early instances of visual figural aftereffects was reported by Gibson (1933); in this case, geometrically straight lines were perceived as curved ones. He also reported a counterpart to this in tactual-kinesthetic perception, one produced by the subject’s rubbing his ringer along a curved edge. As he did this repeatedly, the apparent curvature diminished. Subsequently, when he rubbed his finger along a geometrically straight edge, the edge appeared curved, with the curvature opposite in direction from that of the original.

In Gibson’s example we are dealing with two phenomenal aspects of the situation—the effects of repetition, or previous exposure and response, and the fact that response is illusory. Whereas illusions and figural aftereffects are, in general, independent, we can see that they may be intimately interrelated in given examples.

Theories of figural aftereffects

The explanation of figural aftereffects has taken a somewhat curious turn. Kohler (1940), who (with his colleagues) undertook the most extensive study of figural aftereffects, chose to resort to simple physics to account for them. He utilized what he calls the principle of satiation. (Since he was, by intention, dealing with a physical situation, he was dealing with what physicists and chemists call “saturation,” not “satiation.” Satiation has to do with phenomena at the personalistic or psychological level, matters of appetite, boredom, etc.)

Köhler and Wallach (1944) assumed that “satiation” occurs in the area of the sensory-projection fields of the brain as a function of the geometrical pattern of their involvement during inspection of a visual form. Thus, gradients of satiation, expressed as electrical potentials in a volume conductor (the brain), form the isomorphic bases for the perceptions of distance and shape involved. The satiation theory with its oversimplified conception of isomorphism between visual target and visual perception rides roughshod over the mass of detailed information and understandings of presentday neurophysiology. It also ignores the inadequacy of isomorphism, which has been pointed out on a number of occasions (De Laguna 1930).

The phenomena of figural aftereffects have received a very different explanation by Osgood and Heyer (1952), who utilized conventional neurophysiological concepts. Neither of these, nor any other current theory for explaining figural aftereffects, accounts for very much; hence, it is not profitable to describe them in detail.

S. Howard Bartley

[Other relevant material may be found in Sensesand Vision; and in the biography of Kohler.]

bibliography

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KÜhler, Wolfgang; and Wallach, H. 1944 Figural After-effects: An Investigation of Visual Processes. American Philosophical Society, Proceedings 88:269357.

Osgood, Charles E.; and Heyer, A. W. 1952 A New Interpretation of Figural After-effects. Psychological Review 59:98-118.

Snyder, Frederic W.; and Pronko, N. H. 1952 Vision With Spatial Inversion. Univ. of Wichita Press.

Teuber, Hans-Lukas 1960 Perception. Volume 3, pages 1595-1668 in John Field (editor), Handbook of Physiology. Section 1: Neurophysiology. Washington: American Physiological Society.

Woodworth, Robert S. (1938) 1960 Experimental Psychology. Rev. ed. by Robert S. Woodworth and Harold Schlosberg. New York: Holt.

Wundt, Wilhelm 1862 Beitrdge zur Theorie der Sinneswahrnehmung. Leipzig: Winter. → Contains writings first published between 1858 and 1862.

VI PERCEPTUAL DEPRIVATION

A relatively recent development in experimental psychology has been the study of the effects upon human behavior of a severe reduction in the level and variability of sensory and perceptual stimulation. The attempts to achieve such a reduction in environmental stimulation are often referred to by such terms as sensory isolation, stimulus deprivation, sensory deprivation, and perceptual deprivation. Whatever the terminology, this condition can produce marked behavioral changes, for example, disturbances in perception, thinking, emotions, motivation, and, occasionally, the appearance of hallucinatory-like phenomena. Furthermore, these behavioral alterations are often accompanied by long-lasting disturbances of the electrical activity of the brain.

Although experimental research on this topic is of recent origin, similar phenomena have long been reported. Observations by mystics, prisoners in solitary confinement, miners trapped underground, polar explorers, and solitary sailors have drawn attention to pronounced changes in behavior of people exposed to isolation (Brownfield 1965). Common to many of these are reports of disorientation, delusions, hallucinations, and a variety of emotional and cognitive disturbances. More recently it has been found that prisoners subjected to political indoctrination or “brainwashing” (a term first used in the Korean war) may experience similar reactions (Brownfield 1965). Finally, the medical literature provides numerous examples of behavioral disturbances associated with reduced sensory stimulation—patients with deafness, cataracts, or detached retinas, and orthopedic cases.

Although the existence of these phenomena has long been known, little or no scientific attention was paid to them until shortly after World War II, when a convergence of influences from four major sources resulted in an unusually great interest in the effects of isolation. The first source of interest came from the highly publicized “confessions” extracted by communist interrogators. What little information was available suggested that the re sults were obtained by techniques which often employed solitary confinement and the deliberate impoverishment of the prisoner’s perceptual environment. Drugs and physical torture were apparently not used. A second important source of interest came from the arrival of the space age. The crew of a space vehicle will not only have to live in very restricted quarters under relatively monotonous conditions but also, more importantly, they will be subjected to prolonged separation from society. Other technological advances, as reflected in increased use of submarines, isolated radar and meteorological stations, and of automated equipment in general, also provided considerable impetus to the initiation and development of research programs dealing with reactions to restricted sensory and social environments.

A third major source of interest originated in certain advances in neurophysiology, particularly the discovery of the brain-stem reticular formation, which is important in producing a general state of “arousal” or alertness in the organism. Since this alerting action seemed to be dependent upon constant exposure to sensory stimulation, experiments appraising the behavioral consequences of sensory deprivation were required. Finally, certain developments within academic psychology were instrumental in directing attention to this topic. One of these was in the area of motivation, in which attempts were made to show that animals have an active need for experience, i.e., they possess an exploratory drive or curiosity (Fiske & Maddi 1961, pp. 175-226). A study of human reactions to an impoverished environment might clarify the mechanisms underlying this need for experience, change, or novelty. Another development within psychology came from studies of early sensory deprivation in animals. The experiments of Austin Riesen, D. O. Hebb, and their associates had shown that laboratory animals reared under various forms of isolation subsequently exhibited strikingly abnormal behavior (Beach & Jaynes 1954; Fiske & Maddi 1961, pp. 57-105). These findings suggested the possibiliity of extending this work to humans. But, because of ethical considerations, only adults and short periods of deprivation could safely be employed.

The first experimental work on this topic began in 1951 at McGill University under the direction of Hebb (Bexton et al. 1954). Its purpose was to further our understanding of the mechanisms underlying brainwashing and of the lapses of attention noted under monotonous environmental conditions, such as watching a radar screen. The results of this research were quite startling. The subjects, who were paid to do nothing for several days except lie in a cubicle and wear translucent goggles, soon reported vivid hallucinations, delusions, impaired intellectual efficiency, and an increased susceptibility to propaganda. This pioneer work, which was terminated in 1954, soon precipitated similar research at a large number of institutions in the United States, Canada, and England (Schultz 1965).

Experimental procedures . A variety of procedures have been used to reduce environmental stimulation. Generally speaking, they fall into two main categories, namely, sensory deprivation and perceptual deprivation. In the first category, efforts are made to reduce sensory stimulation to as low a level as possible. This is usually accomplished by the use of a dark, soundproofed room in which the subject, wearing gauntlet-like gloves, is instructed to lie quietly on a cot or a mattress. Earplugs or earmuffs may be used to reduce further the level of sensory stimulation. Communication between subject and experimenter is kept to a minimum, thus reducing social stimulation. Under this category one can also place the water-immersion technique, in which the subject, wearing an opaque mask, is suspended in a large tank of water and instructed to inhibit all body movements (Lilly 1956; Shurley 1960). Because of its severity and numerous methodological problems, this method has received limited attention. Maximum endurance is approximately six hours. In the second category, perceptual deprivation, an attempt is made to reduce the patterning and organization of sensory stimulation while maintaining its level near normal. This is the procedure used in the work at McGill. The subject typically lies on a cot in a cubicle, wearing gloves and translucent goggles which permit diffuse light to enter the eyes but eliminate all patterned vision. A masking sound, usually white noise (a hissing sound like escaping steam), is directed into both ears. The intensity of the light and noise is maintained at a constant level. A less commonly used variation, employed for durations of less than a day, involves placing the subject in a tank respirator used for poliomyelitis patients and exposing him to the blank walls of a screen and the repetitive drone of a motor.

The experimental literature on sensory and perceptual deprivation has been characterized by wide differences in the quality of the studies, ranging from carefully designed experiments employing precise psychophysical measures to vaguely formulated studies using a handful of subjects, and no controls, and relying entirely on the subject’s oral reports, which are often unchecked for their reliability. The presence of numerous contradictory findings is, therefore, not surprising. Large differences also exist in the duration of the reduced sensory stimuation, ranging from several minutes to two weeks. In this article, little attention will be paid to studies employing durations of less than a day, since doubts exist as to whether many of the effects are due to reduced sensory stimulation. These short-term studies, however, have been summarized by Kubzansky (1961) and Fiske & Maddi (1961, pp. 106-144).

Affective changes . There seems little doubt that exposure to either sensory or perceptual deprivation is a stressful experience (Zuckerman 1964). The subjects often report severe boredom, restlessness, irritability, anger, unrealistic fears and anxieties, depression, disorientation in time, and vague physical symptoms that are only rarely reported by control subjects. Also, their dreams, which are often exceptionally vivid, are largely of an anxiety nature whose main theme often concerns death or restricted spaces. Finally, several instances of euphoria of the type sometimes experienced by deep-sea divers and high-altitude flyers have been reported, but usually only after prolonged periods of deprivation.

Hallucinatory-like experiences . Undoubtedly the most dramatic finding of the original McGill study was the report that a variety of vivid hallucinatorylike phenomena, similar to those described for mescal intoxication, could be produced in the majority of normal subjects by exposure to several days of perceptual deprivation. These phenomena were largely visual and ranged in complexity from dots of light, lines, or simple geometrical patterns to meaningful integrated scenes of a picture-like nature. There were also reports of hallucinations involving other senses, as well as occasional delusions and disturbances of body image. The presence of these psychotic-like reactions excited the interest of numerous investigators, particularly clinically oriented groups, since it appeared that a new experimental approach to the study of various abnormal or pathological states which had long resisted scientific analysis had been discovered. Unfortunately, the subsequent research on these phenomena has generated more problems than it has solved.

In general, the early research, largely using short-term isolation, supported the McGill findings. Lilly and Shurley reported similar phenomena after water immersion, except that they occurred in a much shorter period of time. Other investigators also commented on the prevalence of complex visual and auditory imagery, delusions, and other unusual reactions. More recent experiments, however, lasting from 2 to 14 days, have indicated that, in general, these experiences are much less prevalent and complex than was first believed, particularly under conditions of perceptual deprivation. It also appears that numerous variables may influence the complexity and frequency of these phenomena. Among these are the attitudes, suggestion, or “set” resulting from the instructions given a subject prior to isolation, the degree of motor activity permitted during isolation, and the time at which the self-reported experiences are obtained from the subject. Since these variables can influence the results to varying degrees, they may be responsible for some of the contradictory findings. It is also becoming increasingly evident that various unusual experiences, similar to those already described, are quite common even under relatively normal environmental conditions, for example, while lying quietly for several days in an ordinary room. This suggests the necessity of control groups to provide a base line of the incidence of “normally” occurring hallucinatory-like reactions against which the deprivation phenomena can be evaluated. Unfortunately, this has rarely been done in the past, presumably because it was felt that control subjects would not report “seeing” things.

In conclusion, it would appear that a variety of hallucinatory-like experiences can occur during isolation. However, the extent to which they can be attributed solely to reduced sensory stimulation remains to be established. Furthermore, it is doubtful whether these phenomena can be looked upon as mental aberrations resembling those occurring in various pathological and psychotic states. They seem to possess few of the characteristics of the reactions of the mentally ill; rather, they show a greater resemblance to certain types of normal imagery often seen during certain periods of reduced awareness, just prior to sleep, for example. In view of this, it is conceivable that future research may show that these seemingly unusual deprivation phenomena largely represent an accumulation, over time, of a variety of essentially normal experiences.

Perceptual and motor abilities . Another dramatic aspect of the McGill studies was the presence of gross disturbances of the perceptual environment. Subjects, upon emerging from several days of isolation, reported pronounced movement of the visual field; changes in size, shape, vividness, and brightness of objects; afterimages; and distortions of human faces. Although these postisolation phenomena usually disappeared within half an hour, some subjects still experienced them a day later. Subsequent research, however, also of 2 to 14 days’ duration, has not verified the existence of these perceptual distortions. Although various objects are seen as much brighter and more vivid in color, they seem to undergo no gross changes in size, shape, or movement. What distortions do occur are both minimal and transitory in nature and could be due to temporary disturbances of eye movements and equilibrium. The reasons for this discrepancy in results are difficult to find. Perhaps the McGill results were produced by some unique interaction of several variables of a procedural, personal, or motivational nature.

In contrast with the subjective reports, the results of objective tests of perceptual and motor functions show greater agreement. There seems little doubt that various aspects of visual-motor coordination are impaired, e.g., rail walking, handwriting, rotary pursuit, mirror tracing, and various measures of simple eye-hand coordination. Color perception is also uniformly impaired. On the other hand, certain basic perceptual processes, such as depth perception and size constancy, are immune to even prolonged periods of deprivation.

A few measures, surprisingly, are facilitated. Auditory vigilance, as measured by speed of reaction to infrequently presented tones, is improved. An increase in tactual acuity and in pain sensitivity also occurs. Since this cutaneous supersensitivity can result from visual deprivation alone, the possibility exists that a severe reduction in sensory input from several modalities may not be essential for the appearance of certain deprivation phenomena. Some of these may be specific to a particular sense modality or, alternatively, may be produced by interference with any one modality. Finally, there are suggestions that a wider variety of behavioral measures are impaired by perceptual than by sensory deprivation. This particularly applies to prolonged periods of deprivation. For example, both visual and auditory vigilance are disturbed by perceptual deprivation, while only visual vigilance is impaired by sensory deprivation. Again, rate of reversal of ambiguous figures is significantly affected by perceptual but not sensory deprivation.

Cognitive abilities . The extent of cognitive changes also seems to be related, to some degree, to the type of deprivation condition employed. Again, there are indications that disturbances of intellectual functioning are greater after perceptual deprivation than after sensory deprivation. In the original McGill research on perceptual deprivation, nearly all of the subjects reported an inability to concentrate, lack of clarity in thinking, and difficulty in organizing their thoughts. These subjective reports were confirmed by results derived from a battery of objective tests. Significant decrements were observed on tests of word making, anagrams, and various numerical abilities. Subsequent research has not only confirmed these results but has also indicated that other abilities, such as abstract reasoning and space visualization, can be impaired. On the other hand, the cognitive effects of prolonged sensory deprivation are not particularly severe. Few abilities are impaired and these only moderately. Furthermore, certain mental abilities appear to be facilitated. Memory for a prose passage and immediate memory span for digits tend to be superior. Certain types of verbal learning also seem to be facilitated. Thus it would appear that the performance of isolated subjects is not always impaired. Certain perceptual measures as well as some forms of learning and memory can improve in an impoverished sensory environment. The reasons for this differential effect, however, are not known as yet.

Susceptibility to propaganda . The evidence indicates that the beliefs and attitudes of isolated subjects can be altered by appropriate propaganda material. For example, the experimental subjects at McGill showed a greater belief in a variety of psychic phenomena, after listening to recorded lectures advocating the existence of these phenomena, than did nonisolated controls listening to the same material. They also made more requests for the lectures. Furthermore, these propaganda effects were still present to some degree two weeks after termination of isolation. Recently, other laboratories, appraising attitudes toward certain national groups, have also reported a greater frequency of requests for propaganda recordings by the isolated subjects. Attitudinal changes were also observed; but their presence and extent were dependent upon a number of variables, for example, the subjects’ initial attitudes. If these are neutral, the propaganda material is quite effective; but if their initial attitudes are either positive or negative, little or no effect may occur. Intellectual level is another variable, with susceptibility to propaganda being somewhat greater in subjects with lower intelligence. Although the results in this area are as yet meager, they are important in furthering our understanding of the so-called brainwashing phenomena and their dependence on a limited sensory environment.[SeeAttitudes, article onAttitude Change; Brainwashing; Hypnosis; Persuasion; Propaganda; Suggestion.]

Therapeutic effects . Various researchers have claimed that therapeutic benefits can be achieved by merely exposing psychiatric patients to brief periods of sensory or perceptual deprivation. Among these beneficial effects are a reduction in the intensity of hallucinations, an increase in ego strength, less rigid utilization of defenses, a greater recognition that their difficulties originated in themselves, and an increased desire for social contacts and therapeutic relationships. Unfortunately, these results are difficult to evaluate, since control groups are rarely used and agreement on results has not always occurred. Despite these shortcomings, therapeutic benefits probably do occur, particularly since certain perceptual and intellectual processes are known to be facilitated during isolation. It is also possible that even greater benefits might result from the insertion of therapeutic material into the deprivation situation. This method might prove to be extremely effective with psychiatric patients, since it would capitalize on the isolated subjects’ increased need for and receptivity to environmental stimulation.

Physiological changes . Until recently, physiological changes associated with deprivation have received little attention. Furthermore, what studies are available are largely concerned with changes in the electrical activity of the brain occurring during a week of isolation (Zubek & Welch 1963). These have revealed a progressive decrease in occipital lobe frequencies with increasing length of deprivation. They have also shown that this decrease is more pronounced after exposure to perceptual deprivation than sensory deprivation, a fact that may be related to the greater behavioral impairments which are known to occur after perceptual deprivation. The disturbances of brain wave activity are even more striking during two weeks of isolation. Not only is there a progressive decrease in frequencies in the alpha range but this decrease also appears to be approximately twice as great during the second week as during the first. Cumulative effects seem to be indicated. Furthermore, follow-up records reveal that some degree of brain wave abnormality may still be present ten days after termination of isolation. Accompanying these physiological changes, long-lasting motivational losses of up to eight days’ duration may occur. In the light of these results, one can only speculate about the possible physiological and psychological state of prisoners of war and others who, in the past, have been isolated for months or even years.

Large individual differences, however, are present. Some subjects show considerable disturbance of brain wave activity and behavior, while others reveal an almost normal record and few behavioral changes even after 14 days of deprivation. Similar results have also been observed in prisoner isolation practiced by Russian and eastern European state police. Most prisoners develop symptoms of disorganization within three to six weeks; but some have been known to endure this for many months, while others have succumbed within days. Large differences also exist in the capacity to withstand the effects of sleep deprivation. Some individuals can endure 100 hours with their functions largely intact, whereas others become disorganized and ineffective after only 48 hours of sleep deprivation. The reasons for these large individual differences are not fully understood, but it is believed that such factors as the subjects’ genetic make-up, personality, attitudes, and perception of the immediate situation may all play some role.

Data on physiological responses other than the electrical activity of the brain are sparse. Several workers have measured skin resistance, circulatory and respiratory changes, and the output of the adrenal glands, but little of a positive nature has emerged (Schultz 1965; Zubek 1964).

Tolerance of prolonged isolation . Not all volunteers can endure prolonged periods of deprivation. Some can endure the condition for many days, while others terminate it within a few hours. Despite these large individual differences, no satisfactory predictive measures of isolation tolerance have as yet been developed. Various paper-and-pencil tests of personality, together with certain perceptual measures, have been tried but with little or no predictive success (Hull & Zubek 1962). Perhaps other measures, possibly of motivation, attitudes, and values, may prove to be more rewarding. Although predicting success in advance of isolation has been ineffective, certain behavioral responses occurring early in isolation are excellent predictors of later tolerance. For example, volunteers who exhibit considerable boredom, restlessness, and time disorientation early in isolation almost invariably terminate the condition prematurely. This would suggest that preisolation tests utilizing such measures could prove to be excellent predictors of isolation tolerance.

Various factors can make isolation more tolerable and also minimize the impairments, for example, prior exposure to isolation, specification of duration in advance of the experiment, and the introduction of physical exercise while in isolation. Of these factors, exercise seems to be the most effective. Under this condition, virtually all impairments of behavior and of the electrical activity of the brain may be eliminated. This finding, incidentally, substantiates the reports of some explorers and prisoners of war who have claimed that performance of calisthenics is an effective method of combating the effects of isolation.

Theoretical explanations . A variety of theories exist as to how reduced environmental stimulation exerts its effects (Schultz 1965). Briefly, three general types of theories have been formulated: (a) The psychoanalytically oriented interpretations postulate changes in the relationship between the functioning of ego and id or a weakening of the ego for reality testing, (b) The theories of a psychological nature attribute the effects to the organism’s continuous search for order and meaning in an unstructured perceptual environment, or they postulate a disruption of the process of evaluation by which the models and strategies used in dealing with the environment are monitored and corrected. (c) Finally, neurophysiological theories emphasize the function of the reticular system because of its importance in attention, perception, and motivation. According to them, a decrease in the level and variability of sensory stimulation coming into the reticular activating system, via collateral fibers from the sensory systems, disturbs its normal relation with the rest of the brain, thus producing both behavioral and physiological effects. The neurophysiological theories seem to be the most promising but, as yet, we are still in search of an adequate theory of stimulus deprivation.

J. P. Zubek

[Other relevant material may be found in Attention; Nervous System; Sleep; Stimulation Drives.]

bibliography

Beach, Frank A.; and Jaynes, Julian 1954 Effects of Early Experience Upon the Behavior of Animals. Psychological Bulletin 51:239-263.

Bexton, W. H.; Heron, W.; and Scott, T. H. 1954 Effects of Decreased Variation in the Sensory Environment. Canadian Journal of Psychology 8:70-76.

Brownfield, Charles A. 1965 Isolation: Clinical and Experimental Approaches. NewYork: Random House.

Fiske, Donald W.; and Maddi, SalvatoreR. 1961 Functions of Varied Experience. Homewood,111.: Dorsey.

Hull, J.; and Zubek, J. P. 1962 Personality Characteristics of Successful and Unsuccessful Sensory Isolation Subjects. Perceptual and Motor Skills 14:231-240.

Kubzansky, Philip E. 1961 The Effects of Reduced Environmental Stimulation on Human Behavior: A Review. Pages 51-95 in Albert D. Biderman and Herbert Zimmer (editors), The Manipulation of Human Behavior. New York: Wiley.

Lilly, John C. 1956 Mental Effects of Reduction of Ordinary Levels of Physical Stimuli on Intact, Healthy Persons. Psychiatric Research Reports 5:1-9.

Schultz, Duane P. 1965 Sensory Restriction: Effects on Behavior. New York: Academic Press.

Shurley, J. T. 1960 Profound Experimental Sensory Isolation. American Journal of Psychiatry 117:539-545.

Solomon, Philip (editor) 1961 Sensory Deprivation: A Symposium Held at Harvard Medical School. Cambridge, Mass.: Harvard Univ. Press.

Solomon, Philip et al. 1957 Sensory Deprivation: A Review. American Journal of Psychiatry 114:357-363.

Zubek, John P. 1964 Prolonged Sensory and Perceptual Deprivation. British Medical Bulletin 20:38-42.

Zubek, John P.; and Welch, G. 1963 Electroenceph-alographic Changes After Prolonged Sensory and Perceptual Deprivation. Science New Series 139: 1209-1210.

Zuckerman, Marvin 1964 Perceptual Isolation as a Stress Situation: A Review. Archives of General Psychiatry 11:255-276.

VII SPEECH PERCEPTION

Speech perception is the identification of phonemes, that is, the vowels and consonants of language, largely from acoustic clues, and the recognition of the phonemes in combination as a word. Other clues that bear upon the perception of speech relate to rules of syntax, probabilities within the language, precision of utterance, interfering or competing noise or messages, and the listener’s familiarity with the language.

The effects of perceiving speech are both immediate (for example, an ongoing conversation) and long range, even affecting the development of language. That language is constantly changing is a truism, but that this relates to the perception of speech is reason to give the social scientist pause. The listener who fails to detect a phoneme, who erroneously hears a phoneme from afar as being of his own pronunciation dialect, or who seems to hear a sound where none was intended is on the verge of precipitating a change of language when he speaks.

Physical clues to speech perception

English utilizes approximately a dozen vowels and twice as many consonants, no more than onethird of the sounds that are represented in the International Phonetic Alphabet. Each phoneme, in turn, is a singular distribution of acoustic energy among selected frequencies below 7,000 cycles per second (cps). In the instance of vowels this energy is concentrated in two or three bands of frequencies, termed “formants” (Fant 1960).

Vowels have been successfully synthesized on the basis of two formants (Dunn 1950). Synthetic vowels have been made more intelligible by the addition of a third formant, approximately 600 cps above, and of the same intensity as, the second formant. This is particularly true of the front vowels, the ones of seat, sit, met, mate, and sat(R. L. Miller 1953; Delattre 1951). The formant concept is applicable to some consonants as well as to vowels, for example, the four consonants of the word unrolling. In general, “locus” refers to a more useful concept than does formant in describing the crucial concentrations of energy in a consonant. Locus is a “fixed frequency position” and an important clue for the perception of individual consonants in the context of vowels. Another useful concept is “transition,” that is, a gradual shift from the locus to the steady state of the vowel in a syllabic environment (Delattre et al. 1955). These characteristics provide important clues for the perception of the consonant, sufficient in the reproduction of synthetic vowels to suggest which consonant might have preceded a vowel.

There is further evidence that a consonant and a subsequent vowel interact. Individual phonemes have been extracted from recorded speech and set aside as “building blocks.” When reassembled to form new words, some consonants retained their intelligibility; others, such as those in pig, five, make, and thrash, were frequently misunderstood in the new phonetic environment (Harris 1953).

Distortion and attenuation

A common procedure for studying the determinants of recognizable speech is the introduction of distortion and attenuation of physical properties of the signal. The articulation index was developed in this manner, and it, in turn, provides spectrum and acoustic level as further clues for the perception of speech (French & Steinberg 1947).

The study of the duration of a vowel sufficient for recognition led Gray (1942) to present portions of 11 vowels, as spoken by a variety of speakers at different pitches, to a panel of listeners. They differed in their ability to identify the short sounds and found some vowels more perceptible than others; some observers identified vowels reliably on hearing no more than one-quarter of a single sound wave. At the other end of this continuum no advantages for speech perception attend the prolonging of a vowel beyond a normal length (Tiffany 1953).

Discrete segments of running speech have been eliminated by as much as one-half without serious effects (G. A. Miller & Licklider 1950; Garvey & Henneman 1950). The same outcome is accomplished through compressing speech by a procedure that avoids the usual shifts in pitch (Fairbanks 1956).

The intelligibility of speech is resistant to the effects of peak clipping, a process that alters the wave form and reduces the irregular profile of sound waves to a series of square waves (Licklider & Pollack 1948).

Multidistorted speech presents an opportunity to test whether or not the perceptual effects of distortion are predictable, for example, as the joint probability or joint intelligibility of the single distortions (Black & Agnello 1964).

In summary, acoustic clues for the perception of speech sounds and words relate to audible frequencies below 7,000 cps and to the relative amount of acoustic energy momentarily associated with selected frequencies within this range. As speech is attenuated or is distorted (e.g., through filtering) the perception of speech is impaired. However, it is more resistant to distortion than would be expected, an outcome that suggests the importance of clues other than acoustic ones in the perception of speech.

Nonacoustic clues to speech perception

Articulatory clues have been held to be crucial in the perception of speech sounds, resulting in a motor theory of speech perception. Perception of consonants is categorical and not continuous. This categorizing is in agreement with the distinctive features of articulation, especially the place at which the consonant is formed (Delattre et al. 1955). However, such mediation of articulatory correlates has been questioned because of lack of sufficient and integrated acoustic clues in synthetic speech (Fant 1964).

Probability and familiarity

Another nonacoustic feature of speech relates to probability: the more likely an event of oral language, the more readily it is perceived. For example, under circumstances where the first one thousand most frequently used words were 66.9 per cent intelligible, the tenth one thousand most frequently used words were 59.5 per cent intelligible, and the intelligibility of the intervening categories decreased systematically from the most common to the least common ones (Black 1952; Pollack 1964). The familiar vocabulary itself may differ from person to person. For example, listeners were grouped according to their values as shown by the Allport-Vernon scale,

and commonly used words were categorized similarly (Carlton 1953). Each group of listeners identified a higher proportion of words that related to its interest than words that did not. This can readily be interpreted as representing a listener’s greater familiarity with the vocabulary of his interest.

Vocabulary size. Closely akin to the probability that arises from disproportionate usage is one that derives from vocabularies of different sizes. The more limited the number of words, the higher is their intelligibility (G. A. Miller et al. 1951). Figure 1 shows the relative intelligibility of words in vocabularies of 2, 4, 8, 16, 32, and 256 words heard in different signal-to-noise (S/N) ratios. TheS/N ratio states in decibel units (db) the amount by which the speech signal exceeds the noise. Should the two have the same power the ratio would be zero. Words in a 2-word vocabulary were 80 per cent intelligible in a 12 db S/N ratio, while words of a 32-word vocabulary were only 40 per cent intelligible. The typical noise used for controlling conditions of listening is thermal, or white. All frequencies are present. The sound resembles that of a jet aircraft.

Context. A third aspect of probability that provides clues for speech perception derives from context and includes both meaning and the rules of syntax. Figure 2 depicts the intelligibility of words heard both in isolation and in sentences. The advantage provided by context may be compared directly to differences in intelligibility that accompany two S/N ratios (G. A. Miller et al. 1951; O’Neill 1954). The beneficial effects of context is even apparent in groups of words representing second-order word approximation; for example, in “eat mushrooms please speak to drive there” speak

is more intelligible than when it is spoken alone (Traul& Black 1965).

Phonemes and words

The study of phonemes and words reveals further clues to speech perception. First, the different speech sounds have unique values of intelligibility. Some sounds enhance the discrimination of words that contain them; other sounds reduce the likelihood of correct perception. The first phonemes of ought, oat, pat, fat, think, cake, and love are in the latter category. Such a list is dependent on the system in which the tests are made. For example, the phoneme Is/ has been reported as both a deterrent and an aid to intelligibility. The difference lies in the two systems over which words were heard (Black 1952).

The phonetic complexity of a word relates to its perception. In the system cited above that yielded the intelligibility values of more and less familiar words, one-syllable words of two sounds were 55.9 per cent intelligible, and words with 3, 4, 5, and 6 sounds were 56.2, 58.3, 63.0 and 67.7 per cent intelligible, respectively. Similarly, 2-syllable words were more intelligible than one-syllable words of the same number of sounds; 4-sound words of one and two syllables were 58.3 and 66.5 per cent intelligible, and 5-sound words, 63.0 and 68.9 per cent, respectively. Thus, a second syllable was more than the equivalent of an additional sound (Black 1952).

The relation of the intelligible phoneme to the perception of the word is complex, affected by the frequency of usage of the word and the phonetic complexity of the word. Nonsense syllables provide a different model: the intelligibility of the syllable is the joint intelligibility of the constituent phonemes (Fletcher & Steinberg 1929). Applied to words, this gives a highly spurious result: the longer the word, the lower the predicted intelligibility—an outcome not in keeping with the facts. Attempts to predict the intelligibility of a word from the known values of the constituent phonemes have led to the tentative formula: “The intelligibility of the word is approximately equal to the product or joint probability of the intelligibility of the first two phonemes of the word.” This does not imply that all of the phonetic value that contributes to intelligibility lies in two phonemes or that the total potential of the two is utilized (Black 1965a).

Negative and positive reinforcement

Attempts to alter speech perception led to the development of infinite peak clipping, on the one hand, and jamming, on the other (Licklider & Pollack 1948). Some impediments to perception are built into speech and relate to human frailty. For example, in one study listeners were asked to identify the final 3 words of oral sentences from 3 to 17 words in length. Even in quiet, the listeners were progressively less successful in processing the longer sentences, and the detrimental effect of sentence length increased systematically as more and more noise was introduced into the system. Correct scores associated with 5- and 15-word sentences in four conditions of noise (10, 8, 6, and 4 db S/N ratio) were, respectively: 94 and 84 per cent, 83 and 76 per cent, 73 and 47 per cent, and 50 and 27 per cent (Black 1961).

Sound similarity

Similarity in sound (rhyme) may affect speech perception. The letters sound similar to others in the same set but different from those in the other set: (b, c, t, d, v) and (f, m, n, s, x). As the letters were heard and named, the errors of identification were preponderantly within a set, not between sets. This also characterized the confusions in recall of the oral materials (Conrad 1964). Such interference was also evident with listeners who were asked to identify, from a 4-item multiple-choice format, words that they had spoken moments earlier. Recall was poorer when the stimulus word was similar in sound to the multiplechoice foils (Kresheck & Black 1964).

Unfamiliarity

An unfamiliar language has built-in interferences to speech perception. Foreign students are consistently less able to perceive English words than are American listeners, even when marking responses on a simple multiple-choice form. Interestingly, the relative skill of these students in perceiving oral English words relates to other aspects of their orientation to English, for example, the amount of time they devote to pauses in their English speech (Black et al. 1965). In turn, American students have difficulty identifying English words spoken by foreigners; however, with sixty minutes of practice in hearing simple English prose read in the pertinent foreign dialect the listeners improve markedly in their perception of the speech of foreign students (Black & Tolhurst 1955). With respect to American pronunciation dialects, when listeners in a military training program were categorized according to the regional units in which they were inducted into service, the groups varied in their ability to identify words as spoken by their peers from the various regions (Mason 1946).

Interference and delay

An intriguing characteristic of speech communication is that a listener can single out one talker from a group and converse with him in the face of an overridingly unfavorableS/N ratio. This might be termed the “cocktail party effect.” Despite this fortunate circumstance, a competing message may interfere disastrously with speech perception. A control center for air traffic may be a babel of competing messages. An experimental approach led to several recommendations for such a center: separate the sources of speech (loud-speakers) by at least 10 degrees on a horizontal plane; introduce a singular filtering in the most-used circuits; identify message sources with visual lights (Spieth & Webster 1955).

Another investigation treated a primary message and an interfering message that were presented either simultaneously or separately with slight temporal disparities. Phonetic similarity between the two messages was one deterrent; particular temporal differences, another; the primary message was least well received when it was initiated approximately one-third of a second subsequent to the initiation of the interfering message (Peters 1954).

Many gimmicks have been tried for reinforcing speech. For example, speech was presented doubly over a single line and separate lines to a listener’s ears. One rendition was delayed in varying amounts up to 330 msec. Compared to normal reception, words were heard as well, but no better, with the two renditions separated by 150 and 300 msec; other values of delay yielded detrimental effects. In a similar vein, the lower frequencies of the speech spectrum have been put to one ear and the higher frequencies, amplified, to the other, delayed or not. No improvement in speech perception resulted (Camp 1958).

Repetition and instruction

Messages themselves have been tailored for reinforcement. Repetitions gave slightly improved intelligibility but were costly in terms of time (G. A. Miller et al. 1951). Listeners have been warned against highly probable errors, that is, they have heard, “Write faith, not face.” This procedure produced a marked gain in intelligibility; however, face continued to be one of the written responses, although an infrequent one (Traul & Black 1965). The transmission of digits has been enhanced by the repetition of the initial sound of the digit, as in t-two, th-three, f-four; this is termed “voluntary stuttering” (Moser et al. 1956).

Listeners’ agreements in errors

This article focuses on correct perception of speech. An interesting pattern emerges from the incorrect perceptions: listeners tend to agree in their errors. Thus, where words were intelligible to 50 per cent of the listeners, another 25 per cent nonetheless agreed on one wrong response, and about 12 per cent agreed on a second wrong response. As a rule the second-error response occurred with one-half of the frequency of the first, and the third-error response, 0.6 of the frequency of the second. Moreover, the same proportions were present, whether in a closed message set with a specified and limited response form or in an open message set in which a listener could write any word he thought he heard (Black 1965??). That the ratios among the errors were the same in open and closed message sets is taken as an instance of the principle of the constant ratio (Clarke 1957).

This summary of representative work on speech perception has treated particularly (a) the determination of responses by acoustic clues and (b) by nonacoustic clues, (c) interferences and reinforcements that may affect perception, and (d) the tendency of listeners to make the same responses, even in error. Listening behavior generally directs speaking behavior and thus relates to changes in language, which are of interest and importance to the social scientist.

John W. Black

[See alsoHearing; Language; Learning, article onVerbal Learning.]

bibliography

Black, John W. 1952 Accompaniments of Word Intelligibility. Journal of Speech and Hearing Disorders 17:409-418.

Black, John W. 1961 Aural Reception of Sentences of Different Lengths. Quarterly Journal of Speech 47: 51-53.

Black, John W. 1965a Predicting the Intelligibility of Words III. Pages 215-217 in International Congress of Phonetic Sciences, Fifth, Miinster, 1964, Proceedings. New York and Basel: Karger.

Black, John W. 1965b The Language Barrier. Pages 101-128 in North Atlantic Treaty Organization, Advisory Group on Human Factors, Communication Processes. Edited by Frank A. Geldard. Proceedings of a symposium held in Washington, 1963. Oxford: Pergamon.

Black, John W.; and Agnello, Joseph G. 1964 The Prediction of the Effects of Combined Deterrents to Intelligibility. Journal of Auditory Research4:277- 284.

Black, John W.; and Tolhurst, G. C. 1955 Relative Intelligibility of Language Groups. Quarterly Journal of Speech 41:57-60.

Black, John W. et al. 1965 Speech and Aural Comprehension of Foreign Students. Journal of Speech and Hearing Research 8:43-48.

Camp, Robert T. JR. 1958 The Perception of Multiplechoice Intelligibility Items in the Presence of Simulated Propeller-type Aircraft Noise. Research Report No. 73. Pensacola, Fla.: U.S. Naval School of Aviation Medicine; Columbus: Ohio State Univ., Research Foundation.

Carlton, Robert L. 1953 An Experimental Investigation of the Relationship Between Personal Value and Word Intelligibility. Ph.D. dissertation, Ohio State Univ.

Clarke, Frank R. 1957 Constant-ratio Rule for Confusion Matrices in Speech Communication. Journal of the Acoustical Society of America 29:715-720.

Conrad, R. 1964 Acoustic Confusions in Immediate Memory. British Journal of Psychology 55:75-84.

Delattre, Pierre C. 1951 The Physiological Interpretation of Sound Spectrograms. Modern Language Association of America, Publications 66:864-875.

Delattre, Pierre C ; Liberman, Alvin M.; and Cooper, Franklin S. 1955 Acoustic Loci and Transitional Cues for Consonants. Journal of the Acoustical Society of America 27:769-773.

Dunn, H. K. 1950 The Calculation of Vocal Resonances and an Electrical Vocal Tract. Journal of the Acoustical Society of America 22:740-753.

Fairbanks, Grant 1956 Experimental Studies of Time Compression of Speech. Journal of the Acoustical Society of America 28:592 only.

Fant, Gunnar 1960 Acoustic Theory of Speech Production: With Calculations Based on X-ray Studies of Russian Articulations. The Hague: Mouton.

Fant, Gunnar 1964 Auditory Patterns of Speech. A paper given at the Air Force Communication Research Laboratory Symposium on “Models for the Perception of Speech and Visual Form,” Nov. 11-14, 1964. Unpublished manuscript.

Fletcher, H.; and Steinberg, J. C. 1929 Articulation Testing Methods. Bell System Technical Journal 8: 806-854.

French, N. R.; and Steinberg, J. C. 1947 Factors Governing the Intelligibility of Speech Sounds. Journal of the Acoustical Society of America 19:90-119.

Garvey, William D.; and Henneman, Richard H. 1950 Practical Limits of Speeded Speech. AF Technical Research Report No. 5917. Dayton, Ohio: U.S. Air Force, Air Material Command, Wright-Patterson Air Force Base.

Gray, Giles W. 1942 Phonemic Microtomy: The Minimum Duration of Perceptible Speech Sounds. Speech Monographs 9:75-90.

Harris, Cyril M. 1953 A Study of the Building Blocks in Speech. Journal of the Acoustical Society of America 25:962-969.

Kresheck, Jan D.; and Black, John W. 1964 Approriate Materials for Self-administered Training in Intelligibility. Journal of Speech and Hearing Disorders 29:70-75.

Licklider, J. C. R.; and Pollack, Irwin 1948 Effects of Differentiation, Integration and Infinite Peak Clipping Upon the Intelligibility of Speech. Journal of the Acoustical Society of America 20:42-51.

Mason, Harry M. 1946 Understandability of Speech in Noise as Affected by Region of Origin of Speaker and Listener. Speech Monographs 13, no. 2:54-58.

Miller, George A.; Heise, George A.; and Lichten, William 1951 The Intelligibility of Speech as a Function of the Context of the Test Materials. Journal of Experimental Psychology 41:329-335.

Miller, George A.; and Licklider, J. C. R. 1950 The Intelligibility of Interrupted Speech. Journal of the Acoustical Society of America 22:167-173.

Miller, R. L. 1953 Auditory Tests With Synthetic Vowels. Journal of the Acoustical Society of America 25:114-121.

Moser, Henry M. et al. 1956 Effects of Repeating the Initial Sounds of Words on the Intelligibility of Air Messages. Air Force Cambridge Research Center, Washington, D.C., Operational Applications Laboratory, Technical Report No. 30. Columbus: Ohio State Univ., Research Foundation.

O’Neill, John J. 1954 Recognition of Intelligibility Test Materials in Context and Isolation. Joint Project Report No. 23. Pensacola, Fla.: U.S. Naval School of Aviation Medicine; Columbus: Ohio State Univ., Research Foundation.

Peters, Robert W. 1954 Competing Messages: The Effect of Interfering Messages Upon the Reception of Primary Messages. Research Report No. 27. Pensacola, Fla.: U.S. Naval School of Aviation Medicine; Columbus: Ohio State Univ., Research Foundation.

Pollack, Irwin 1964 Message Probability and Message Reception. Journal of the Acoustical Society of America 36:937-945.

Spieth, W.; and Webster, J. C. 1955 Listening to Differently Filtered Competing Voice Messages. Journal of the Acoustical Society of America 27:866-871.

Tiffany, William R. 1953 Vowel Recognition as a Function of Duration, Frequency Modulation and Phonetic Context. Journal of Speech and Hearing Disorders 18:289-301.

Traul, G. Nygaard; and Black, John W. 1965 The Effect of Context on Aural Perception of Words.Journal of Speech and Hearing Research 8:363-369.

VIII PERSON PERCEPTION

Person perception concerns the study of processes by which we come to know and think about other persons, their characteristics, qualities, and inner states. This area has been variously named social perception, person cognition, interpersonal perception, connaissance d’autrui, to mention only some of the phrases used. It overlaps, but is not coterminous with, psychodiagnosis by the expert. In the phrase “person perception” used here, the term “perception” is used in a very loose way, most often meaning apperception and cognition.

As a physical object a person is, of course, not different from other stimuli, and the processes of perception and cognition of social and nonsocial objects are probably basically the same, as Egon Brunswik repeatedly maintained ([1947] 1956). In the sense that we perceive or infer primarily intentions, attitudes, emotions, ideas, abilities, purposes, traits, thoughts, perceptions, memories, consciousness, and self-determination—events that are, so to speak, inside the person and strictly psychological—persons are doubtless special objects. Person perception is unique in that the similarity between the perceiver and the object perceived is greater than in any other case. This fact has far-reaching consequences in that the perceiver is probably maximally inclined and able to use his own experience in perceiving, judging, or inferring another’s state or intentions.

The process by which we know people did not receive formal and separate attention until the latter part of the nineteenth century. It was the work of Darwin (1872) on emotional expressions and their recognition that gave scientific impetus to this problem area. At the beginning of this century the approach was extended to the process by which we know any characteristics of another and to determining what are the characteristics of the “good judge” of other persons. How these processes are related to action was seen as basic to an understanding of interpersonal behavior.

The general structure of this problem area becomes apparent when the various elements involved are set out for consideration. In the basic situation of person A’s perception or cognition of person B’s characteristics or states, the major components involved in the process are: (1) person B’s characteristics or state (say, fear, courage, intelligence, happiness, attractiveness to others, or intention to help); (2) the concomitants of B’s characteristics; (3) the distal cues or manifestations of B’s characteristics that are, so to speak, available to A (these in practice, include cues from the environment and are external to the person); (4) the proximal cues or manifestations of B’s characteristics that are utilized by A; (5) the cognitive processes that utilize these proximal cues; and (6) the percept or judgment by A of B’s characteristics.

These elements and their relationships correspond to the various lines of investigation that have developed around this problem area: whether emotional or mental states and personal characteristics have consistent expressions, whether these manifestations yield usable cues, the question of which cues the observer uses, and by what process he forms his judgments, and, finally, whether there is a valid connection between the state or characteristic of B and the judgment of it by A.

Common to all these approaches is the problem of finding good ways of systematically and objectively specifying the “stimulus,” the object of the perception or cognition, which is often a covert and distal variable, such as an emotion, trait, or intention. This presents difficulties not usually encountered in the more traditional regions of psychology. For example, the judgment that a person is “kindly” is not related to one specific stimulus constellation but is based on greatly diverse observations, such as appearance, gestures, and actions that radically differ from one another, and on contexts, simultaneous and sequential, that are equally diverse. The events that produce the judgment of “kindliness” in any single situation are most often too complex to be described in terms of physical dimensions, although the physical dimensions of another person and of his behavior, such as his facial features or movement in space, are regularly used by the beholder as cues of the psychological characteristics and state of the person. One line of attack is to use a consensus of judges as the criterion. In some cases these might be “experts.” Still another way of defining the criterion, especially when studying the perception of certain internal states or attitudes and feelings, is to use self-reports by the stimulus person. One may also resort to psychological test results as the criterion (e.g., when certain personality traits are judged). One other possibility is to start with the perceived stimuli and coordinate these with the further cognitive judgments they generate. By taking this approach, Heider (1958) and Jones (Jones & Davis 1965), among others, have been able to make remarkable progress in explicating how we understand others. Still other approaches have been used.

These diverse definitions of the criterion need not be equivalent, and the comparability of studies performed on the basis of such different operations cannot be assumed.

There seem to be two major directions of effort in modern studies of person perception. In one, questions are asked about the process of perceiving or knowing another person, about stimulus and perceiver characteristics and their interactions. In the other, the processes leading to the percept or judgment are not of primary interest. The focus is, instead, on the veridicality of the judgment and its correlates, on what is sometimes called empathic ability, or “accuracy/* In specific instances these two streams of work blend, and, of course, there are contributions that fit poorly into this simple dichotomy. It is, however, a useful one for surveying the work done.

The “ability” to judge others

The interest in empathic ability and its generality stems principally from an amalgam of two trends of thought: on the one hand, the work of such writers as Ludwig Klages, Theodor Lipps, Wolfgang Kohler, and Max Scheler, who were trying to explain how one person understands another in terms of such concepts as intuition, inference, and empathy; and on the other hand, the work of Charles H. Cooley, James Angell, George H. Mead, Sigmund Freud, William McDougall, Harry Stack Sullivan, and Leonard Cottrell, who, among others, were stressing the importance of recognizing others’ feelings for the process of socialization and evolution of the self. The quantitative approach, added to the fusion of the above two lines of thought, led to a search for simple operational indices of empathy and accuracy. Thus, the ability to understand others was defined as the discrepancy between a judgment and a criterion, for example, that between the self-ratings by a subject and the ratings attributed to him by a judge. Many variations of this operational definition of empathic ability or accuracy have been used in a substantial number of investigations [seeSympathy and Empathy; see also Bruner & Tagiuri 1954; Taft 1955; Sarbin et al. I960; Allport 1961; Kaminski 1959; 1963; Shrauger & Altrocchi 1964; Cline 1964; Secord & Backman 1964, for reviews of this literature].

Characteristic of these approaches is the accuracy or empathy score or rating that results from the comparison of the judge’s performance with the criterion. It soon became apparent, however, that these scores were far from being pure measures of the ability under investigation. In addition to differences in accuracy dependent upon the particular judgmental task, which suggested that several types of abilities might be involved, several complications made the interpretation of these scores difficult. For example, the tendency toward assumed similarity (i.e., the general inclination to attribute to others responses one would give oneself) generates high accuracy scores for judges who happen to be similar to the persons judged and low scores for judges who are not; thus, accuracy as an ability is confounded with a fortuitous event. Other response set phenomena and sampling characteristics of judges and judged were observed to influence the results. In 1955 Cronbach analyzed these “dyadic” global scores into a number of components. He showed that in considering a judge’s response in social perception, the theory needs to take into account “the traits being perceived, the constant tendencies in this perceiver with respect to those traits and finally the effect of the particular other as a social stimulus to this perceiver” (Cronbach 1958, pp. 375-376). He argued that instead of a global score covering all traits or themes, the procedure ought to allow for differential results depending on the characteristics involved. In the light of his analysis prior work on accuracy as an ability and on its correlates proved very difficult to interpret, and new approaches had to be sought.

While it has been impossible to design a study that takes into account all the elements, effects, and artifacts that have been identified, several investigators with more appropriate methodologies have since made new and vigorous approaches to the problem. Their reports support the view that the ability to judge others is complex; that, for instance, “stereotype accuracy” (ability to judge the generalized other) and “differential accuracy” (the ability to judge individual differences) are two of the components; and that they are to some extent independent (e.g., see Bronfenbrenner et al. 1958).

The process of knowing others

The difficulties with the problems of accuracy and of the “good judge,” and the hope that better understanding of these matters might be reached indirectly, strengthened interest in the process by which we perceive and think about others, irrespective of its veridicality. How can we know the personality or inner states of others when we do not have access to their experience? In a review of the major approaches to this broad issue Allport (1961, chapter 21) considers several explanations. One theory, founded in British empiricism, suggests that we know about others by a swift process ofinference, or analogy, of which we are seldom aware. We infer the state or characteristics of another person because the circumstances, the behavior, the sequence of events, are similar to those with which we ourselves have had experience in previous situations.

The inference theory of interpersonal knowledge accounts well for the findings that similarity between judge and other, and breadth of life experience, aid in the accuracy of judgments. Yet, according to some writers, inference theory fails to account for several other aspects of the process, particularly for the fact that we seem to comprehend transitory moods or states of mind of others without necessarily having had relevant personal experiences—a phenomenon often seen in children. This suggests that additional processes need to be postulated to account for such observations. One of these is Einfilhlung, or “empathy,” introduced by Theodor Lipps in 1907 and denned as “objective motor mimicry.” By slight movements we partially imitate the other, thus creating for ourselves cues that give us an understanding of his feelings or characteristics. It may be argued that this process can be explained, at least in part, in terms of inference theory. We draw inferences from cues derived from our own mimicry. Yet the mimicry itself cannot be accounted for in this manner, and it is worth considering it as a possible component of the process in its own right.

Finally, many writers, especially those in the phenomenological and gestalt traditions, insist that inference, even modified and supplemented by empathy as denned by Lipps, does not account for the observation that certain external objective patterns or configurations (gestalten) have an immediate meaning, which is mediated by physiological and psychological processes that do not necessarily include inferences but are of a more direct nature. According to this view, the state of the other person’s mind is related to a patterned expression on his part that directly produces in the beholder a patterned sensory and central excitation that represents or conveys the person’s state of mind.

A combination of the explanations reviewed above may be necessary to account for the phenomenon of how we know and understand others. We do it through inference and analogy, through sensory cues derived from empathic responses, and through the immediate response to external configurations and patterns that are expressions of qualities of a person “out there.” And all of these elements are intensified by the unusually demanding effect that a person has upon our attention, perception, and thought.

Yet explanations in terms of inference processes have been gaining ground, partly as a result of the increased clarification of cognitive processes, as, for example, in the approaches of Brunswik, Tolman, and Bruner. The most extensive and thoroughgoing application of this genre of cognitive theory to how we know others has been presented by Sarbin, Taft, and Bailey (1960) in their work on clinical inference. These writers argue forcefully against intuition and empathy as necessary elements in explanations of how we know others; they propose that the process of clinical inference and of person cognition can be adequately represented by a broadened syllogistic inference model with both major and minor premises stated in probabilistic terms.

On certain properties of the process

Regardless of the general explanation or combination of explanations they favor, students of person cognition seem to be in some agreement about certain characteristic features or properties of the process and about certain factors that influence it.

As mentioned, studies of accuracy of social perception indicate that the process involves several different types of judgment that had not been distinguished in most of the earlier empirical studies of empathic ability. The data used by the perceiver may correspondingly differ, and the question arises as to what are the major cues, information, or concepts that people use in perceiving or thinking about others. We should really know a great deal about this, since literature, the theater, movies, as well as our daily thoughts and conversations, are replete with such content. Some writers have begun to treat this aspect of the problem systematically, but little as yet is known about it on the basis of empirical work. The task is a difficult one, since it seems that the categories of social perception and cognition and the inferential sets mobilized vary not only with the particular nature of the judgment but also with the purpose of the social interaction under which the judgment takes place and with the role of the judge. It is also unquestionable that people can arrive at some evaluation from almost any data about the other person and that they do so with a high degree of consensus. This occurs in spite of the great variety of cues that have been used, such as actual persons, photographs, schematic representations of persons, voice, trait information, and paths followed by a person, to mention just a few.

As Brunswik ([1947] 1956) has made clear, cues are highly interchangeable, and a great variety of them can lead to the attribution of a trait or disposition. It is the attributed distal, covert disposition of the other that serves to guide behavior in relation to him, and it is with this level of event that the perceiver is concerned. Yet some dispositions and states are more important for interaction than others, and cues to these may be objects of special attention. (See Jones & Davis 1965 for a discussion of attributing dispositions from acts.)

Among the aspects of the other to which a person particularly attends are his intentions, especially when the action is directed toward the beholder. Indeed, there is a general tendency to see others as origins and responsible agents of actions. Two other aspects that seem to have strong demand for the beholder’s attention are the other person’s good— bad qualities and his relative power. These distal characteristics of the other—intention and responsibility, goodness-badness, and relative power— can be seen as crucial dimensions along which a person orients himself vis-a-vis the other. From a functional point of view, special sensitivity to these aspects of the other may be assumed.

To continue with considerations of the process, there are two sources of information external to the beholder about the states, feelings, attributes, and intentions of the other: the object person and the situation or context of the object. The other person himself, insofar as we can divorce him from the context, seems to allow the formation of impressions on which there is a measure of agreement among judges. Discriminations of emotions from facial expressions alone are possible, for example, if the categories used are broad enough, such as those proposed by Robert S. Woodworth: love, happiness, and mirth; surprise; fear and suffering; anger and determination; disgust; contempt; and a residual category (see Bruner & Tagiuri 1954 for a summary).

While there is a clear determinant of the impression in the configuration of the stimulus person himself, the interpretations of the other’s state may not be unique. In fact, judges will frequently offer two or three possible interpretations, although there are other possibilities they will not advance. The situation or context by itself is often sufficient to permit indeterminate estimates of the state of another person. We do not need to have information on the behavior and appearance of a person who has lost a loved one to make a good guess at how he feels.

Either the person or the situation, then, taken separately, allows nonrandom but indeterminate judgments. We “know” both that certain situations tend to evoke feelings A, B, or C and that a certain expression or behavior reflects feelings B, D, or E. So we judge that in this particular situation, given the person’s expression, the feeling is probably B. By combining cues derived from the person with those derived from the situation, the observer typically arrives at highly consensual and functional judgments in the great majority of ordinary situations. These two sources of cues, of course, are not often independent and are frequently redundant, thus increasing the likelihood of a veridical response. It may be that the transaction between the other person and the situation operates as a higherorder informational variable and is closely and parsimoniously attended to.

The interpersonal judgment depends not only upon the way in which person and situation cues are jointly utilized but also upon certain other operating characteristics of the typical judge himself. For example, the tendency toward “assumed similarity” is characteristic of a great deal of social perception, as is the tendency to attribute to the other a high degree of invariance by thinking of him in terms of enduring dispositions.

As in all complex cognitive processes, in the case of person objects there is a tendency to maximize cognitive balance and to avoid dissonance of elements. Thus, the characteristics of other persons are viewed as more homogeneously good or bad than they can be shown to be when independently measured. The other person is thought of as a configuration of highly integrated characteristics (traits, emotions, etc.). Knowledge of one or two traits, for example, leads to a strong expectation that other particular traits are present. The “halo effect” and the “logical error” are well-known instances of this more general phenomena. [SeeCognitive TheoryandThinking, article onCognitive Organization and Processes.]

The tendency to “make sense” of the other person’s behavior is seen again in the inclination to attribute intentionality to others even where such attribution is objectively unwarranted, to see others as origins of actions, thus forcibly integrating the person and the situation in which the events are taking place. Indeed, a very strong determinant of how the behavior of a person is understood is whether the cause of his actions is believed to be internal or external to him and, consequently, whether he is or is not responsible for it (Heider 1958; Pepitone 1958; Jones & Davis 1965). Thus, the same behavior may be interpreted as malicious or clumsy, depending upon the perceived locus of causality.

Also well known is the general inclination toward quickly placing a person into categories according to some easily identifiable characteristics—sex, age, ethnic membership, nationality, occupation—and then to attribute to him, correctly or incorrectly, qualities believed to be typical of members of that category. The term “stereotyping” is often used to refer to this form of categorizing and should not be regarded as necessarily misleading, for it has been shown that it sometimes leads to more accurate inferences about others than does detailed information about each individual person [seePrejudiceandStereotypes; see also Sarbin et al. 1960, chapter 9].

Neglected in empirical studies, but easily observed, is the tendency to adopt a base-line characteristic of the other person, against which judgments are made; to deal, that is, with variations and gradients rather than with absolute values. A mean man is judged to feel kindly toward someone if he does not hurt him. Another form of utilization of a base line is carefully described by Jones and Davis (1965), who point out that the certainty with which a disposition is inferred from observed actions and their consequences increases with their atypicality and unexpectedness.

These general tendencies can be viewed as properties of the typical implicit personality theory: the assumptions we make about the nature of other persons. These assumptions presumably affect the way we perceive and understand others, much in the way conceptions of any type of event influence what we perceive and the manner in which we perceive and understand it. [SeePersonality, articleOil The Field.]

How the other is perceived also depends with some regularity upon the interpersonal relationship between him and the judge. The qualities he is seen to have depend upon whether the perceiver likes him or not; when the perceiver does like him, the tendency toward assumed similarity is strengthened (see Secord & Backman 1964, p. 81). Differences in the relative status of the perceiver and the other person influence the attributed qualities, usually adversely (Pepitone 1958). There is also a connection between how we feel toward others and our perceptions of their feelings for us. We seldom feel liked by people we do not like ourselves (Tagiuri & Petrullo 1958, pp. 316-336; Moreno et al. 1960, part m C). The role of the beholder vis-a-vis the object person (e.g., colleague versus superior) affects the dimensions in terms of which the other is apprehended and evaluated (see Secord & Backman 1964, pp. 85-87).

The tendencies to attribute intentionality and causality, to categorize and stereotype, to integrate discordant information, to attribute particular dispositions on the basis of diverse and variant cues—all can be seen as efforts at simplification and stabilization of a highly variable human environment. These efforts are comparable in function to such phenomena as the constancies in visual perception, where invariance is achieved in spite of drastic changes in the stimulus characteristics.

Several studies have been carried out on developmental trends in person perception. A baby seems able to distinguish facial expressions made by its mother, although only extreme differences are recognizable early in life. As the child matures, the tendency to describe persons in terms of outstanding visible features—big, old, thin—decreases, and the capacity to discriminate increases rapidly, while physiognomic perception, the immediate response to stimulus configurations, tends to recede[See Bruner & Tagiuri 1954, pp. 638-639, for review of main references; see also Developmental Psychology, article onthe field; Infancy].

Sex differences in person perception have been well established. Among children, females describe adult figures in a less differentiated and more favorable manner than do males. Boys focus more on aggression, nonconformity, and physical recreation in describing others, while girls refer more to nurturant behavior, physical appearance, and social skills. Women seem to use more stereotyping than men, to be less analytical and more intuitive, and to use more psychological (as opposed to physical) terms than men (see Shrauger & Altrocchi 1964). Women seem to focus visually on those with whom they interact more than men do, perhaps because they rely on visual cues more than men do.

By and large, investigators have reported sex differences irrespective of research approach or judgmental task, and it is clear that the sex of judge and judged should always be taken into account in the design of research and analysis of data in this area. [SeeIndividual differences, article onsex differences.]

Individual differences

The processes mentioned so far are typical of how people in general perceive, think about, and form impressions of others. Even within the same cultural environment, however, there are demonstrable individual differences (see Shrauger & Altrocchi 1964).

There is some evidence that individual cognitive styles apply to both person and nonperson concepts, such as the degree to which people integrate cognitive elements, the cognitive complexity with which they conceive of human and nonhuman objects, and the extent to which they prefer analytic approaches to global ones. [SeeSystems Analysis, article onPsychological Systems.]

In addition to formal differences in cognitive approach, variations have been shown in the “theories” people have about human nature and personality. More often than not implicit, such notions affect what is attended to in others, as well as the inferential process itself. There is, for instance, considerable evidence that individuals give importance to, and tend to use, different traits in their perceptions and thoughts about others. Some persons, for example, tend to describe others in terms of external and physical traits; some do so in terms of internal and psychological traits.

The patterns of trait relationships assumed by the individual perceiver have been shown to have consistency and individuality, although there is also high consensus on trait interdependencies. Furthermore, people seem to have available a range of “personality theories” upon which they draw, depending upon variations in the stimulus person and the situation. Methods appropriate to the analysis of such multidimensional processes and interactions have only recently been developed (Jackson & Messick 1963).

The matter of individual differences in ability in understanding others has already been treated above in the course of discussing accuracy of person perception (see also Shrauger & Altrocchi 1964). The issues involved, such as the effects of response sets, the likelihood that interpersonal accuracy has several independent components, and the failure to show that accuracy is stable over a variety of object persons, make it difficult to draw general conclusions on the basis of past empirical studies. Some earlier interpretations of this literature (Bruner & Tagiuri 1954; Taft 1955) are based on evidence that now seems equivocal. Allport (1961, pp. 497-522), writing with awareness of the post-1955 literature on the complexity of the accuracy concept, still concludes that there is evidence that “good judges” have particular characteristics, such as breadth of personal experience, intelligence, cognitive complexity, self-insight, social skill and adjustment, detachment, aesthetic attitude, and intraceptiveness. The relationship of personal characteristics to accuracy, however, is probably complex; different types of persons may be accurate, depending upon the form of interpersonal judgment and the type of object persons involved. This does not necessarily deny that some individuals may be better judges than others. It does show, however, that one probably cannot describe a person’s accuracy in terms of a single dimension. Instead, accuracy may have to be expressed in terms of variations of a profile of components, where scatter and shape must be taken into account. Those who seem to be extraordinarily good judges may well be individuals who happen to be accurate on most of the major components. Such individuals may be quite rare, and most studies so far have not been designed to identify large numbers of them so that they can be compared with average persons.

Overview

In 1872 Charles Darwin published The Expression of the Emotions in Man and Animals, in which he discussed not only the manifestations of emotions but also the recognition of emotional expressions. Although the matter of recognition was secondary in Darwin’s work, his was the best documented naturalistic treatment of this subject up to that time. It placed prior contributions in the category of speculative work. The problem has since expanded to encompass the recognition of practically all psychological states and human qualities. For about a decade the issue of accuracy of person perception and its correlates preoccupied many investigators. It became clear in the early 1950s, however, that a much better understanding was needed of the processes involved in this kind of judgment, an approach that had been advocated and followed by some investigators from the beginning. Much progress has been made in this direction. Serious attention has been given to the elements, factors, and artifacts that enter into the process, to fresh analyses and reformulations of the phenomenon, and to the development of methods more suitable for its study. No one had ever seriously underestimated the subtlety and delicacy of the process whereby we come to know other minds and personalities. However, empirical, naturalistic, and theoretical work has indicated that it is even more complex, and that more elements and variables enter into it, than had been surmised. What is needed next?

Generally speaking, three directions of development would seem particularly beneficial: certain types of theoretical formulation, greater stress on certain forms of inquiry, and intensification of exchange and contact with other relevant areas of knowledge.

Many theories, both in psychology and philosophy, are more or less directly relevant to the process of knowing others. Darwin’s genetic-learning theory of recognition of emotions, Scheler’s theory of direct understanding, Brunswik’s lens theory and its variations, Cronbach’s theory of dyadic and configuration al components, the theory of clinical inference of Sarbin, Taft, and Bailey, and Kaminski’s taxonomic formulations and integrative proposals should serve as examples. There is, however, a dearth of theories spanning the available data and retaining contact with neighboring or more generally relevant fields. The contributions of Kaminski (1959; 1963), Rommetveit (1960), and Sarbin, Taft, and Bailey (1960) illustrate, in very different ways, the kinds of theoretical effort needed both to clarify and integrate the large quantities of past observations and to point to new questions for research, while keeping in contact with germane lines of inquiry. New methodologies, more imagination, and better analytic tools are needed. More work has to be done tc analyze what people actually do when forming impressions of, and understanding, others.

The treatment of person perception has often >suffered from separation from other areas of psychology with which it overlaps. This relative isolationism is diminishing, however, and more attention is being given to the relationship between knowing other persons and more general processes, such as cognition, perception, and theories of personality, and to more specialized subjects, such as clinical diagnosis of personality, as well as to appropriate quantitative tools.

Renato Tagiuri

[Other relevant material may be found inExpressive Behavior; Interviewing, article OnPersonality Appraisal; Personality Measurement; Sympathy and Empathy; and in the biographies ofBrunswikandWood Worth.]

bibliography

Allport, Gordon W. 1961 Pattern and Growth in Personality. New York: Holt.

Bronfenbrenner, Urie; Harding, John; and Gallwey, Mary 1958 The Measurement of Skill in Social Perception. Pages 29-111 in David C. McClelland et al. (editors), Talent and Society. Princeton, N.J.: Van Nostrand.

Bruner, Jerome S.; Goodnow, J. J.; and Austin, G. A. 1956 A Study of Thinking. New York: Wiley.

Bruner, Jerome S.; and Tagiuri, Renato 1954 The Perception of People. Volume 2, pages 634-654 in Gardner Lindzey (editor), Handbook of Social Psychology. Cambridge, Mass.: Addison-Wesley.

Brunswik, Egon (1947) 1956 Perception and the Representative Design of Psychological Experiments. 2d ed., rev. & enl. Berkeley: Univ. of California Press.

Cline, Victor B. 1964 Interpersonal Perception. Volume 1, pages 221-284 in Brendan A. Maher (editor),Progress in Experimental Personality Research. New York: Academic Press.

Cronbach, Lee J. 1958 Proposals Leading to Analytic Treatment of Social Perception Scores. Pages 353-379 in Renato Tagiuri and Luigi Petrullo (editors), Person Perception and Interpersonal Behavior. Stanford Univ. Press.

Darwin, Charles (1872) 1965 The Expression of the Emotions in Man and Animals. Edited by Francis Darwin. Univ. of Chicago Press.

Heider, Fritz 1958 The Psychology of Interpersonal Relations. New York: Wiley.

Jackson, Douglas N.; and Messick, Samuel 1963 Individual Differences in Social Perception. British Journal of Social and Clinical Psychology 2:1-10.

Jones, Edward E.; and Davis, Keith E. 1965 From Acts to Dispositions: The Attribution Process in Person Perception. Volume 2, pages 219-266 in Advances in Experimental Social Psychology. Edited by Leonard Berkowitz. New York: Academic Press.

Kaminski, Gerhard 1959 Das Bild vom Anderen. Berlin: Liittke.

Kaminski, Gerhard 1963 Die Beurteilung unserer Mitmenschen als Prozess. Pages 51-67 in Deutsche Gesellschaft fur Psychologie, Bericht uber den 23. Kongress. Gottingen (Germany): Verlag fur Psychologie.

moreno, Jacob L. et al. (editors) 1960 The Sociometr reader. Glencoe, 111.: Free Press. → See especially Part III C.

PagÈs, Robert 1965 La perception d’autrui. Pages 101— 169 in Paul Fraisse and Jean Piaget (editors), Traiti de psychologie exp4rimentale. Volume 9: Psychologie sociale. Paris: Presses Universitaires de France.

Pepitone, Albert 1958 Attributions of Causality: Social Attitudes and Cognitive Matching Processes. Pages 258-276 in Renato Tagiuri and Luigi Petrullo (editors), Person Perception and Interpersonal Behavior. Stanford Univ. Press.

Rommetveit, Ragnar 1960 Selectivity, Intuition an Halo Effects in Social Perception. Oslo Univ. Press.

Sarbin, Theodore R.; Taft, Ronald; and Bailey, Daniel E. 1960 Clinical Inference and Cognitive Theory. New York: Holt.

Scheler, Max (1913) 1954 The Nature of Sympathy. London: Routledge; New Haven: Yale Univ. Press. → First published as Zur Phdnomenologie und Theorie der Sympathiegefilhle. The second revised and enlarged edition—which was later translated into English—was published in 1923 as We sen und Formen der Sympathie.

Secord, Paul F.; and Backman, Carl W. 1964 Social Psychology. New York: Mc-Graw-Hill. → See especially Chapter 2.

Shrauger, Sid; and Altrocchi, John 1964 The Personality of the Perceiver as a Factor in Person Perception. Psychological Bulletin 62:289-308.

Taft, Ronald 1955 The Ability to Judge People. Psychological Bulletin 52:1-23.

Tagiuri, Renato; and Petrullo, Luigi (editors) 1958 Person Perception and Interpersonal Behavior. Stanford Univ. Press.

IX SOCIAL PERCEPTION

Social perception is a field of study notoriously difficult to define. It is generally concerned with the effects of social and cultural factors on man’s cognitive structuring of his physical and social environment. This very broad statement is needed as a first approximation in order to encompass the variety of interests represented in the study of the social context of human perception.

To the psychologist concerned with perceptual theory, social perception is of interest for two reasons. First, cultural and social differences in perceptual functioning can serve as demonstrations of the role of experiential factors in perception. Second, the importance of motivational processes has been stressed in many recent theories of perception. Perceptual selectivity emerges as a common denominator of much of this work, especially when the distinction is made between what can be perceived by an organism and what is perceived in a variety of conditions. A selection exercised at any given moment depends upon a great number of antecedent factors, including interests, needs, values, and goals. Many of these are of social origin; and thus, “social” has often come to mean, explicitly or implicitly, motivational factors in perception.

In much of social psychological writing, social perception has acquired yet a different meaning: it is the study of “person perception” or “perception of people.” Heider wrote: “We shall speak of ’thing perception’ as ’nonsocial perception’ when we mean the perception of inanimate objects, and of ’person perception’ or ’social perception’ when we mean the perception of another person” (1958, p. 21).

The field so defined extends well beyond the conventional use of the term “perception“; it includes highly complex inferences about characteristics of persons. It is true, however, that such a blanket use of the term is based on more than a mere transplantation of its current everyday meaning. The point of transition from perception to other cognitive activities, such as inferring, recognizing, categorizing, and judging, is difficult to specify. In addition, the role of inference and categorization in perceptual activities has been a central preoccupation in several theories, and from this an interest has developed in showing that basic similarities of process exist at various cognitive levels.

Some phenomena of person perception have been used as contributing evidence in these attempts at unification. Brunswik was interested in the inferences about personal attributes made on the basis of physiognomic cues and in the validity of such inferences under various conditions (1947). In the application of his theoretical premises to the perception of people, he was able to provide some evidence for the unity of process in phenomena that differed in their degree of complexity. Thus, in a view such as Brunswik’s, social perception appears as perception of people, and the use of the term perception finds its justification on systematic grounds rather than as a metaphor [see Brunswik].

In social psychology, efforts were also made to demonstrate that perceptual phenomena embedded in a social context do not require for their explanation a set of principles distinct from those used in general perceptual theory. The building of bridges between general and social psychology became particularly important as increasing stress was laid on the determination of social behavior by the manner in which an individual perceives and interprets his social environment.

This trend of thought found one of its most prominent expressions in the textbook published by Krech and Crutchfield in 1948. The frequent equation of the study of motivational factors in perception with the study of social perception can quite possibly be traced to the influence of the movement of which this book was an expression. Value judgments and social norms are powerfully present in nonveridical perception, and thus social, in the area of perception, has tended to become identified with various forms of perceptual biases and distortions.

And yet, efforts toward a theoretical integration of social with nonsocial in perception need not rely exclusively on this particular link. If it is true that many perceptual activities “depend upon categorizing activities and upon the construction of an adequate system of categories against which stimu: lus inputs can be matched” (Bruner 1957, p. 127), then we may expect that such a category system will strongly influence the perception of social objects and events that, in their complexity, offer to the perceiver a large number of alternative possibilities of assignment. This has important implications for the “perception of the social.” It means, for example, that social stereotyping and related phenomena can be analyzed on the same lines as the categorization of complex sensory inputs.

So far, the term social perception has been seen to subsume in the psychological literature three principal varieties of empirical content: it has been used to designate some aspects of nonveridical perception; it has referred to the perception of people; and it has served as a link in the theoretical unification of a wide range of cognitive phenomena. Any ex-cathedra pronouncement on the proper definition of this field encounters further difficulties when one considers the interests of social anthropologists in cultural differences in cognition. For example, in his excellent review of the relations between culture and behavior, Kluckhohn (1954) devoted most of his section on perception to a discussion of culturally distinct modes of thought concerning time and space. In one of his earlier papers, Hallowell wrote: “If a culture does not provide the terms and concepts, spatial attributes cannot even be talked about with precision. Individuals are left to fend for themselves, as it were, on the level of elementary discriminatory reactions. This limits the possibilities for the mental manipulation of more refined and developed concepts that require symbolic representation in some form” (1942, p. 77). It is just possible that this quotation defines our problem. There is no doubt about the cultural determination of conceptual systems concerning the view of the world, but is this also true of what Hallowell calls the “elementary discriminatory reactions“?

A discussion of the problems of social perception must rely on the uses of the term in the empirical literature. But not all such usage can be included, since the consequent stretching of the term would finally lead to a consideration of the social and cultural determinants of all knowledge about the world. There is not much difficulty in the specification of the antecedent term: the influence of society and culture. The dependent variable must be a category of human behavior that can be shown to be affected by these cultural and social determinants and that at the same time falls within a roughly acceptable range of perceptual phenomena.

For these reasons, social perception will be understood here to include those aspects of discrimination, identification, recognition, and judgment that satisfy two conditions: (a) they refer to the sensory information received at the time of responding, and (b) the response does not consist of complex and abstract inferences based on conscious choice and deliberation. The second characteristic of this range of phenomena—their subjective immediacy—points to one of the main reasons for their importance and interest when they are considered in a cultural and social context. We know that systems of values and beliefs are to a large extent a product of an individual’s culture and society. We know much less about the limits of these influences, about their capacity to reach those regions of human experience that appear as immediately given, as the incontrovertible evidence of our senses. The intervention of culture and society in our experience of aspects of the world that appear unquestionably as “out there” is not clearly established; but a good deal of relevant research is available, and some tentative conclusions can be drawn.

Social context of motivational factors

The frequent use of the term “social perception” to indicate the effects of motivational factors on perception has already been mentioned. But in many cases, the effects of needs on perception can be considered independently of their social source or context. Similarly, in the studies on individual consistencies in perceptual and cognitive styles, personality becomes a determinant of a set of perceptual responses, not in virtue of conditions common to a social or a cultural group but rather in terms of individual differences superimposed on a common social background. And yet, many of these studies are, or could be, directly relevant to the investigation of social or cultural contexts of perception. For example, group membership that determines differences in the subjective importance of values, conflicts, and needs could conceivably lead to cultural or social regularities in recognition phenomena, which are usually discussed under the headings of perceptual defense and perceptual sensitization. But research in this field tended to focus on intracultural phenomena, although cultural differences in recognition related to a variety of nonmotivational factors have been extensively studied.

Emotional and motivational factors determine in a great many ways our inferences about personality attributes and traits of other people. They also sometimes influence our perception of their physical characteristics. With the appropriate adaptation of experimental procedures employed in some studies of perceptual illusions, it can be shown that the perception of people who are familiar and/or important to the perceiving individual tends to be relatively resistant to various distortions. The use of visual characteristics of persons as stimuli presents obvious advantages in the attempts to demonstrate the effects on perception of emotional investment in the stimulus (Ittelson & Slack 1958). It is true, however, that although the effect of familiarity in resisting distortion finds a confirmation in these experiments, the role of emotional factors is not so clearly demonstrated. The attempts to investigate the effects of emotion as distinct from familiarity (Ittelson & Slack 1958) yield results that are suggestive and promising, but an unambiguous separation of the two effects is not always easily achieved.

In addition to the above, there exists another group of studies that is concerned with the perception of visual characteristics of people and that can also be related to the general trend of findings on motivational factors in perception. Under some conditions, it seems that the association of needs or values with a stimulus leads to a perceptual overestimation of its magnitude. An alternative interpretation of these findings is possible: when differences in magnitude between stimuli are associated with differences in their value or in their emotional significance to the subject, the stimuli tend to be perceived as physically more different from each other than they really are. This interpretation relates the phenomenon directly to one of the basic aspects of social stereotyping—the accentuation of differences between members of different social groups in those characteristics that are subjectively related to the criterion for classification (Hochberg 1957; Tajfel 1959). There is some evidence that this accentuation of relevant differences is not confined to judgments of personality characteristics but also extends to physical attributes of human groups (e.g., Secord et al. 1956). Similar polarization has also been found to operate in the identification of racial membership of human faces presented in pairs under conditions of binocular rivalry (Pettigrew et al. 1958).

The sensitivity to selected visual cues and their use in the identification of a human being as belonging to an ethnic group was well known before World War n in some European countries with a strong tradition of anti-Semitism and a large Jewish minority. One of the difficulties experienced by Jews who attempted to pass for non-Jews under Nazi occupation was the ease with which they were identified. An experimental equivalent of this can be found in a few studies concerned with identification of Jews from photographs. Despite inconsistencies in the findings, the trend of evidence provides some justification for Allport’s conclusion: “It is a striking fact that prejudiced people are better able to identify members of the disliked outgroups than are nonprejudiced people” (1954, p. 133). Here also it is difficult to decide to what extent these identifications are based on familiarity with relevant cues rather than on selection of cues because of emotional involvement. At any rate, it is likely that emotional involvement breeds familiarity and thus leads to increased efficiency in the detection of identifying cues.

The implications for social perception of studies on the role of motivational factors of perception are threefold. The first, and most general, can be stated as follows: Much of human motivation is of social origin; it can be shown that motivation affects some perceptual responses; therefore it can be expected that social factors are part of the causal chain. Second, if it is true that appropriate motivational antecedents vary predictably from one social group to another, then some corresponding social regularities of perceptual responses would be expected to occur. Finally, there is some evidence that motivational factors intervene in the perception of physical features of the social environment.

Cultural differences in perception

Cultural determination of cognitive systems and of behavior relating to them ranges all the way from views about man, nature, and the universe to the organization of sensory data. The limits of cultural relativism are bound to vary according to whether one considers a cultural system or the range of variability in the behavior of individual members of a culture. From the psychological point of view, the main problem in this field is to discover techniques that would allow the transition from the analysis of a cultural system to that of behavior relating to this system. When this is done, it is often found, as Asch (1952) pointed out, that it is easy to underestimate the extent of free play that a cultural system of values and beliefs gives to individual variability.

But regularities do undoubtedly exist, and it may be expected that cultural influences become more marked as the individual has fewer opportunities to engage in his own independent checking of what comes to him from social sources. Thus, it is fairly obvious that supernatural beliefs or aesthetic preferences will show more evidence of uniform cultural impact than, for example, perceptual recognition or identification. However, there are many intermediate cases, and because of them the empiricist-nativist controversy loses much of its meaning in the context of social perception. The capacity to receive sensory information is not, in general, culturally determined; what tends to be selected and how the selection is interpreted will be so determined to varying extents. It would be a bold investigator who would undertake to decide for each case whether it should properly be called perceptual or not.

Within the rough boundaries previously specified, it is possible to distinguish three classes of variables determining some of the known cultural differences in perceptual responses. They are the functional salience of selected aspects of physical environment, the familiarity with the material products of a culture, and the communication systems employed in a culture.

Functional salience

The general relationship between functional salience and various communication systems used in a society turns out to be, more often than not, a case of the hen-and-egg situation. For example, functional importance of some aspects of the environment presumably leads to the growth of an appropriate and specialized terminology. The existence of a terminology may become in turn a facilitating factor in the efficiency of discrimination or recognition; and it is often very difficult to decide whether a system of labels has an independent causal status in the production of a set of perceptual responses. The problem of the direction of causal relationship becomes even more complicated when one considers that sometimes the existence of a terminology is sufficient reason for the need to learn a set of perceptual discriminations. Then the communication system becomes itself a property of the environment, which determines the relative salience of a set of cues.

It is true, however, that in some cases a distinction is possible between the direct effects of functional salience and the effects of such salience mediated through a set of linguistic labels. The problem is that of conditions for learning. In the case of the Eskimos and Laplanders who “discriminate” between more kinds of snow than do members of other cultures, it may well be difficult to decide whether this is due to the learning of labels or to the direct consequences of misperception, although it is likely that the consequences of a perceptual mistake concerning snow are fairly independent of linguistic labeling. The phenomenon is in no way different from those usually discussed under the headings of acquired distinctiveness of cues and perceptual differentiation. Professional competence based on learning to make discriminations that are very difficult for the layman is a familiar phenomenon; the reading of X-ray plates or wine tasting are two examples among many.

It is in this area of functional salience that there exists the only experimentally established case of a culturally determined regularity in discrimination between simultaneously presented stimuli. Liberman and his colleagues (1961) found clear evidence of the effects of learning on the distinctiveness of speech sounds. They were able to show that, with objective acoustic differences kept equal and the stimuli (synthetically produced speech sounds) presented in pairs, the subjects perceived as the same those phonemes that belonged to the same phoneme category in English, and as different those that were on different sides of a boundary between two phoneme categories.

This is an important phenomenon because it probably provides the most striking example of cultural differences in discrimination that we are ever likely to discover. The range and selectivity of sounds in various languages impose probably more opportunities for alternative modes of experience than is the case for any other aspects of the sensory information that we receive. As a result, cultural regularities occur in the perception of simultaneously presented stimuli as “same” or “different“—a phenomenon not easy to replicate for any other culturally divergent aspects of experience.

Familiarity

Cultural variation that might influence perceptual responses can also be found in the general type of man-made products peculiar to a culture. One example is the use of straight lines and right angles in urbanized societies. Is it possible that the accumulation of corresponding habits of visual inference would lead to differences in perceptual functioning? In order to explore this possibility, Allport and Pettigrew (1957) used one of the illusions devised by Adelbert Ames, the rotating trapezoid window, in which the inferred objectquality of the stimulus is in conflict with its objective features. If the visual inference of a windowlike object is made by the observer, the rotation is perceived as a swaying to-and-fro movement. Therefore the illusion should be less marked in cultures where there is less familiarity with the type of object represented by the trapezoid.

Allport and Pettigrew tested this prediction, using as their subjects groups of rural and urban Zulus and a group of South Africans of European origin. They found that under conditions optimal for the experience of the illusion “virtually as many primitive Zulus report the trapezoidal illusion as do urban Zulus or Europeans” (p. 111). But under marginal conditions the subjects familiar with the products of Western culture tended to experience the illusion more frequently. We may perhaps note that in this case evidence concerning the existence of cultural differences in perception seems to be confined to marginal conditions within an already marginal and fluid situation.

In the study by Allport and Pettigrew the nature of past experience emerges clearly as a determinant of cultural differences. In many cases it is more difficult to distinguish between the possible causal factors. This is so in the investigation by Segall, Campbell, and Herskovits (1966), who undertook the study of the extent of geometrical illusions experienced by subjects from nearly twenty different societies. For the Miiller-Lyer and the Sander parallelogram illusions, the authors predicted more susceptibility in the urbanized than in the nonurbanized societies. The opposite prediction was made for two versions of the horizontal—vertical illusion.

Three hypotheses underlie the study. The first of these, referred to by the authors as the “carpenteredworld hypothesis,” is similar to that put forward by Allport and Pettigrew. The second relates the extent of horizontal-vertical illusion to “aspects of the physical environment of peoples, specifically the presence or absence of broad, horizontal vistas.” The third hypothesis concerns cultural differences in the amount of experience with two-dimensional schematic representations of reality. This can be related to familiarity, but it can also be assigned to the category of cultural differences in communication systems. It seems, however, preferable to distinguish research designs that use directly a system of notation employed in a culture (such as language or conventional visual representations) from those in which cultural differences in perception are attributed to such a system but not directly shown to be related to it.

Segall et al. were able, on the whole, to provide support for their hypotheses despite some inconsistencies in the pattern of findings. It seems that on the basis of assumptions about the kind of visual experience prevailing in a society, predictions can be made about susceptibility to different types of illusions. It should be noted that at the turn of the century similar findings were reported by Rivers (1905).

Three unresolved problems in the results of Segall et al. should be mentioned: for example, some discrepancies between the ranking of societies in terms of susceptibility to the illusions and the ranking that should be expected on the basis of the hypotheses; the difficult issue of perceptual development emerging from the ambiguous data on children; and discrepancies with the findings of other investigators who used slightly different procedures. These are discussed in detail in the original monograph.

Another trend of evidence concerning the role of past experience in cultural differences in perception is provided with techniques of binocular rivalry. Bagby (1957) presented stereoscopically pairs of photographic slides to groups of American and Mexican subjects. In each pair one photograph was of a typical Mexican scene and the other of a typical American scene. The subjects reported what they saw. The differences between the two groups were highly significant, each group identifying more easily the culturally familiar scene. Bagby makes a fairly convincing case that his results are due to genuine perceptual dominance of the scenes characteristic of one’s own culture and not just to selective reporting.

The range of evidence concerning cultural differences in perception determined by familiarity is fairly limited. Indeed, it would be surprising if large and important differences were discovered; human beings in different cultures share the most fundamental aspects of experience of their physical environment. The marginal differences that have been discovered simply provide a supplementary, and perhaps minor, source of evidence for the perceptual adaptability of man. There is a simple but instructive example of this in an experiment conducted by Tresselt (1948), who found discrepancies between groups of weightlifters and watchmakers in their initial judgments of heaviness of a series of weights. These discrepancies gave way very rapidly, however, to a convergence of judgments because of common experience of both groups with the experimental stimuli. It can safely be assumed that the cultural differences discussed in this section would share the same fate under conditions of cultural change and intercultural contact. This has already been shown in some of the previously described experiments in which urbanized members of non-Western cultures served as subjects. These studies do not really show that what we “see” is determined by our cultural environment; their importance lies in stressing our capacity to make, under conditions of perceptual choice, an appropriate selection of those aspects of the environment that matter.

Communication systems

Languages are the most important cultural notation systems. A good deal of controversy has been created by the ideas put forward by Whorf (1956), but also expressed previously by several eminent anthropologists and linguists, that the structure of a language deeply influences the view-of-the-world characteristic of a culture. The main psychological problem is to determine whether nonlinguistic cognitive behavior can be related to language. Within the approximate domain of perception some knowledge about the simpler relationships must be gathered independently of conclusions that may be drawn about organizations of higher order. The possible effects of language on the conceptual organization of the world must be distinguished from its effects on the organization of the available sensory data.

Most of the relevant experimental work .was done with the view of determining the effect of linguistic labels on perceptual recognition. This research is best represented by the well-known study of Brown and Lenneberg (1954). Different languages tend to impose different groupings on such physical continua as the range of colors. These groupings may be described in terms of three principal characteristics: (a) the number of labels assigned by a language to a continuum, (£→) the regions on the continuum where the assignment of one label tends to shade over into the assignment of another, and (c) the consistency with which a particular label is assigned to a particular stimulus on the continuum. Brown and Lenneberg determined the relative linguistic “codability” of a number of colors and found that the degree of codability related to the ease of recognition of a color, but only when the conditions of performing the task were rendered a little difficult. This finding raises a number of problems. Is it really that the more codable colors are better remembered and recognized, or is it possible, as Lenneberg (1961, p. 379) suggests in a later paper, that “confronted with the color context, S would now tend to remember the word ’green’ but not its exact shade and would therefore ask himself ’what is a good green here.’” In addition, it is found that colors that show “singularly low communality” in labeling also tend to be recognized with relative efficiency.

In the Netherlands, Frijda and van de Geer (1961) were able to confirm Brown and Lenneberg’s results and to take them one step further. They showed that what is true of recognition of colors is also true of recognition of facial expressions of emotion. Those that are more consistently codified in a language are also more accurately recognized.

Some work has been done on communication systems other than language. Conventional representations of the visual world vary not only from culture to culture but also from one historical period to another. They present two problems, both of them common to the art historian and to the psychologist: (a) the development of stylistic conventions and (b) the problem of communication, or the matching of the reproductive or creative intentions and their interpretation by the viewer. The first of these is not really perceptual. Albrecht Diirer’s woodcut of a rhinoceros showing the beast as armor plated does not allow us to draw the conclusion that Diirer would not have been capable of perceiving a real rhinoceros without these dramatic embellishments. As Gombrich (1960) points out, there are many examples of drawing or painting from nature in which distortions occur. These distortions are not of a random nature, otherwise we could not talk about styles in art. Presentations of the visual world are guided by the stereotypes concerning the object represented and by a choice in which some sets of relationships are conveyed among many possible ones.

Many experiments on reproduction point in the same direction. But we cannot conclude that there is a parallel between perceiving and building a notation system out of what is being perceived. It can hardly be assumed that the ritualistic Egyptian manner of drawing human figures corresponded to the Egyptians’ perception of human shape. There remains, however, the second problem: How are these cultural artifacts perceived?

Conventional representation of visual aspects of the world becomes in many cultures an important part of an individual’s visual environment. There is little doubt that within a culture these visual messages communicate the information that they are supposed to convey. Therefore, perceptual interpretation of visual notation systems acquires some importance. It provides perhaps the only case of sharp discrepancies in the modes of visual experience acquired by individuals who are members of different cultures.

Most of the evidence gathered by art historians is concerned with the makers of symbols rather than with the receivers of messages; and, indeed, it would be difficult to infer the modes of perception of paintings in the past from the way these paintings appear to us at present. There exists here an opportunity for psychological research that has not been exploited: namely, investigating how the unsophisticated observers of today perceive the spatial features of visual representations created in the past.

Several anthropologists reported difficulties experienced by those unfamiliar with photographs in recognizing in them thoroughly familiar objects, but there are few experimental studies. The clearest evidence of cultural divergences in the perception of two-dimensional representations comes perhaps from the work recently reported by Hudson (1960). Using simple drawings of scenes whose content was familiar to his African subjects, he found that failure to perceive the scenes threedimensionally was related to the amount of formal schooling and intelligence, but even more clearly to the general opportunities that the subjects had in the past of familiarizing themselves with pictorial material of all kinds.

It seems fair to conclude that some marginal aspects of perception can be shown to differ predictably in various cultural contexts. In some cases, it seems that the ecology of the environment and conditions of survival related to it are responsible for some differences in the degree of close inspection that various human groups undertake of one or another aspects of their surroundings. In other cases, the visual environment acquires distinctive properties in large measure through man’s own activities. Finally—and perhaps in its implications this is the most important of these phenomena—perceptual interpretations of notation systems are not given; they are embedded in appropriate past experience. This points to difficulties that may be encountered in the introduction of educational techniques using unfamiliar notation systems in new cultural contexts. It seems that an urgent research priority in this field is not only the ascertaining of the existence of cultural differences; for theoretical as well as for practical reasons, it would be essential to concentrate on problems of perceptual development in children, which until now have received very little attention in cross-cultural research.

Social pressures

In addition to the information we receive through sensory channels, we have at our disposal information that we receive from social sources. One of the most important aspects of social learning is in the reliance of the individual on the consensual information imparted to him by other people.

The problem is to discover whether our perception of the physical world is influenced by social consensus. This is undoubtedly so in the case of moral and aesthetic judgments or of the interpretation of highly ambiguous information. Visual illusions such as the autokinetic phenomenon, which consists of an impression of movement given by a stationary point of light presented in total darkness, have been used abundantly to demonstrate the effects on perception of socially derived norms. In a classical series of experiments, Sherif (1935) has shown that individual judgments of the extent of subjective movement tend to converge when the individual is in a group and also that this convergence persists subsequently to the group situation. Jacobs and Campbell (1961) used the same phenomenon for an elegant demonstration of the cultural transmission of social norms. They created a laboratory “microculture,” in which they first established the convergence of subjective movement. Subsequently they eliminated from the experiment, one by one, the subjects who had served originally and replaced them with new subjects. The effect of the social norm tended to persist several “generations” after the last of the original subjects had gone.

In one sense perceptual illusions such as the autokinetic phenomenon lend themselves admirably to the demonstration of the effects of social norms on perception. The responses are perceptual, and there is no difficulty in the quantitative assessment of the extent of social influence. But this is an extreme condition, in which the subject has no possibility to assess the accuracy of his response. Therefore, it is only too easy to draw from these cases unwarranted conclusions about the impact of social influence in less extreme conditions. The need for caution is well shown in a careful series of experiments by Luchins (e.g., Luchins & Luchins 1955). Using sets of ambiguous figures as stimuli, he was able to demonstrate the converging effect of social influence and also to demonstrate that the magnitude of this effect depended on the degree of fit between the information received from the group and the actual properties of the stimuli.

This is one of the more obvious variables determining the extent of influence of social norms. There are others, such as the size of the group, its unanimity, its emotional significance to the individual, etc. (for a detailed review, see Graham 1962). From the point of view of perception, the main problems concern the nature of the response and the mode of operation of social norms. Are converging responses only verbal or do they represent a perceptual change? Is the subject aware that his responses are socially influenced, or is he convinced that he is still reporting aspects of his own experience in undiluted form?

These questions are not easy to answer. They become even more difficult in the case of drastic effects of group pressures on judgment, such as those shown in the famous experiments by Asch (1952) and many that followed. In these situations, a group of the experimenter’s confederates announce comparative judgments of length that are obviously wrong. Despite this, a sizable proportion of subjects tends to yield to the influence of the group.

There is little doubt that in many cases this yielding is only verbal. Many subjects feel uneasy and uncertain about the unexpected differences between what they see and what they think the others see, but this happens without a concomitant perceptual or judgmental change. The conflict introduced in such cases has important social implications, but it is not a perceptual phenomenon.

In some of the postexperimental interviews there appears to be no awareness that the subject’s judgment has been determined by the judgment of the group, but this is not sufficient evidence of a change in judgment. There is, however, some evidence that suggests that this could be the case. It has been shown, for example, that judgments in accordance with the norms created in the group situation tend to persist subsequently. This is not easily explained on the basis of verbal yielding alone. A study by Flament (1958) is illustrative of those attempting to provide evidence that some judgmental change is taking place. He argued that when group influence of a verbal nature is followed by verbal reports of the individual who is subjected to the influence, a conscious link is established between the two. In order to minimize this, he used a procedure in which subjects, after being told the mean response of a fictitious group in a task consisting of detecting differences in length between two lines, were asked to adjust manually the length of a variable line to a constant one. In this situation the effects of a group norm were at least as marked as in the case of verbal responses.

The literature on the effects of group pressures on judgment is very large, but most of these studies are mainly concerned with aspects of conforming behavior rather than with perceptual change. Many of the results do not allow one to infer with any degree of certainty that such change has occurred. On the other hand, some suggestive evidence does exist. This is not fully convincing, and much more work will have to be done to provide the perceptual and nonperceptual alternatives with a clear empirical meaning. For the present it seems that the occurrence of judgmental or perceptual change as a function of group influences, particularly in conditions of some uncertainty and ambiguity, is at least likely.

Henri Tajfel

[Other relevant material may be found inCommunication; Language; Groups, articles onGroup BehaviorandGroup Performance; Norms.]

bibliography

Allport, Gordon W. 1954 The Nature of Prejudice. Reading, Mass.: Addison-Wesley. → An abridged paperback edition was published in 1958 by Doubleday.

Allport, Gordon W.; and Pettigrew, Thomas F. 1957 Cultural Influence on the Perception of Movement: The Trapezoidal Illusion Among the Zulus. Journal of Abnormal and Social Psychology 55:104-113.

Asch, Solomon E. (1952) 1959 Social Psychology. Englewood Cliffs, N.J.: Prentice-Hall.

Bagby, James W. 1957 A Cross-cultural Study of Perceptual Predominance in Binocular Rivalry. Journal of Abnormal and Social Psychology 54:331-338.

Brown, Roger W.; and Lenneberg, Eric H. 1954 A Study in Language and Cognition. Journal of Abnormal and Social Psychology 49:454-462.

Bruner, Jerome S. 1957 On Perceptual Readiness. Psychological Review 64:123-152.

Brunswik, Egon (1947) 1956 Perception and the Representative Design of Psychological Experiments. 2d ed., rev. & enl. Berkeley and Los Angeles: Univ. of California Press.

Flament, Claude 1958 Influence sociale et perception. Anne psychologique 58:377-400.

Frijda, Nico H.; and Van De Geer, John P. 1961 Codability and Recognition: An Experiment With Facial Expressions. Ada psychologica 18:360-367.

Gombrich, Ernst H. (1960) 1961 Art and Illusion: A Study in the Psychology of Pictorial Representation. 2d ed., rev. London: Phaidon; New York: Pantheon

Graham, D. 1962 Experimental Studies of Social Influence in Simple Judgment Situations. Journal of Social Psychology 56:245-269.

Hallo Well, A. I. 1942 Some Psychological Aspects of Measurement Among the Saulteaux. American Anthropologist New Series 44:62-77.

Heider, Fritz 1958 The Psychology of Interpersonal elations. New York: Wiley.

Hochberg, Julian E. 1957 Psychophysics and Stereotype in Social Perception. Pages 117-141 in Conference in Social Psychology. University of Oklahoma, 1954, Emerging Problems in Social Psychology. Norman, Okla.: University Book Exchange.

Hudson, W. 1960 Pictorial Depth Perception in Subcultural Groups in Africa. Journal of Social Psychology 52:183-208.

Ittelson, William H.; and Slack, Charles W. 1958 The Perception of Persons as Visual Objects. Pages 210-228 in Renato Tagiuri and Luigi Petrullo (editors), Person Perception and Interpersonal Behavior. Stanford (Calif.) Univ. Press.

Jacobs, Robert C ; and Campbell, Donald T. 1961 The Perpetuation of an Arbitrary Tradition Through Several Generations of a Laboratory Microculture. Journal of Abnormal and Social Psychology 62:649-658.

Kluckhohn, Clyde 1954 Culture and Behavior. Pages 921-976 in Gardner Lindzey (editor), Handbook of Social Psychology. Reading, Mass.: Addison-Wesley.

Krech, David; Crutchfield, Richard S.; and Bellachey, Egerton L. (1948) 1962 Individual in Society: A Textbook of Social Psychology. New York: McGrawHill. ã First published as Theory and Problems of Social Psychology by Krech and Crutchfield.

Lenneberg, ERIC H. 1961 Color Naming, Color Recognition, Color Discrimination: A Re-appraisal. Perceptual and Motor Skills 12:375-382.

Liberman, Alvin M. 1961 Some Results of Research on

Speech Perception. Pages 142-153 in Sol Saporta (editor), Psycholinguistics. New York: Holt.

Luchins, Abraham S.; and Luchins, Edith H. 1955 Previous Experience With Ambiguous and Non-ambiguous Perceptual Stimuli Under Various Social Influences. Journal of Social Psychology 42:249-270.

Pettigrew, Thomas F.; Allport, Gordon W.; and Barnett, Eric O. 1958 Binocular Resolution and Perception of Race in South Africa. British Journal of Psychology 49:265-278.

Rivers, William H. R. 1905 Observations on the Senses of the Todas. British Journal of Psychology 1:321396.

Secord, Paul F.; Bevan, William; and Katz, Brenda 1956 The Negro Stereotype and Perceptual Accentuation. Journal of Abnormal and Social Psychology 53: 78-83.

Segall, M. H.; Campbell, D. T.; and Herskovits, M. J. 1966 The Influence of Culture on Visual Perception. Indianapolis, Ind.: Bobbs-Merrill.

Sherif, Muzafer 1935 A Study of Some Social Factors in Perception. Archives of Psychology 27, no. 187.

Tajfel, Henri 1959 Quantitative Judgment in Social Perception. British Journal of Psychology 50:16-29.

Tresselt, M. E. 1948 The Effect of the Experience of Contrasted Groups Upon the Formation of a New Scale of Judgment. Journal of Social Psychology 27: 209-216.

Whorf, Benjamin L. (1927-1941) 1956 Language, Thought and Reality. Cambridge, Mass.: Technology Press of Massachusetts Institute of Technology.

X UNCONSCIOUS PERCEPTION

The term “unconscious perception” has two distinct and separate meanings with quite different implications for a science of behavior. One meaning assumes that the nervous system is capable of differentially responding to stimulus energies whose intensity is below that at which a discriminated verbal report could be obtained. This meaning is frequently referred to as subliminal perception. The second meaning makes no assumption concerning a lower sensitivity for unconscious discrimination but is instead concerned with situations in which an observer’s behavior is governed by perceptual stimuli whose presence he is unable to report when subsequently interrogated. In this situation the cues that guided the behavior would have stimulus energies sufficiently great that, had they been examined in a standard threshold situation, they would have been found to be above a threshold for discriminated verbal report. This meaning of unconscious perception is designated as incidental perception.

These two separate meanings of unconscious perception have seldom been made explicit. The Freudian concept of the unconscious has had a major influence in current psychological usage of this term, and as with most Freudian theory, the concepts are so loosely defined and of such a general nature that they provide only a starting point for scientific inquiry. It is not surprising that both meanings of unconscious perception seem to be involved in the Freudian conception of the unconscious with no attempt to separate or distinguish these two distinct possibilities.

The neurophysiology of our sensory systems provides a certain plausibility for the existence of subliminal perception. Stimulus energies that initiate activity at the receptor have to pass through several relay stations in the nervous system before reaching the cortex and other higher brain centers assumed to mediate consciousness. Thus, at a theoretical level it is plausible that an energy too weak to transmit all the way to the cortex might nonetheless be able to produce nervous activity up to several of the lower relay stations. Then if effector mechanisms are capable of responding to input at these lower way stations, discrimination without awareness would be possible [seeMental disorders, article onBiological Aspects; Nervous System, article onStructure and Function of the brain; Senses].

Some who have advocated the existence of subliminal perception have characterized it as only a gross discriminating system. Thus it is assumed that subliminal discriminations are only of rather gross emotional or evaluative types, such as danger versus no danger and pleasant versus unpleasant.

Attributing only gross discriminative capacity to subliminal discrimination provides the concept with somewhat more plausibility. It avoids the concept of the “Judas Eye” type of unconscious that screens and evaluates incoming information before it is allowed to pass into the conscious realm.

However, the assumption of only gross discriminative capacities for subliminal discrimination has certain logical difficulties. The meaning of a stimulus is conveyed not by one or two gross physical characteristics of the stimulus but rather by the total aggregate of characteristics, which at times can be quite subtle. In the case of words, which are frequently used as stimuli in experiments on subliminal perception, the correct perception of the emotional tone of the word depends upon accurate discrimination of very slight differences in letters and in the sequence of letters. For subliminal discrimination to detect the affective tone of the stimulus would require a very precise, highly developed discrimination capacity.

Before consideration of the evidence on subliminal and incidental perception, it is necessary to define such terms as “awareness,” “threshold,” and “subliminal” in order to evaluate the experimental evidence properly.

Awareness . Most writers on the subject of awareness have distinguished between conscious and unconscious processes in terms of verbalization. Conscious processes are subject to verbal report; unconscious processes are not. Freud, in his extensive writings, on several occasions equated consciousness with verbalization, and Dollard and Miller (1950), among contemporary writers on the subject, are most explicit in defining consciousness in terms of verbal report or verbalization.

The scientific value of a definition of awareness, or consciousness, in terms of verbalization and verbal report depends critically upon the precaution taken in experimental investigations with respect to assuring adequate motivation on the part of the subject to report with precision. Care has to be taken to make sure that the subject understands what is being asked of him, and consideration must also be given to the effects of the questioning upon awareness itself. But most important, an adequate schema for classifying the subject’s verbal reports along relevant dimensions is necessary if progress is to be made in clarifying the concept of consciousness. Some promising steps toward this objective have been reported by Dulany (1962).

Although one can use verbalization as an indicator of awareness, one must not make the error of equating the two or of considering them synonymous. Words are highly abstract symbols and bear little or no physical resemblance to the objects, events, and relationships that they denote. It would be placing an excessive burden upon language to expect it to convey fully the richness of images and perceptual experience. Instead, consciousness, or awareness, at our present stage of knowledge must be treated as a concept whose properties are inferred from the observable verbal reports, but as a concept it remains distinct from the responses from which it is inferred. This use of consciousness as a concept is similar to the treatment of perception as a concept that has been advanced by Garner et al. (1956).

There are many limitations and disadvantages to the definition, or criterion, of awareness stated above, but it must be recognized that this is an area of little knowledge and the above definition serves as a beginning point for experimentation that will enrich the knowledge of the concept of awareness as well as provide a criterion for evaluating work that has been done in the general field.

Thresholds, or limens . Before one can talk meaningfully about subliminal discrimination or discrimination below threshold, it is necessary to examine what is meant by the concept of a threshold. In order to do so, it will be helpful to consider in a skeletonized form the psychophysical procedures for determining a sensory threshold.

Let us consider an auditory study in which the intensity or energy necessary for the subject to detect a thousand-cycle tone is being investigated. The subject is in a controlled environment where relevant environmental stimuli are under the experimenter’s control. The subject would typically be presented with a “ready” signal, perhaps in the form of a flash of light, to inform him that the auditory signal is about to occur. He would further be instructed to report “yes” if he heard the tone and “no” if he did not. The subject would then be presented with a thousand-cycle tone at various intensity levels in a random order. Traditionally, the threshold has been defined as the stimulus intensity that leads to the subject’s report of a stimulus 50 or 75 per cent, or more, of the time.

Two characteristics of such a definition of threshold are immediately apparent. First, the stability or sensitivity of such a threshold will depend critically upon the number of observations made; and second, the threshold represents an arbitrary statistical decision. It does not represent anything absolute in a neurophysiological sense.

Statistical nature of thresholds. The statistical arbitrariness of such a definition of threshold has certain consequences for experimentation on subliminal perception. The subject often makes correct verbal discriminations even below the 50 per cent point. How much better his discrimination is than chance can be determined if the experimental procedure is modified to include control intervals in which, unknown to the subject, no signal is presented. The frequency with which the subject gives “yes” responses to the zero-intensity signal is then a measure of his “false alarm” rate, or his tendency to say “yes” in the absence of stimulation. Any increase in “yes” responses over this “false alarm” rate indicates some capacity for verbal discrimination. Consequently, one should not be impressed by an experiment on subliminal perception demonstrating that a nonverbal indicator, such as a galvanic skin response, will show better than chance discrimination at a level below the 50 per cent threshold. What must be asked of such an experiment is how often was the verbal response a response to the signal at this intensity level. Only if it is found that the nonverbal indicator is appreciably or significantly more accurate than the verbal response is there evidence on the side of subliminal discrimination [seePsychophysics].

Effects of “set” upon thresholds. Another, but less apparent, difficulty with the threshold concept is that the subject’s tendency to say “yes” depends upon the instructions given to him. If a subject is instructed to make certain that he does not miss a signal when it occurs, then there is a tendency for the over-all number of “yes” responses to increase. On the other hand, if the subject is instructed to make certain that he does not report a signal when none is present, then there is a tendency for the over-all number of “yes” responses to decrease. This shifting of the threshold depending upon instructions reflects the fact that the threshold obtained under a given experimental procedure depends quite pronouncedly upon the subjective criterion the subject adopts for what he will consider a signal. In a poorly denned psychophysical situation the subjective criterion adopted by different subjects will vary rather markedly. Some subjects will adopt a cautious criterion; others will be more radical in terms of the internal standard that they adopt for a “yes” response [seeResponse Sets].

Procedures for eliminating difficulties. Two procedures are available that circumvent most of the difficulties of these individual differences in subjective criterion. One procedure requires that the subject give a confidence rating with a “yes” response. Thus, if the subject was highly certain that he had heard the signal he would assign a rating of “1,” whereas if he felt it was just a pure guess he would assign it a rating of “5.” The intermediate numbers would be used to reflect varying degrees of his subjective confidence.

The other procedure is the “forced choice technique” that has been intensively investigated by Blackwell (1953). Under this procedure the subject is told that the stimulus can occur in one of several intervals, marked off by the flashing of a light, and he is asked to indicate in which of the intervals the stimulus occurred. The stimulus, of course, occurs in only one interval. With four intervals the subject’s chance level of accuracy would be 25 per cent. Blackwell has shown that this method leads to lower thresholds than are obtained by the more traditional procedure and also that thresholds so obtained are more stable and less subject to fluctuation on different sessions.

Subliminality . The considerations just discussed abundantly make clear that the term “subliminal” is devoid of scientific value unless the specific operational context in which it is used is specified. It is not sufficient for an experimenter to say that a stimulus was subliminal. One needs to know the procedure, criterion, and instructions.

Subjective confidence

Before considering the experimental procedures that have been employed in studying subliminal perception, one should examine a phenomenon that has occasionally been construed as evidence of unconscious perception. In sensory investigations in which the subject is required to give his confidence rating along with his report of the stimulus, it is not uncommon to find that certain subjects show above chance accuracy on the responses they rate as pure guesses.

There are two serious objections to accepting this phenomenon as unconscious discrimination. First, it violates our definition of unconscious since the subject is making a discriminated verbal report. If consciousness is equated with verbalization, then these reports, irrespective of the subject’s subjective confidence in them, must be considered as conscious. A more serious objection is that acceptance of this phenomenon places the criterion of consciousness at the mercy of what any subject subjectively wants to term a guess. This latter objection would not be as serious were it not for the marked individual differences that exist in the subjective standard of guessing.

Subliminal perception

Having clarified our concepts and our terminology, we are now in a position to evaluate the experimental evidence on subliminal perception. We can begin by noting that in terms of our criterion for consciousness, the question of subliminal discrimination resolves into the question of whether or not a nonverbal response can be found that will be a more accurate indicator of the presence of stimulation than a verbal response. Experimenters have been quite ingenious in the variety of methods they have devised in their attempts to demonstrate subliminal discrimination. For the purpose of evaluation, these various experimental approaches have been classified into the four basic methodologies considered below.

Subliminal conditioning

In subliminal conditioning, the experimenter attempts to condition a nonverbal response to a stimulus whose intensity is lower than that capable of eliciting a discriminated verbal report. The conditioned response may be either a voluntary or an autonomic response, although the latter is more typical (for example, galvanic skin response, heart rate, pupillary contraction).

From an over-all evaluation the numerous studies that have attempted to demonstrate subliminal conditioning have been unsuccessful. In the few instances in which positive results have been obtained, other investigators have been unable to replicate the findings or threshold criterion, for verbal report was inadequate.

Conditioned nonverbal responses

The method for assessing thresholds by generalizing conditioned nonverbal responses involves the conditioning of some nonverbal response to a suprathreshold intensity of a stimulus, and then observing whether this response generalizes to values of the stimulus at threshold intensities. One could condition a galvanic skin response to a suprathreshold tone and then observe the frequency of conditioned responses as the tone varies in intensity around the threshold level. The frequency of conditioned responses as a function of the intensity of the stimulus could be used to compute a threshold that would then be compared with the threshold obtained by verbal report.

Experiments in which a conditioned nonverbal, but voluntary, response has been compared with the threshold obtained by verbal report have generally given virtually identical thresholds or detection functions. Experiments that have employed autonomic responses, such as the galvanic skin response, typically have found that the autonomic response is a less sensitive indicator of perception or discrimination. The instability and error that exist in autonomic responses as currently measured almost ensure that such responses would be less sensitive indicators than a verbal report (see Dulany & Eriksen 1959).

Subception

The methodology used in subception is virtually identical with the preceding one; the difference lies primarily in how the results are analyzed. To each stimulus presentation the subject gives both the conditioned nonverbal response and a verbal response concurrently. The subception effect consists in the demonstration that the conditioned nonverbal response shows above chance accuracy, on the average, over those trials in which the verbal response was wrong.

Although the subception effect has been interpreted as demonstrating subliminal perception (Lazarus & McCleary 1951), it can be more parsimoniously explained in terms of noncorrelated sources of error in the two concurrent responses or response systems. For example, if a verbal response is being studied concurrently with a galvanic skin response, when the stimulus is presented there will be some occasions in which the verbal response is in error but the galvanic skin response gives the correct discrimination, and conversely, there will be other trials in which the verbal response is correct but the galvanic skin response gives a wrong response. The less than perfect correlation between these two concurrent responses can be attributed to different sources of error that effect one response and not the other at a given moment in time for a given stimulus presentation. That certain variables will differentially effect one type of response and not another has been shown for several different response systems (Eriksen 1960). Further evidence that the subception effect does not represent subliminal discrimination is the fact that in these experiments, if the performance in the experiment as a whole is examined, it is typically found that the verbal response conveys more discrimination or more information than does the nonverbal response.

Subliminal stimulation and behavior

Rather heterogeneous methodologies are united by the fact that subliminal stimulation has indirect effects on subsequent behavior. The assumption uniting these methodologies is that subliminal stimulation does not lead to precise discrimination but rather, through the distortion of unconscious processes and defense mechanisms, reveals itself in subsequent behavior in indirect forms. The subject is presented with a subliminal stimulus, and the effects of this stimulation are sought in subsequent dreams, fantasy productions, or mental associations. Representative of such experiments are those of Fisher (1954), Hilgard (1962), and Shevrin and Luborsky (1958).

Since the effect of the subliminal stimulation is expected to be distorted or transformed, or to appear in a symbolic rather than the identical form of the stimulation, the detection of this subliminal influence poses severe methodological problems. Numerous sources of artifact are present in such experimental designs, as have been pointed out by Johnson and Eriksen (1961). Interpretation of these experiments is further hindered by the failure of experimenters in most cases to ensure that the stimulation was indeed incidental or to specify the criterion by which they determined that the stimulation was subliminal. In many of the experiments it is quite obvious that the stimuli were not subliminal, and the subject’s lack of awareness of the stimulation was due to the nature of the experiment, which directed his attention to other stimuli. In this case the experiments are more concerned with incidental stimulation than with subliminal stimulation and bear a strong resemblance both methodologically and conceptually to the traditional work on incidental memory.

Incidental perception

Although the evidence for subliminal perception is largely negative, the possibility that behavior can be directed by above-threshold cues of which the subject is unaware is not only more plausible but has somewhat better experimental substantiation. We know by common sense that we are constantly utilizing cues of which we are unaware in our perception of depth and of shape and size constancy, as well as a number of cues that guide such complex motor habits as playing the piano and driving an automobile.

The methodology employed in experiments on incidental perception makes use of one or another of a few basic techniques. In most cases the perceptual cue is presented without the subject’s being forewarned that it will be present. Typically, it is presented at an intensity so low that it does not command attention. In other experimental arrangements the cue may be of a more obvious nature, but the subject is prevented from attending it because of misdirection about the purpose of the experiment or the nature of the experimental task. In fact, misdirection of attention seems to be a crucial consideration in experiments on incidental perception; this suggests that perhaps these experiments are dealing more with the concept of attention than with one of awareness.

Two separate lines of investigation using incidental perceptual cues can be distinguished. The first is concerned with the effect of incidental cues upon the behavior that was learned outside the experimental situation. For example, the effect of the incidental stimulation of the word “angry” upon the interpretation of ambiguous profiles might be studied. The other approach is concerned with the problem of perceptual learning without awareness. The first approach we will term “performance without awareness” and distinguish it from the problem of “learning without awareness.”

Performance without awareness

There have been several experiments demonstrating that cues of which the subject is unaware are capable of producing perceptual illusions (Dunlap 1900; Bressler 1931). Bressler, using the Mueller-Lyer illusion, found evidence of illusory effects in the judgment of the length of the two lines when the directional arrows were so faint that the subjects reported that they were unaware of their presence. Perky (1910) and Miller (1939) projected geometric forms on the back of a ground-glass screen at low intensities and found that the patterns presented were influential in determining the subject’s choice of a geometric pattern. The effect was found even when the subjects reported that they had not been aware that a pattern was actually being projected on the screen.

In more recent work, Smith et al. (1959) presented their subjects with a line drawing of an expressionless human face. The face was continuously presented to the subjects except for short interruptions during which the word “happy” or “angry” was briefly flashed on the screen. Although nearly all of the subjects reported that they had not noticed the flashing of the words, some suggestive evidence was found that the subjects’ interpretations of the face were more pleasant following exposures of the word “happy” than following exposures of the word “angry.” Exposures of the word “angry,” however, had little or no effect upon the subjects’ interpretations of the ambiguous face.

These experimenters, as well as others, have interpreted the results as demonstrating the effects of subliminal cues upon perception. In terms of our criterion of subliminal, however, none of these experiments permit such interpretation. In Bressler’s experiment even the faintest lines he employed, when studied independently of the illusion, were found to yield better than chance discrimination by his subjects. Vinacke’s data show clearly that Miller’s and Perky’s results are not obtainable when the geometric forms are shown at an intensity level below even a rather crudely determined psychophysical threshold (Vinacke 1942). And in the study by Smith et al. (1959), observations upon a control group of subjects, who had been alerted to the fact that the words were being flashed, indicated that most of the word exposures for the experimental group were above the level for recognizability of the word.

Even when the above studies are considered as demonstrations of incidental rather than subliminal perception, the conclusions must be accepted with caution. In most cases the effects are slight, and careful replication of these studies is needed before confidence can be placed in the effect.

Although there is an extensive literature on incidental learning and learning without awareness, most of this effort has been concerned with verbal learning. Little work has been done on perceptual learning without awareness. Brunswik and Herma (1951) found that an incidental color cue associated with the lighter or heavier weight in a weightlifting experiment resulted in illusory judgments when a subject was confronted with equal weights that differed in color. Since the subjects were unable to report an awareness of the correlation between color and weight, it would seem that they had learned the association of the cue unconsciously. However, a more carefully controlled replication of this experiment by Levine (1953) failed to confirm the finding.

Eriksen and Doroz (1963) have reported a series of experiments using perceptual tasks containing incidental cues that the subject could use as a guide for his perceptual judgments. Although a certain percentage of the subjects learned to use the cues, all of those who did were readily able to verbalize the nature and the presence of the cue. The results of these experiments on incidental perceptual learning seem to be consistent with the increasing evidence that human adults do not learn unless awareness can be verbalized.

Charles W. Eriksen

[Other relevant material may be found in Attention; Learning, articles on Classical Conditioning, Discrimination Learning, and Instrumental Learning; Psychophysics.]

bibliography

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Brunswik, Egon; and Herma, Hans 1951 Probability Learning of Perceptual Cues in the Establishment of a Weight Illusion. Journal of Experimental Psychology 41:281-290.

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Dulany, Don E. JR. 1962 The Place of Hypotheses and Intentions: An Analysis of Verbal Control in Verbal Conditioning. Pages 102–129 in Charles W. Eriksen (editor), Behavior and Awareness. Durham, N.C.: Duke Univ. Press.

Dulany, Don E. JR.; and Eriksen, Charles W. 1959 Accuracy of Brightness Discrimination as Measured by Concurrent Verbal Responses and GSRs. Journal of Abnormal and Social Psychology 59:418-423.

Dunlap, Knight 1900 The Effects of Imperceptible Shadows on the Judgment of Distance. Psychological Review 7:435-453.

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Eriksen, Charles W.; and Doroz, Lee 1963 Role of Awareness in Learning and Use of Correlated Extraneous Cues on Perceptual Tasks. Journal of Experimental Psychology 66:601-608.

Fisher, Charles 1954 Dreams and Perception: The Role of Preconscious and Primary Modes of Perception in Dream Formation. Journal of the American Psychoanalytic Association 2:389-445.

Garner, Wendell R.; Hake, H. W.; Eriksen, C. W. 1956 Operationism and the Concept of Perception. Psychological Review 63:149-159.

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Miller, Jyames G. 1939 Discrimination Without Awareness. American Journal of Psychology 52:562-578.

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Shevrin, Howard; and Luborsky, Lester 1958 The Measurement of Preconscious Perception in Dreams and Images: An Investigation of the Poetzl Phenomenon. Journal of Abnormal and Social Psychology 56:285-294.

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International Encyclopedia of the Social Sciences

Perception

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PERCEPTION

The term perception may be used generally for mental apprehension, but in philosophy it is now normally restricted to sense perception—to the discovery, by means of the senses, of the existence and properties of the external world. Philosophers have been concerned with the analysis of perception—that is, the study of its nature and of the processes involved in it—and with its epistemological value—that is, how far, if at all, it can be regarded as a source of knowledge about the world. Their answers to these closely interrelated questions have been formulated in various theories: the commonsense theory and other kinds of direct realism, the representative or causal theory, critical realism, the sense-datum theory, and phenomenalism. This entry will be devoted to the main features of perception that underlie the various theories and that have raised philosophical problems and controversy. It will discuss both the initial evidence that may be analyzed without recourse to scientific findings and the causal and psychological process revealed by scientific investigation.

Initial Evidence and Analysis

reflective examination

As percipients we are all familiar with perception, and so the first evidence should come from reflection on our own experience. The following points may thus be made about perception.

First, it is awareness of the external world—of material objects, to use a technical term for physical objects in general, animals, plants, and human beings insofar as they are perceptible (their bodies, in fact). The main characteristics of such objects are that they are external, independent of the percipient, and public, meaning that many people can perceive them at once. Perception, in being the awareness of such objects, may be contrasted with imagery, bodily sensations, or having dreams.

Second, perception is, or seems to be, intuitive—immediate and normally undoubting, a direct face-to-face confrontation with the object in sight or a direct contact in touch. Nor are we normally conscious of any processes of reasoning or interpretation in it. On the rare occasions when we reason or we have doubts about what an object is, the reasoning or doubts are about the identity or character of something already perceived—for instance, a rectangular red object or something white on the hillside.

Third, perception is variable in quality and accuracy; we may fail to notice something, to see clearly, to hear distinctly, and so forth. Three types of variation may be involved: variations in attention, in what we notice or discriminate; variations in quality or distinctness (for instance, where there is nearsightedness or fog); and variations in liability to err—we may misidentify what we perceive or mistake its qualities.

Fourth, perception nevertheless normally gives us knowledge of material objects and properties. With a few fairly obvious tests, like touching and looking closely, or using the evidence of other percipients, we can establish certainty or else correct the first sight or hearing.

Fifth, perception often issues in some judgment or assertion (to others or perhaps only to oneself)—for example, "There is a green fly on the roses" or "Here's the milkman"—but it may not.

illusions

Illusions, comprising illusions proper, hallucinations, and cases of the relativity of perception, have traditionally been the most important origin of the major problems of perception. The two main claims of the argument from illusion are (1) illusions show that perception is never absolutely certain, that tests are never final, and (2) the appearances we are aware of in illusions, especially hallucinations, cannot be identified with the real properties of objects and must therefore be private objects of awareness, or sensa (indeed, all perception involves awareness of sensa that in correct, or veridical, perception belong to the object or correspond to its properties).

The first claim was long thought to rule out perceiving as a source of knowledge; instead, one had to turn to pure reason, or rational intuition, which was held to provide mathematical knowledge. But, since the absolute certainty of mathematics came to be generally ascribed to its ultimately analytic, or even tautological, character, the tendency now would be to stress the negligibility of the possibility of error in tested perception and to use a different standard of certainty and knowledge concerning matters of fact, one that allows perceptual statements to qualify.

The second claim, concerning the existence of sensa, is vital in that almost all theories of perception either found their analyses on it (as does the sense-datum theory) or seek to controvert it or explain it away (as does commonsense realism). The seeds of this conflict already lie in the results of the reflective examination. Insofar as perceiving seems to vary in quality and accuracy, it is easy to say that in illusions we merely see the object looking different from what it is. But if perception is a direct intuitive confrontation, the illusory appearance must be a genuine existent, perceived as it really is, a sensum in fact; "looking different from what it is" must be interpreted as "presenting sensa different from the standard ones." In any case, some phenomena—for example, the integration of hallucinatory images with a perceived background—are difficult to explain without supposing awareness of private sensa in all perception, and almost all the phenomena require scientific and psychological findings for their full explanation, thus pointing beyond this initial evidence.

perceptual consciousness and perceiving

The occurrence of illusions may lead to ambiguity in the use of perceive and allied terms. Thus, in double vision a man may be conscious of two bottles where there is only one. Do we say "He perceived two bottles" or "He perceived one bottle"? Each alternative has been adopted philosophically, and to avoid ambiguity, it is safer to distinguish (1) "X is perceptually conscious of Y " from (2) "Y is present to X 's senses (or light from it is acting on his sense organs)" and use "X perceives Y " only when both are meant.

This recommendation is claimed to have the further advantage of enabling us to discuss as perceptual consciousness the state of mind (or mental act) occurring in both veridical perceiving and illusions. Perceptual consciousness of, for example, a dagger might occur when only a stick was present or even, as in Macbeth's case, when nothing was there. The notion of such consciousness as a common factor in perceiving a real dagger, in having hallucinations of one, and in mistaking something else for one fits in best with dualist theories, such as the sense-datum theory (especially H. H. Price's version) or critical realism, since it suggests that the contents of such consciousness differ from the external object perceived. Direct realists are suspicious of it; for them having hallucinations is something (imagery, perhaps) quite different from normal perception, even if confused with it, whereas in illusions they want to stress that one is perceiving the real object present—seeing a stick as a dagger or the round table as elliptical.

But even if perceiving a round table as round or in perspective as elliptical is taken as immediate confrontation needing no further analysis, seeing a stick as a dagger (or a piece of wax as a tomato or a bush in a fog as a man) can hardly be equally simple and immediate. In such cases and in hallucinations one has to admit that one seems to see an object quite different from that present to the senses. This can fairly be described as perceptual consciousness of the (ostensible) object (dagger, wax, or man) and distinguished in analysis from actually perceiving an object (dagger, wax, or man). And in view of the subjective similarity it is but a short step to suppose that perceptual consciousness of X also occurs in perceiving X as X, the difference between illusory and veridical perception of an X lying not in this common consciousness but in whether X is present and acting on the sense organs. Any philosophy of perception should analyze this perceptual consciousness and explain how it may occur without the presence of the corresponding object.

analyses of perceptual consciousness

Three major analyses are integral parts of the theories of perception mentioned above. First is the traditional notion that perceiving—that is, perceptual consciousness—is the interpretation of sensations as properties of external objects. Second, the sense-datum theory claims that perceptual consciousness is taking for granted that the sense datum one is sensing belongs to a material object. Third, the analysis of the critical realists, though stated as an analysis of perceiving, amounts to saying that perceptual consciousness is taking an intuited datum or character complex to characterize an external object.

The essential difficulty in these three analyses is that they contradict introspective evidence by splitting up perceptual consciousness into the awareness of some private data, recognizable by analysis as such, and the act of interpreting them as, or taking them to be, objects or object properties. In experience there is no such core of sensing or intuiting data distinguishable from the consciousness of a material object, even if only subsequently; still less is there any passage of the mind from awareness of sensation as such to object perception. And if some critical realists are less liable to this difficulty because they do not treat their data or character complexes as existents readily distinguishable from material objects, they do this only at the expense of obscurity or disagreement as to what the data are. Attempts at a remedy must be postponed until the psychological processes in perception are considered.

A fourth analysis is the idealist claim that all perceiving is judging, which is really an analysis of perceptual consciousness but is easier to follow if stated in terms of perceiving. It is that perceiving (perceptual consciousness) consists in making a judgment, which has an implicit sensory basis, about the real existence of an object or property. Thus, perceiving a tomato on a plate or perceiving that the dog has hurt its leg are the sensorily grounded judgments "There is a tomato on a plate" or "The dog has hurt its leg." The "perceiving that" description of a perception (for example, "He saw that the dog was hurt") certainly seems to suggest judgment, though the form may be misleading and may only be for emphasis of the feature noticed. But the main reasons for this analysis are (a ) that perceiving is true (veridical) or false (erroneous) and only judgments or assertions can be true or false and (b ) that perception is more than just sense experience, for we identify and interpret what is given (that is, it involves inference from implicit data, and the conclusion of such an inference must be a judgment).

One may object that truth characterizes what is asserted, not the asserting—the judgments, in the sense of propositions, to which perceiving may lead, but not the act of perceiving itself. Perceiving may be proper, correct, clear, or accurate, but not true or false. Many other things we do may be done correctly and be liable to errors without being forms of judging, such as playing the piano, playing games, tying knots. False judgment is not the only form of error. Also, the idealist doctrine that all perceiving is judging is open to the general objection to the first three analyses above, particularly because the nature of the implicit data or sensory grounds is very obscure. Attempts to elucidate it—for example, Brand Blanshard's—turn them into sensa. Furthermore, the term judgment suggests something intellectual, explicit, and considered, with consciousness of the evidence for the assertion—conditions inappropriate to much perception. Also, we may correct a faulty judgment on learning the truth, but such knowledge does not enable us to correct illusory perceptions; we still see the mirage, and the railroad tracks still appear to meet in the distance.

Fifth, there is a causal analysis of perceptual consciousness—namely, that it is inferring that one's sensa are caused by an external object. This may be associated with representative realism but is not essential to it; representative realism's main thesis is that the sensa and the consciousness are externally caused by objects that the sensa "represent." One may accept this thesis along with any of the analyses of perceptual consciousness—the causal inference it involves is subsequent to the perceiving, and so is a claim about perception. The difficulties of supposing that perceptual consciousness consists in such an inference from effect to cause are that (a ) we are not conscious of such an inference; (b ) if we started only with private sensa, any inference to external causes would be too difficult and complex to be automatic and unconscious—it would have to be conscious; and (c ) it leads to paradoxes, such as that children, being ignorant of the supposed causation of perception, cannot therefore perceive or be perceptually conscious of anything.

conceptual analysis of perceiving

A rather different approach to perceiving is adopted by those who advocate conceptual analysis—that is, a close study of the ordinary meaning (or use) of expressions. This analysis is naturally associated with commonsense realism, for ordinary language tends to reflect ordinary views on perception or at least what once were such views. Such analysis, however, may well indicate features of perception that are not normally realized and so supplement or even correct reflective examination of an introspective kind.

Much attention along these lines has been directed to the categorization or classification of perceiving. Previous philosophers have referred to perception in various ways: as an act, even an operation, as a process, and as a mental state. None of these is satisfactory. "Act," at least as activity or operation, suggests listening or watching rather than just hearing or seeing; "state" and "process" suggest something long-term, and "process," like "activity," suggests something open to public observation—yet whereas one may observe X looking at Y one cannot observe X seeing Y. (One can perhaps claim that the best description of perception is "mental act," which would put perceiving in a special category with realizing, noticing, deciding and so on, but mental acts as such are suspect to these philosophers.) One suggestion is that perceiving is simply having an experience, but this neglects the active side of recognizing and identifying involved in it. A more popular suggestion is that perceiving is a skill or art, or, rather, since seeing X or hearing Y occur at a definite time, perceiving is the exercise of a skill.

Oddly enough, the evidence for this is not linguistic. We may speak of a skilled observer, one who can direct and coordinate a series of perceptions, but not a skilled perceiver; we do not say that X is an expert at the art of seeing or hearing things. Rather, this suggestion is based on the fact that perceiving can be improved by learning and experience, so that one recognizes things easily, avoids mistakes, or can make allowances for such factors as distance. Although this may occur to one on reflection, however, its full and precise extent has been established only by psychological investigation. As soon as one seeks out this and other psychological evidence about perceiving or even asks how one learns by and exploits experience in perceiving, one is carried far beyond language and conceptual analysis to a scientific study of the subject. Also, to maintain that perceiving is the exercise of a skill brings one back to the suggestion that it is an operation or activity.

More striking perhaps was the earlier claim of Gilbert Ryle that "perceiving" is an achievement verb, like "finding" or "winning," and indicates the scoring of an investigational success. This means that perceiving is not an activity or process, though it may be the successful termination of the activity of looking for something; it is instantaneous, not something that takes time or can be observed. Ryle's aim was to attack representative realism and its associated dualisms of mind and body, sensa and object, by claiming that (a ) perceiving, usually thought to be a private mental activity because it is not an overt one, is not an activity at all and thus provides no evidence of a mental world and that (b ) since it is not a process, perceiving is not the final stage or effect of a process, particularly not of the causal process from object to person. Hence, there is no need to suppose that science proves that perceiving is awareness of private sensa.

These are not very convincing arguments. As to the first, winning or scoring involves some activity such as kicking a ball. Likewise, perceiving involves experiences of colors or sounds and the psychological processes discussed below; these are normally claimed to be mental. The second is a non sequitur—instantaneous success may be the end and result of a causal process. Thus, scoring and finding may be observed and may be the result of a process or series of activities; other conditions may also be required but do not rule out their being effects. More generally, if perceiving is an achievement, what are misperception, illusion, failure to see properly, a casual glance? An analysis of perceiving must take these into account and not apply only to veridical perception. Ryle also failed to show how perceiving is related to the causal processes representative realism emphasizes. Thus, if instantaneous, perceiving can no longer be the relation across time and space that direct realism would need to claim in view of the factually verified time lag, the time taken by the causal transmission from a distant object. Indeed, contrary to Ryle's intention, a dualist interpretation of his claim is possible. Perceptual consciousness is instantaneous; when it is also successful, that is, when its content corresponds to the properties of the object causing it, it is perceiving; when unsuccessful, it is misperception or illusion.

The Causal Processes in Perception

the causal chains

The causal processes involved in perception form causal chains from the external object to the percipient's brain. In sight a complex system of light waves, sometimes emitted by the object but normally a differential reflection of light from the object's surface, travels from the object to the percipient's eyes. This system is diversified in intensity and wavelength according to the shape, brightness, and color of the object surface and, on striking the eyes, is focused so that an image of the object is cast (upside down) on each retina. Each retina has a mosaic of more than 120 million receptors, which are activated by the light cast on them in this image. The light causes chemical changes in the receptors; these changes, in turn, cause electrical impulses to pass along the nerve fibers that lead from the receptors to one of the two visual receiving areas of the brain. The impulses set up activity there and in certain other association areas; this done, the person then sees the object. More than one million such fibers form the optic nerve from each eye, and each fiber consists of a succession of cells that are made to conduct by a chain reaction; the resultant impulses can be picked up and reproduced on a cathode-ray tube.

In hearing, a pattern of sound waves is emitted or reflected from the object and strikes the eardrum; this causes vibrations to be transmitted through a series of bones to the liquid filling in the inner ear, thereby setting up vibrations in the basilar membrane of the cochlea according to the frequency (waves per second, corresponding to pitch) and intensity of the sound. The receptors in the cochlea then transmit electrical impulses along the nerve fibers to another receiving area in the brain. (These impulses are not at the frequency of the incoming sound.)

In smell and taste there is a chemical stimulation of receptors in the nose and tongue by particles of the substance perceived, and the receptors, in turn, send neural impulses to another area of the brain. For touch, the brain is linked to receptors all over the skin, some of which respond to the pressure of direct contact with the object, some to heat, and some to cold (or, rather, to rate of change in skin temperature). Other receptors in the skin and the body respond to a wide range of stimuli by transmitting to the brain impulses that ultimately cause a sensation of pain. There are also other senses—for example, a kinesthetic sense by which receptors in the muscles send impulses to the brain so that the position of the limbs is sensed or unconscious adjustments are made to guide and make efficient voluntary movement. There are also receptors in the vestibule and semicircular canals behind the ear that assist balance and give us information about head position.

The chain process (object-[waves]-receptor-nerve impulses-brain activity) is a necessary condition of perception of an external object, for if it is interrupted by damage to the sense organ, no perception occurs. It is not a fully sufficient condition in that other areas of the brain must be suitably active so that the person is conscious and minimally attentive—that is, not wholly absorbed in thought. The interesting question is whether or how far the chain process is necessary and sufficient for perceptual consciousness of an object, granted conditions of consciousness and attention. At least the brain activity is clearly necessary, but theoretically one might insert stimulation at some point on the chain and thus cause experiences the same as those that would normally be attributed to the external object. This apparently happens naturally in illusions and hallucinations, including phantom limbs, and electrical stimulation of the appropriate areas of the brain may cause sensations of color, smell, or touch. (The sensations are not like the contents of perceptual consciousness of objects, but this difference may be due to the comparative crudity of the artificial stimulation by an electrode; also, activity in the association areas is necessary for normal perception.) Thus, it seems probable that suitable activity in the nervous system is a necessary and sufficient condition of perceptual consciousness, though it may be that some kind of external stimulation, even one quite unlike the object perceived, is required to trigger it.

time lag

Causal processes take time. In the case of distant objects this is marked. Thus, because sound waves travel much more slowly than light waves, the flash of some distant gunfire or explosion may be seen appreciably before the sound is heard. Even at its great speed light takes eight minutes to reach us from the sun and four years and four months from the nearest star. Consequently, we may well be "seeing" a star long after it has disintegrated, for the perceptual consciousness occurs at approximately the time of the arrival of the star's light on Earth. But as time is required for the sense organ to be activated and for the nerve impulses to travel to and spread in the brain, there is a slight but variable time lag in all perception; an accurate estimate is not possible but the delay is probably of the order of one-tenth of a second for nearby objects.

uniformity of nerve impulses

One surprising fact is that the nerve impulses are of a similar type for all the senses. All that travels along any nerve from any receptor to the brain is a sequence of such impulses varying normally between 10 and 100 per second. The frequency variation is, in fact, a mark of intensity; the stronger the stimulus, the more impulses per second. Consequently, what distinguishes causation of an experience of sound from that of smell or an experience of a high pitched sound from that of a low one is not the impulse itself but the connections of the nerve fibers excited and conducting—where they start in the sense organ and where they end in the brain. (Though if one imagines a cross section across a bundle of nerve fibers, the pattern of some conducting and some not conducting can be regarded as a changing code.) Thus, excitation by nerve impulses of one tiny portion of the brain results in awareness of a loud shrill sound, excitation of another in awareness of a blue line. Various areas of the body are mapped in the brain, a group of receptors (or sometimes an individual receptor) in the skin and tissues corresponding to each point in the cerebral receiving area. Similarly, the retinal image is reproduced point by point in the brain, though with each half reproduced in a separate area, duplicated there, and distorted. Again, a strip of brain tissue is activated at different points according to the frequency of the sounds heard, as if it were a keyboard.

complexities

There are nevertheless many complexities in the system, only a few of which we can mention here. The nerve connections are intricate, with feedback fibers from the brain to the incoming sensory fibers and cross connections between the sensory fibers; in the grouping of receptors and in the brain there is summation—several nerves join one that conducts only when all or most of them do. (In fact, neurologists constantly use such terms as selecting, integrating, summating, and coding.)

Binocular vision involves retinal disparity (a slight difference in the images cast on the two retinas) and the operation of two visual receiving areas reached by crossed-over nerve fibers so connected that the left-hand receiving area receives the signals coming from the right half of each retina and the right-hand area receives those of the left half. As a result we somehow normally see one object with depth and solidity rather than two two-dimensional ones.

Constant small eye movements are necessary for vision, with a shifting of the retinal image and of the resultant pattern of impulses in the fibers of the optic nerve, yet the object is seen as steady.

Most of the impulses reaching the brain from the eye come from a small portion of the retina (the fovea) that has relatively many receptors giving great distinctness; for exact vision the image is focused on the fovea by eye movement.

Color vision is particularly complex, and its mechanism is disputed. All the colors we know can be produced by suitable mixtures of red (long-wave), green (medium-wave), and blue (short-wave) light. White light can be formed by an appropriate combination of three colors or even of two widely separated ones. (Light, or "spectral" colors, mix differently from paint colors.) The simplest theory is that there are receptors in the eye reacting to each primary light color (red, green, and blue) and the brain, by summating the three color inputs, is enabled to cause the final color sensation. Thus, grass looks green because it absorbs red and blue light but reflects green, and a buttercup is yellow because it absorbs blue but reflects green and red, which combine to produce yellow. There are many difficulties in this theory. For instance, no receptors for blue can be positively located in the eye, only for red and green ones; the light from a green surface actually contains a mixture of wave lengths, with green predominating; the light wave lengths cover the spectrum of all the colors of the rainbow; the brightness and purity of the color also affect its hue. A final theory must therefore be very complicated.

The auditory receiving area gets impulses from both ears. This enables us to locate the source of a sound. If a sound is to the left, then sound waves reaching the left ear differ in phase (that is, timing of the wave crests) and in intensity from those reaching the right ear. The brain apparently combines the different inputs so that the location is done unconsciously and we just hear the sound as if it came from a certain direction.

limitations of the senses

Radiant energy is known to range from short cosmic rays to long radio waves, but the eye responds only to visual light, which is a narrow band occupying about one-seventieth of the whole range. Even then we cannot distinguish light of different polarizations, as bees and birds apparently can, or see very small objects or fine structures. Similarly, in hearing we can distinguish only waves between 20 and 20,000 cycles per second; dogs, cats and rats can hear higher notes. Our sense of smell is obviously very inefficient compared with that of most other animals. Hence, though we can extend our range of observation by microscopes, infrared or X-ray photographs, radiotelescopes, and so on, it is clear that our senses themselves are very limited as a direct source of knowledge of the external world.

the causal argument

The causal argument maintains that the existence and character of these causal processes refute direct realism and force the adoption of a dualist position. Perception of an external object cannot be the direct contact or immediate confrontation it seems to be, since it requires this causal chain from object to the percipient's brain and is prevented if that is interrupted—for instance, if the optic nerve is cut or one of the small bones in the ear does not move properly. In this sense directness or immediacy must mean no intermediary and no possibility of interruption. The causal chain suggests that perceptual consciousness and its objects are generated, or brought into being, by the causal process, presumably by its last stage, the brain activity. In other words, insofar as perceptual consciousness is intuitive, it is awareness of some content or object quite distinct from the external object.

This suggestion is supported by various points. First, the time lag—perceptual consciousness may occur after the external object has disappeared or moved, so its content cannot be identified with the object. Second, the possibility of perceptual consciousness without any external object at all or without one at all similar seems confirmed by the production of sensations by stimulation of the cerebral cortex and seems actualized in hallucinations. Third, the enormous complexity involved shows that the subjective simplicity of perception is illusory, at least insofar as a relation to an external object is concerned. Fourth, illusions and the relativity of perception are often explicable in terms of the causal processes. Unless the contents of perceptual consciousness are generated and conditioned by the causal process, one has to attribute bizarre and contradictory properties to the external object. Fifth, the simplicity and uniformity of nervous impulses show that they cannot transmit all the various secondary qualities that make up objects as we know them. These qualities must thus characterize contents of consciousness generated by the causal process. (This point is supported by such other limitations as the purely mechanical transmission through the bones of the ear.) Hence, it follows from the fourth and fifth points that one must abandon the other assumption of direct realism—that even when we are not perceiving them, objects continue to exist with the exact qualities we normally observe in them.

There is a good deal of resistance to these conclusions. One obstacle is that they seem to require a self-refuting type of representative realism. This fear is unjustified. It must also be noted that granted the dualist conclusion that the causal process generates the sensory experience whose content is (numerically) different from the external object, the nature of that experience and its content is still open. It may be that the awareness is of sensa, or it may be a full-fledged perceptual consciousness of percepts or ostensible objects. One is not even forced to adopt a mind-body dualism, though it is normally thought that sensa or percepts are mental. One might claim that though apparently distinct objects, they are in fact only the contents of sense experience, not existing apart from the sensing of them (adverbial analysis), and that they and the brain activity are two aspects of the reaction of the organism or person as a whole. This would mean that sensa are only a correlated aspect of brain activity, not effects of it, though still conditioned by the rest of the chain. In this way one might bypass one of the notorious difficulties of ordinary dualism—the unique and obscure causal relation supposed to exist between material brain and immaterial mind.

Sometimes, however, the opposition takes the form of denying the relevance of the scientific evidence to philosophy; it tells us only what the causes of perceiving are, not what perceiving itself is. Philosophers must investigate the latter and leave the causal processes to the scientist. But scientists normally hold that these processes require the adoption of representative realism, thus giving them philosophical relevance; also, those philosophers who wish to concentrate on the nature of perception alone usually come up with some answer (the sense-datum analysis or a view that perceiving is the exercise of a skill or an investigational success) that is compatible with or even supports a dualist interpretation of the causal processes. But, above all, to achieve full understanding of anything so vital as perception, one must consider its causes and conditions, particularly as their study has traditionally been claimed to transform our concepts of perceiving itself and of our knowledge of the external world.

The Psychological Processes

It is clear from experimental psychology that perceptual consciousness involves a whole range of adjustments and selective or quasi-interpretative processes. The main evidence for this lies in differences between what psychologists often call the phenomenal properties of an object (those we are perceptually conscious of) and its stimulus properties. In this context the stimulus is the pattern of light rays from the object striking the eye, of sound waves striking the ear, or of heat or pressure from touching the object. The stimulus properties are those that we should observe in the stimulus (such as shape, color, pitch) could we observe it directly and in itself. This is difficult to achieve, and in fact the evidence of cameras, tape recorders, and other instruments is used, plus knowledge of the nature of the object and reasoning from the laws of perspective or of physics generally. The difference between the two kinds of property is presumed to be the result of modifications by the percipient.

attention and selection

It is a simple fact of experience that the quality and accuracy of perception vary with our attention. We often look inattentively and fail to notice pronounced features of a scene, yet we may carefully observe and thus notice unexpected details—a mark on the wallpaper, a printer's error, a wrong note in a recorded symphony. From the evidence of other people, from photographs, and from other means there may be no doubt that these features appeared all the time in the stimulus properties even when we were unconscious of them. Besides confirming this, psychologists have shown how greatly what we do or do not notice depends on habits of attention or interests, on often unconscious "priming" or "set." A mother will hear her baby cry but not notice much louder noises; an architect may notice features of buildings, and a boy notices makes of cars, both being oblivious to much else. Thus, perceptual consciousness is very selective, and this selection is usually largely unconscious, though voluntary attention can greatly modify it. One special case of voluntary attention is of importance—"perceptual reduction" or "phenomenological observation," where we concentrate on the sensible qualities of what we perceive and not, as is usual, on the identification of the object concerned. An artist must do this when he has to paint a scene, and this kind of observation may reveal all sorts of previously unnoticed details of color, shape, and so on. It is open to question whether this kind of reduction reveals an element present in all perception—namely, sensing—or whether, and this is more plausible, it is simply a special kind of perception of external objects not found in normal perceiving.

errors and enrichment

Some errors in perception can be attributed to psychological factors—misidentifications because of careless observation, seeing what one expects to see rather than what is actually present, thinking that one hears the expected visitor coming when no one is there, and the like. These point to a common characteristic of perception and one apparent only when it goes wrong—the enrichment of perception by imagery and thought. Many psychological experiments have been concerned with this. For example, vague or ambiguous stimuli (pictures or sounds) are presented to different groups of people who see or hear them as definite objects or words, and the direction in which they are thus unconsciously supplemented or altered can be shown to be caused by suggestion or by the interests, emotional state, or physical state of the person. Another kind of case is the divergence between several eyewitnesses' accounts of an incident, which may all differ from a filmed record. Again, blind spots or other visual defects are often not apparent to the subject, who unconsciously fills in the gap (this happens to us all if we look with one eye, for there is a blind spot where the optic nerve leaves the retina). Extreme cases are hallucinations where the apparition is integrated with the background or casts shadows. Unnoticed supplementation by imagery, which is admittedly private and mental, seems strong evidence for the dualist claim that the contents of perceptual consciousness are similarly private and must be distinguished from object properties.

learning and cues

Our perception is clearly affected by learning and experience. Identification and discrimination afford obvious examples; one can learn to identify objects seen or photographed from unusual angles, to detect animals in natural camouflage, to distinguish different birds' songs. Driving a car involves perceptions of distance and relative speeds, perceptions that are acquired by experience. Psychological investigation has shown the role of learning to be far greater than this. Perception of spatial relations generally depends to a large extent on learning (normally unconscious and in childhood) to harmonize sight and touch and to use various cues. This is shown by various experiments, such as those with distorted rooms or inverted spectacles, and by the evidence of blind men who recover their sight. Among the various cues used for perception of distance and of solidity are shadows, aerial and linear perspective, parallax (or relative movement), and the interposition of objects. These assist binocular vision and enable us to see depth even with one eye.

figure-ground and gestalt

In perception our immediate consciousness is of an organized or structured whole. Some shape or feature stands out and is seen as the figure against a background, and if discrete units such as dots are presented, we see them as grouped or patterned in some way. This characteristic of experience has been particularly stressed by Gestalt psychologists, who produced much experimental evidence to show that we see wholes or structures (Gestalten, literally, "forms") and that perception develops by discriminating these in and from a background and not by synthesis of atomic elements or point sensations first perceived separately. Such organization of the visual field, though little affected by learning, is nevertheless largely the result of processes in the percipient himself. The clearest evidence of this comes from the reversals, or "alternating illusions," where the stimulus (picture or succession of sounds) is constant but is perceived differently at different times; thus, sometimes one pattern or shape stands out as the figure, sometimes another. Examples are the goblet that may appear as two faces in profile, Edwin Boring's wife–mother-in-law figure, Ludwig Wittgenstein's duck-rabbit, and the staircase that seems to be seen now from above, now from below.

perception of motion

Perception of motion was closely investigated by the Gestalt psychologists, who drew attention to the Phi phenomenon, which is the impression of movement between adjacent stationary stimuli that are activated in succession. This underlies the consciousness of movement on a motion picture or television screen and is used in illuminated advertisements in which if groups of lights are successively switched on for a brief time, one is perceptually conscious of a moving figure or even of words moving along. Intermittent illumination may also make moving objects appear stationary. Thus, when illuminated by the flashing light of a stroboscope, a moving crank in a machine may, if the flashing is properly adjusted, be seen as stationary and examined for defects; if there is a slight maladjustment of the flashing, it may seem to rotate slowly backward like the wheels of coaches in Western films. There is a clear distinction in these cases between the properties of the stimulus and the contents of consciousness. Figure-ground effects also occur in movement perception, such as when the moon seems to sail through the clouds or when one's stationary train seems to move if an adjacent one starts.

object constancy

The widespread phenomenon of object constancy in perception differs from the above in that the phenomenal properties of an object tend to remain constant or nearly so even though the stimulus properties vary considerably. Thus, when we look at a round object—for example, a dish—from an angle, it often still looks round and not elliptical, although by the laws of perspective the stimulus (light-ray pattern) or retinal image is elliptical, as would appear on a photograph taken from the percipient's viewpoint. (This causes complication in stating the argument from illusion and perspective realism.) Only if the angle is very marked does the dish look elliptical. ("Look" here refers to the sensible quality, not to what we judge to be the object's shape.) Similarly with size, brightness, and color—a man looks much the same size at ten yards' distance as at five even though the image cast on the retina is half as high in the former case; a white patch in the shade reflects less light than a dark one in bright sunlight, but it still looks white; a white patch in a yellowish light still looks white although it is reflecting yellowish light (one may be surprised by color photographs taken in the evening, for the camera cannot adapt itself to the yellower light).

In general, over a range of varied stimuli we tend to see something corresponding to the property of the object or at least some compromise between this and the stimulus property. Experiments show that this constancy depends not on knowledge of the object but on the visibility of its background, for if the background is cut off by a screen so that only the object is visible, constancy does not hold and the stimulus property is seen. It is as though we made unconscious allowance for distance, angle of sight, and illumination as revealed by the whole scene. But this is not a learned or intelligent adjustment; children and even chickens or fish apparently see things with constancy, though to some extent it can be counteracted by adopting a stimulus attitude (trying to see the stimulus property).

philosophical significance

The existence of these many complex processes that underlie perceptual consciousness and affect its content reinforces the causal argument by making even more incredible the direct-realist notion of perceiving as a straightforward direct confrontation with the actual properties of objects. If perception were a simple intuitive awareness of such properties, there would be no place in it for variations in quality; for the effects of interests, priming, and learning; and for the use of cues for enrichment by or integration with supplementary imagery, for constancy adjustments (especially where they produce a compromise between object and stimulus properties), for changing figure-ground effects, or for the Phi phenomenon.

The range of these processes is far greater than that which would be compatible with the usual analyses of perceptual consciousness—namely, that it is the interpretation of sensations (or inference from implicit grounds) or the taking for granted that a sensed datum belongs to an object. These views were mainly influenced by the possibility of error in perception, particularly in identification, although they took some account of the use of cues and of the role of learning. But they seem inadequate to cover the part played by attention and unconscious selection or by such organization adjustments as figure-ground, grouping, object constancy, or the Phi phenomenon, whereas some of the imaginative supplementation goes far beyond what can be called interpreting a datum. It is sometimes claimed that these adjust-ments are interpretations. But this is implausible, for they seem little affected by learning and are not intelligent since lower animals make them. Nor can many of the illusions or adjustments be overcome by knowledge of the facts or by conscious interpretation; where some counteraction is possible, as in object constancy, it is very difficult, and for most people the presence or absence of screens in experiments is compelling in its effect.

The final objection to such analyses concerns the alleged pure sensory data; interpreting or taking for granted, insofar as we are aware of it, is of something we are conscious of as distinct and external and which is thus already the effect of many of these processes. Normally, however, perceptual consciousness seems intuitive—that is, without interpretation and quite unanalyzable; except in perceptual reduction its content almost always consists of ostensible objects. All the same, psychological evidence shows that there is a range of subjective processes. The only answer seems to be a genetic hypothesis, not an analysis into elements. Perceptual consciousness is introspectively a whole but must be supposed to be a product of a range of selective, supplementary, integrative or organizational, and quasi-interpretative processes acting on a supposed basic sentience. But—and this is the point—both processes and sentience are unconscious and so may plausibly be regarded as cerebral activities or adjustments of the nervous system. However, since we cannot as yet give any precise neurological statement of these processes, we have to describe them as if they were conscious, basing the description on the difference between the input to the senses and the finished product, but this product (perceptual consciousness) does not reveal within itself the processes that may be supposed to form it.

The suggestion that perceptual consciousness is the product of many unconscious processes is controversial, and any general conclusions about perception are bound to be personal. Hence, the main attention in this entry has been on the facts that have to be taken into account in any fully adequate view of perception, and the reader is also referred to the statement of the various theories here and in other related entries. In this way one has the material for assessing the general view here adopted—namely, that the causal and psychological processes essential to perception, as well as its liability to illusion, require abandonment of direct realism for a dualist position. One must distinguish perceptual consciousness, whose content or objects are subjective and private to the percipient, from perception that occurs when this perceptual consciousness is caused by an external object with properties corresponding to its content. But one must not confuse this dualism with the traditional representative realism, which is only a variant of it, some form of critical realism being superior; the sense-datum theory's dualism of sense data and objects (perceptual consciousness is not thus analyzable, and its content consists of ostensible material objects); or the Cartesian mind-body dualism (it is possible also to adapt this view of perception to a double-aspect account of mind and body).

Bibliography

general introductions

Perception is discussed, though usually less extensively than in the related entries in this encyclopedia, by most introductory books on philosophy. For example, see Bertrand Russell, Problems of Philosophy (London: Williams and Norgate, 1912) and Outline of Philosophy (London: Allen and Unwin, 1927); Charles Harold Whiteley, Introduction to Metaphysics (London, 1955); John Hospers, Introduction to Philosophical Analysis (New York: Prentice-Hall, 1953; London: Routledge and Paul, 1956); and Arthur Pap, Elements of Analytic Philosophy (New York: Macmillan, 1949). A clear and useful outline with emphasis on the associated epistemological problems is A. J. Ayer's The Problem of Knowledge (London: Macmillan, 1956). A more detailed introductory treatment is given by R. J. Hirst, The Problems of Perception (London: Allen and Unwin, 1959), Chs. I–VI; the later chapters of this book develop a more advanced treatment of perceptual consciousness and the scientific evidence on the lines adumbrated here.

perceptual consciousness

For analyses of perception in the sense of perceptual consciousness, see H. H. Price, Perception (London: Methuen, 1954) on the sense-datum theory; Durant Drake and others, Essays in Critical Realism (London: Macmillan, 1920), and Roy Wood Sellars, The Philosophy of Physical Realism (New York: Macmillan, 1932), on critical realism; Brand Blanshard, The Nature of Thought, Vol. I (London: Allen and Unwin, 1939), on idealism; and Roderick M. Chisholm, Perceiving: A Philosophical Study (Ithaca, NY: Cornell University Press, 1957), which is a causal analysis with a general consideration of perceiving and epistemological questions. Roderick Firth, "Sense-Data and the Percept Theory," in Mind 58 (232) (1949): 434–465, and 59 (233) (1950): 35–36, is critical of any such analyses.

conceptual analysis

For the approach by conceptual analysis see "On Seeing and Hearing" by Winston H. F. Barnes, "Identification and Existence" by Stuart Hampshire, and "Sensation" by Gilbert Ryle in Contemporary British Philosophy, Vol. III, edited by H. D. Lewis (London: Allen and Unwin, 1956); Gilbert Ryle, The Concept of Mind (London: Hutchinson, 1949), Ch. 7, and Dilemmas (Cambridge, U.K.: Cambridge University Press, 1954); Anthony M. Quinton, "The Problem of Perception," in Mind 64 (253) (1955): 28–51; and D. W. Hamlyn's discussion of psychological theories, The Psychology of Perception (London: Routledge and Paul, 1957). John Langshaw Austin's Sense and Sensibilia (Oxford: Clarendon Press, 1962) is an outstanding defense of common sense against the argument from illusion, but unfortunately he does not discuss the scientific evidence. Another direct-realist study is D. M. Armstrong's Perception and the Physical World (New York: Humanities Press, 1961).

scientific background

Useful elementary accounts of the causal and psychological processes are given in such textbooks as Ernest Ropiequet Hilgard, Introduction to Psychology (New York: Harcourt Brace, 1953); David Krech and Richard Stanley Crutchfield, Elements of Psychology, 3rd ed. (New York: Knopf, 1962); and Robert Sessions Woodworth and D. G. Marquis, Psychology (New York: Holt, 1947; London: Methuen, 1949). Rather more detail, with a philosophical discussion as well, is given in George McCreath Wyburn, Ralph William Pickford, and R. J. Hirst, Human Senses and Perception (Edinburgh: Oliver and Boyd, 1964). Two significant but unconventional psychological books that raise philosophical questions of interest to the advanced student are James Jerome Gibson, The Perception of the Visual World (Boston: Houghton Mifflin, 1950), and Friedrich August von Hayek, The Sensory Order (Chicago: University of Chicago Press, 1952; London: Routledge, 1953).

interpretations of scientific evidence

See the works by Price, Chisholm, Hirst and the critical realists mentioned above and Arthur O. Lovejoy, The Revolt against Dualism (La Salle, IL: Open Court, 1930), also a critical realist. C. D. Broad's Perception, Physics and Reality (Cambridge, U.K.: Cambridge University Press, 1914) and Scientific Thought (London: Kegan Paul, 1923), sense-datum analyses, are very technical at times. William Pepperell Montague, The Ways of Knowing (New York: Macmillan, 1925), gives a general comparison of the different realist (and other) views. See also the more sophisticated types of representative realism—namely, John Raymond Smythies, Analysis of Perception (London: Routledge and Paul, 1956), or Bertrand Russell, Analysis of Matter (London: Kegan Paul, 1927) and Human Knowledge (London: Allen and Unwin, 1948). Of Russell's two books, the first is often difficult, and the second is more for the general reader.

other recommended titles

Alston, William P. The Reliability of Sense Perception. Ithaca, NY: Cornell University Press, 1993.

Armstrong, D. M. Belief, Truth, and Knowledge. Cambridge, U.K.: Cambridge University Press, 1973.

Ayer, A. J. The Foundations of Empirical Knowledge. London: Macmillan, 1940.

BonJour, Laurence. The Structure of Empirical Knowledge. Cambridge, MA: Harvard University Press, 1985.

Brewer, Bill. Perception and Reason. Oxford: Clarendon Press, 2002.

Burnyeat, Myles. "Conflicting Appearances." Proceedings of the British Academy 65 (1979): 69–111.

Dancy, J., ed. Perceptual Knowledge. Oxford: Oxford University Press, 1988.

Dicker, Georges. Perceptual Knowledge. Dordrecht: Reidel, 1980.

Dretske, Fred. Seeing and Knowing. London: Routledge and Kegan Paul, 1969.

Fales, Evan. A Defense of the Given. Lanham, MD: Rowman & Littlefield, 1996.

Fumerton, Richard. Metaphysical and Epistemological Problems of Perception. Lincoln: University of Nebraska Press, 1985.

Goldman, Alvin. "Perceptual Knowledge and Discrimination." Journal of Philosophy 73 (1976): 771–791.

Grice, H. P. "The Causal Theory of Perception." PAS, Supp. 35 (1961): 121–168.

Jackson, Frank. Perception: A Representative Theory. Cambridge, U.K.: Cambridge University Press, 1977.

Perkins, M. Sensing the World. Indianapolis: Hackett, 1983.

Sellars, Wilfrid. Science, Perception, and Reality. Atascadero, CA: Ridgeview Press, 1991.

R. J. Hirst (1967)

Bibliography updated by Benjamin Fiedor (2005)

Encyclopedia of Philosophy Hirst, R.

Perception

views updated May 23 2018

Perception

Perceptual systems

Historical background

Classical perceptual phenomena

Innate and learned

Broad theoretical approaches

Current research/future developments

Resources

Human perception is the active reception and coordination of information received through our sensory systems in order to make sense of the environment and to behave effectively within it. In contrast with the direct and immediate sensations actually received and transmitted, perception is the transformation of that information into nerve cell activity that is transferred to the brain where further processing occurs. Our perceptual systems do not passively receive stimuli from the world, instead they actively select, organize, interpret, and sometimes distort sensory information. The real world then may not be the same as the one we perceive. Broadly, perception can be said to be the study of the human organism’s relation to the physical world.

Perceptual systems

Human beings possess five basic perceptual systems. The basic orientation system informs us of the position of the body in relation to the environment through receptors sensitive to gravity, such as those in the vestibular mechanism in the inner ear. The haptic or tactual system responds to pressure and temperature sensations using the skin, muscles, and joints. The haptic system uses the kinesthetic sense (information from the muscles), and the proprioceptive sense (information from the skeletal joints). The auditory perceptual system allows us to locate the sources of sounds, and to recognize organized sound structures such as speech and melodies. What is often called the savory system combines the senses of taste and smell which are intimately related. They both respond primarily to chemical aspects of the environment. The visual system responds to light received through the eye, or more precisely, the light sensitive cells of the retina. Fully functioning visual systems perceive color, distance, depth, motion, and form.

While perception involves information coming in from all of the sense modalities, psychologists have tended to focus on visual perception. This is because many aspects of visual perception are not easily explained by sensory processes alone, they seem instead to involve more higher-level brain processing than, for instance, taste, or scent perception. For the same reasons, this entry emphasizes visual perception.

Historical background

Perception is studied by philosophers, physiologists, physicians, and psychologists. Physiologists and physicians focus primarily on sensation and the underlying physical processes connected to perception. Systematic thought about perception began with ancient Greek philosophers who were interested primarily in the sources and validity of human knowledge. This still active branch of philosophy, known as epistemology, explores whether a real physical world exists independent of our experience of it, and whether our perceptions are an accurate reflection of that world. Epistemologists also question whether we are born with certain innate forms of knowledge, or whether all knowledge is learned through experience.

The systematic examination of mental organizations of physical sensations falls largely within the domain of psychology. In contrast with philosophers, psychologists use scientific methods to investigate perceptual questions. Many of the questions raised by philosophers are reflected in the larger issues that are still investigated by psychologists today. These issues include how our perceptions are formed from the interaction of the physical environment and our sense organs, the accuracy of our perceptual systems in perceiving the world, and what, if any, aspects of perception reflect innate properties of the brain versus being learned through experience.

Classical perceptual phenomena

Many aspects of perception are, quite simply, amazing, often denying easy explanation based on stimulation of the sensory systems alone. The perceptual phenomena discussed below have historically been the subject of much research and they pose a continuing challenge to researchers and theoreticians in perception.

Constancy

One of the most striking aspects of perception is constancy. Constancy refers to how our perception of objects remains the same despite changes in their image on the retina. Constancy is seen in the perception of a number of different properties of objects such as size, shape, color, and orientation. We will discuss only size and shape constancies.

Size constancy refers to perceiving familiar objects as approximately the same size regardless of their distance from the observer. Thus, for example, a person’s size does not appear to expand or contract as they come toward you or move away, even though their image on the retina does become larger and smaller based on their distance.

Shape constancy refers to an object’s shape being perceived as remaining the same despite being viewed from different perspectives with different shapes being projected onto the retina. A circular shape such as a pie on a table for example, is still perceived as circular even when you sit down at the table and perceive it from the side. This is despite the fact that circles viewed from the side produce not circular but elliptical images on the retina.

Perceptual constancy is one of the hallmarks of the field of perception, for it strongly indicates that visual perception involves more than the direct registration of the retinal image in the brain. Without perceptual constancy the world would be perceived as a booming blur of chaotic confusion in which the sizes, shapes, and colors of objects would be constantly shifting. Thus it can be seen that perceptual constancy serves an important adaptive function.

Despite constancy’s great importance and prevalence across much of visual perception, there still no widely agreed upon explanation of it. There is, however, great agreement that constancy is based in part on the observer using appropriate contextual cues in the environment. For example, in size constancy it seems that in most instances we use estimates of detected distance based on various cues (for instance, haze, and a smoothing of textured surfaces such as grass are indicators of distance) to estimate an object’s true size. And it seems that without cues enabling an accurate estimation of distance from an object, the perceived constancy of size ceases. For example, if one is extremely distant from objects without many intermediate visual cues enabling an accurate assessment of that distance, their perceived size would decrease. This is evident when looking at the world from a very high mountain top, or from a very tall building. From this distance, houses, cars, trees, and people below look very small, as if they were in miniature.

Perception of motion

The perception of motion has been the subject of much research. The mystery lies in how perceived movement cannot be accounted for by the movement of an object’s image across the retina. If that were so, movement of the observer, or eye movement would lead to perceived object movement. For example, when riding a bike the rest of the world would be perceived as moving. Another phenomenon of motion perception that cannot be fully explained by sensory processes involves saccades, which are rapid directed eye movements. Because the eye sees detail only in a small area in the center of the eye called the fovea, in order to obtain detailed information from any object or scene, the eye must perform saccades so that the fovea receives enough information. Yet the images of stationary objects do not appear to move even when their retinal image moves due to saccades.

Evidence suggests motion perception can be partially explained by our apparently automatic use of numerous specific spatial and sequential relations between stimuli. Perceived motion then depends on such factors as the change in angular direction of the object from the observer, and the relation of the object in motion to the field in which it is perceived. For example, as an object moves through space it systematically covers and uncovers the background through which it is moving. Thus if a lion is running toward you across an open grassy plain, the grass will appear to be blocked out at the lion’s leading edges as he moves toward you, and the grass that was not visible behind him will become visible as he gets closer.

In addition to contextual environmental factors, specific visual receptor cells that detect different types of movement such as up and down have been discovered. Despite this knowledge, many questions about the exact mechanisms of motion perception remain unanswered.

Form perception

Form perception is what enables us to identify objects and distinguish them from each other. Rather than a loose grouping of apparently separate stimuli, we see the world as organized with interrelated objects having definite shapes and forms. And as with many other perceptual phenomena, the light projected onto the retina from objects cannot account for our visual perception of the world. It seems perceiving form involves certain organizational principles, many of which were discovered by the Gestalt school of psychology. These rules or principles illustrate our tendency to organize and group separate elements of the visual world.

In the figure-ground rule, Gestalt psychologists found that when looking at unfamiliar scenes, familiar or consistent shapes tend to stand out as figures, and unfamiliar or undifferentiated shapes are perceived as the background. So, when looking at an abstract painting in which there are very few clearly defined forms, those forms that appear familiar, or that are repeated, will tend to be perceived as standing out from the rest of the painting which is then perceived as the background for those forms.

Gestalt psychologists also described a number of perceptual grouping principles. They found that when we perceive various stimuli we tend to group them according to their similarity, or according to their closeness to one another. Another principle, that of good configuration, is a very general organizing tendency that incorporates a number of figural characteristics. These include a tendency toward closure or perceiving a whole figure when there are actually gaps in its contour, and continuation in form where smooth continuous contours tend to be perceived over uneven or irregular contours.

The perception of form can be said to result, in part, from characteristics of the nervous system, as well as learning and experience. Yet there is no single theory of form perception that can fully account for the ability to perceive form. Nor is there a general principle that can pull together the many different types of form perception.

Depth perception

This entails perceiving the three-dimensionality of the world and objects. This clearly involves more than the nature of images sent to the retina since the retina has a two dimensional surface and images projected onto it are two-dimensional.

In the 1800s researchers discovered that our binocularvision greatly aids depth perception. Binocular vision refers to having two eyes that are at slight distances from one another, so that each receives a slightly different perspective of the object or scene being focussed upon. It seems that these small differences in perspective greatly aid depth perception. The ears also use the slight differences in time between stimulation received to locate the source of sounds.

In addition, most environments have common patterns corresponding with varying distances that provide visual clues about space and depth. Such clues include blocking of a far object by a near one, increasing haze with increasing distance, perspective, and shadow.

In sum, binocular vision and environmental cues can account for many aspects of depth perception. In addition, based on research with animals and human infants too young to have had experience with depth perception, it appears that humans and various species of animals are born with some innate visual mechanism to perceive depth.

Illusions

Illusions are misperceptions of stimuli, where what is perceived does not correspond to the actual dimensions or qualities of the physical stimulus. Geometrical illusions usually involve the misperception of the direction or size of parts of figures.

The mechanisms that produce many types of illusions are as yet not understood, but they seem to involve the misapplication of perceptual phenomena like constancy. Illusions are natural, occurring regularly and following regular rules. Illusions should not be confused with hallucinations which are responses in the absence of any external stimulus, or with delusions which are basically mistaken beliefs.

Innate and learned

A theme running throughout the study of perception since the time of the ancient Greeks has been whether perceptual processes are learned or innate. Innate means existent or potential at birth due to genetic factors. Learned means that the ability is based on remembered past experience with similar or relevant stimuli.

One way to test these ideas is to examine humans or animals who from birth had no visual experience, and thus no opportunities for visual learning, and to test them when their sight is restored. Perceptual functions are then tested to see which, if any, are intact. This was done with human beings born blind because of cataracts before surgical methods were developed to safely remove them. Cataracts are a disease of the eye in which the crystalline lens or its capsule are or become opaque. It was found that after their cataracts were removed they were normally responsive to changes in color and light, but they were unable to tell when a figure was present, or to discriminate between simple shapes. It took a period of two to three months before they were able to perform these tasks with ease.

Along the same lines, research with animals deprived of visual experience from as close to birth as possible, finds that even without visual experience, some of the animals can perceive visual depth cues. Research also finds that animals raised without opportunities to see (for example if reared in the dark) sustain long-lasting deficits in their perceptual abilities. Indeed, such deprivation can even affect the weight and biochemistry of their brains.

Studies with human infants find that at even one or two days of age, they are able to perform detailed visual discriminations, and they show preferences for visually complex or novel stimuli. While this line of research cannot prove the ability is not learned, it does lend support to these abilities being present at birth in some form.

In sum, it seems that while some fundamental visual perceptual abilities are innate, visual experience is necessary to maintain and further develop them.

Broad theoretical approaches

Over the last century a number of theories have been proposed to account for perception. Each theory, however, has encountered difficulties in accounting for some of the above-discussed phenomena of perception. And most perception researchers today do not adhere to one theory, instead they believe those aspects of the theories that have some experimental support, or that seem most logical and sound.

Classical theory

What is sometimes referred to as classical theory is usually associated with Hermann von Helmholtz who believed perception results from a process of unconscious inference about what the stimulus affecting the sense organs is most likely to be. He thought these unconscious inferences are formed by past experiences and learning, and they are unconscious because people are clearly not aware of making them.

Gestalt theory

Probably the most well-known theory of perception, Gestalt theory, developed partly as a reaction against the view that perception could be broken into simpler elements and that it was the result of learned mental associations between simple sensations. This view, the basis of Helmholtz’s theory, was also put forth without the process of unconscious inference by such famous psychologists as W. Wundt and E.B. Titchener. Gestalt theory, founded by K. Koffka, W. Köhler, and M. Wertheimer, argued that while simple sensations could be seen as making up organized perceptions, our nervous system is primed to perceive the organization of sensory stimuli over the individual sensory elements themselves. The process of organization is basic to perception, and the common saying, “the whole is greater than the sum of its parts,” illustrates this important concept.

Moreover, while Gestalt theorists believe learning may play a role in perception, perceptual organization results from innate organizing processes in the brain itself. To Gestaltists then, studying perception was in effect studying the brain.

Psychophysical or direct theory

This theory as put forth by J. J. Gibson holds that perception may be fully explained by the properties of the stimulation we receive from the world interacting with our sensory capabilities. Characteristics of scenes and events in the physical world may give sufficient information for the nervous system to be able to specify them. Thus, there is no need to posit unconscious mechanisms of inference as put forth by the Helmholtzian theory, or higher order organizations of stimuli as proposed by Gestalt theory.

Modern sensory physiology

This theory proposed by E. Hering and E. Mach believes the structure of the nervous system may fully explain at least some perceptual constancies as well as depth perception. E. Hering also proposed that there may be visual receptor cells organized into certain functional patterns that provide color sensation. And in fact these functionally patterned receptor cells have been found. These findings have strongly influenced current views of color perception, and the study of perception and sensation in general. It is still unknown, however, whether similarly organized receptor cells may exist for, or contribute to, perceptual phenomena such as the constancies and illusions. But their demonstrated existence may indicate that many perceived qualities of the physical world are based on such specific sensory mechanisms.

Current research/future developments

Some of the more recent theoretical and research developments fall within the areas of emotion, neuro-psychology, ecological psychology, and artificial intelligence.

Emotion

Reflecting a trend across psychology as a whole, there has been a renewed and increased interest in how emotion influences perception and attention. This research investigates such questions as how emotion influences the focus and duration of attention, how quickly the emotional meaning of various stimuli can be processed, and whether individuals attend to positive and negative stimuli in different ways. Unlike most past research on perception this evolving area often researches socially meaningful perceptual stimuli, such as the perception of emotion in facial expressions and in vocal tones.

Neuropsychology

Neuropsychologists study changes in thinking due to brain injury, and use brain imaging techniques such as magnetic resonance imaging (MRI) and positron emission tomography (PET) scans, to examine the activity of the brain while performing high-level mental tasks such as problem-solving. A number of their findings have challenged explanations of perception based on behavioral studies. And improvements in brain imaging techniques hold the promise of shedding even more light on the neural basis of perception.

Ecological psychology

Ecological psychology attempts to specify the unchanging and limiting aspects of perceptual stimuli in the environment. They also stress how the nature of perceptual stimuli supports perception. This approach

KEY TERMS

Constancy— A striking aspect of perception, constancy refers to how our perception of properties of objects such as size, shape, and color, remains the same despite changes in their image on the retina.

Delusions— Fixed, false beliefs that are resistant to reason or factual disproof.

Fovea— Tiny hollow in the retina and area of acute vision.

Gestalt theory of perception— A theory holding that organization is basic to perception; discovered many rules of organization used in perceiving form.

Hallucination— A sensory experience of something that does not exist outside the mind. A person can experience a hallucination in any of the five senses.

Illusions— Naturally occurring misperceptions of stimuli that follow regular rules.

Retina— An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.

Saccade— Rapid directed eye movement, often used to obtain detailed information about an object or scene.

is most closely associated with the psychologist J.J. Gibson.

Artificial intelligence

This is an interdisciplinary field combining research and theory from cognitive psychology and computer sciences. It focuses on the development of artificial systems, such as computers, that show thinking processes similar to humans. This approach believes that for a complete explanation of perception it is necessary to divide it into three levels of analysis: 1) hardware, or its physiological aspects, 2) algorithms for operation, or what the processes of perceiving are, and 3) the theory of the task to be performed, or what are the qualities of our environment that enable perception. It is hoped that these divisions will serve as an important intellectual tool and aid our understanding of perception.

Perception is a field ripe with unanswered questions that continues to fascinate researchers who may greatly benefit from new technologies and new perspectives. Indeed, recent technological advances in the measurement of eye movements (saccades) have made their study much easier for researchers interested in changes in the focus of visual attention.

See also Hearing; Touch.

Resources

BOOKS

Masin, S.C., ed. Foundations of Perceptual Theory. New York: Elvesier Science, Inc., 1993.

Schiffman, H.R. Sensation and Perception: An Integrated Approach, 5th ed. New York: John Wiley & Sons, 2001.

Sekuler, R. and R. Blake. Perception. 3rd ed. New York: McGraw-Hill Humanities/Social Sciences/Languages, 2005.

Marie Doorey

The Gale Encyclopedia of Science

Perception

views updated Jun 08 2018

Perception

Human perception is the active reception and coordination of information received through our sensory systems in order to make sense of the environment and to behave effectively within it. In contrast with the direct and immediate sensations actually received and transmitted, perception is the transformation of that information into nerve cell activity that is transferred to the brain where further processing occurs. Our perceptual systems do not passively receive stimuli from the world, instead they actively select, organize, interpret, and sometimes distort sensory information. The real world then may not be the same as the one we perceive. Broadly, perception can be said to be the study of the human organism's relation to the physical world.

Perceptual systems

Human beings possess five basic perceptual systems. The basic orientation system informs us of the position of the body in relation to the environment through receptors sensitive to gravity, such as those in the vestibular mechanism in the inner ear . The haptic or tactual system responds to pressure and temperature sensations using the skin, muscles, and joints. The haptic system uses the kinesthetic sense (information from the muscles), and the proprioceptive sense (information from the skeletal joints). The auditory perceptual system allows us to locate the sources of sounds, and to recognize organized sound structures such as speech and melodies. What is often called the savory system combines the senses of taste and smell which are intimately related. They both respond primarily to chemical aspects of the environment. The visual system responds to light received through the eye , or more precisely, the light sensitive cells of the retina. Fully functioning visual systems perceive color , distance , depth, motion , and form.

Historical background

The systematic examination of mental organizations of physical sensations falls largely within the domain of psychology . In contrast with philosophers, psychologists use scientific methods to investigate perceptual questions. Many of the questions raised by philosophers are reflected in the larger issues that are still investigated by psychologists today. These issues include how our perceptions are formed from the interaction of the physical environment and our sense organs, the accuracy of our perceptual systems in perceiving the world, and what, if any, aspects of perception reflect innate properties of the brain versus being learned through experience.

Classical perceptual phenomena

Constancy

Size constancy refers to perceiving familiar objects as approximately the same size regardless of their distance from the observer. Thus, for example, a person's size does not appear to expand or contract as they come toward you or move away, even though their image on the retina does become larger and smaller based on their distance.

Shape constancy refers to an object's shape being perceived as remaining the same despite being viewed from different perspectives with different shapes being projected onto the retina. A circular shape such as a pie on a table for example, is still perceived as circular even when you sit down at the table and perceive it from the side. This is despite the fact that circles viewed from the side produce not circular but elliptical images on the retina.

Despite constancy's great importance and prevalence across much of visual perception, there still no widely agreed upon explanation of it. There is, however, great agreement that constancy is based in part on the observer using appropriate contextual cues in the environment. For example, in size constancy it seems that in most instances we use estimates of detected distance based on various cues (for instance, haze, and a smoothing of textured surfaces such as grass are indicators of distance) to estimate an object's true size. And it seems that without cues enabling an accurate estimation of distance from an object, the perceived constancy of size ceases. For example, if one is extremely distant from objects without many intermediate visual cues enabling an accurate assessment of that distance, their perceived size would decrease. This is evident when looking at the world from a very high mountain top, or from a very tall building. From this distance, houses, cars, trees, and people below look very small, as if they were in miniature.

Perception of motion

The perception of motion has been the subject of much research. The mystery lies in how perceived movement cannot be accounted for by the movement of an object's image across the retina. If that were so, movement of the observer, or eye movement would lead to perceived object movement. For example, when riding a bike the rest of the world would be perceived as moving. Another phenomenon of motion perception that cannot be fully explained by sensory processes involves saccades, which are rapid directed eye movements. Because the eye sees detail only in a small area in the center of the eye called the fovea, in order to obtain detailed information from any object or scene, the eye must perform saccades so that the fovea receives enough information. Yet the images of stationary objects do not appear to move even when their retinal image moves due to saccades.

Evidence suggests motion perception can be partially explained by our apparently automatic use of numerous specific spatial and sequential relations between stimuli. Perceived motion then depends on such factors as the change in angular direction of the object from the observer, and the relation of the object in motion to the field in which it is perceived. For example, as an object moves through space it systematically covers and uncovers the background through which it is moving. Thus if a lion is running toward you across an open grassy plain, the grass will appear to be blocked out at the lion's leading edges as he moves toward you, and the grass that was not visible behind him will become visible as he gets closer.

Form perception

The perception of form can be said to result, in part, from characteristics of the nervous system , as well as learning and experience. Yet there is no single theory of form perception that can fully account for the ability to perceive form. Nor is there a general principle that can pull together the many different types of form perception.

Depth perception

In the 1800s researchers discovered that our binocular vision greatly aids depth perception . Binocular vision refers to having two eyes that are at slight distances from one another, so that each receives a slightly different perspective of the object or scene being focussed upon. It seems that these small differences in perspective greatly aid depth perception. The ears also use the slight differences in time between stimulation received to locate the source of sounds.

In sum, binocular vision and environmental cues can account for many aspects of depth perception. In addition, based on research with animals and human infants too young to have had experience with depth perception, it appears that humans and various species of animals are born with some innate visual mechanism to perceive depth.

Illusions

Illusions are misperceptions of stimuli, where what is perceived does not correspond to the actual dimensions or qualities of the physical stimulus . Geometrical illusions usually involve the misperception of the direction or size of parts of figures.

Innate and learned

A theme running throughout the study of perception since the time of the ancient Greeks has been whether perceptual processes are learned or innate. Innate means existent or potential at birth due to genetic factors. Learned means that the ability is based on remembered past experience with similar or relevant stimuli.

One way to test these ideas is to examine humans or animals who from birth had no visual experience, and thus no opportunities for visual learning, and to test them when their sight is restored. Perceptual functions are then tested to see which, if any, are intact. This was done with human beings born blind because of cataracts before surgical methods were developed to safely remove them. Cataracts are a disease of the eye in which the crystalline lens or its capsule are or become opaque. It was found that after their cataracts were removed they were normally responsive to changes in color and light, but they were unable to tell when a figure was present, or to discriminate between simple shapes. It took a period of two to three months before they were able to perform these tasks with ease.

In sum, it seems that while some fundamental visual perceptual abilities are innate, visual experience is necessary to maintain and further develop them.

Broad theoretical approaches

Classical theory

Gestalt theory

Probably the most well-known theory of perception, Gestalt theory, developed partly as a reaction against the view that perception could be broken into simpler elements and that it was the result of learned mental associations between simple sensations. This view, the basis of Helmholtz's theory, was also put forth without the process of unconscious inference by such famous psychologists as W. Wundt and E.B. Titchener. Gestalt theory, founded by K. Koffka, W. Köhler, and M. Wertheimer, argued that while simple sensations could be seen as making up organized perceptions, our nervous system is primed to perceive the organization of sensory stimuli over the individual sensory elements themselves. The process of organization is basic to perception, and the common saying, "the whole is greater than the sum of its parts," illustrates this important concept.

Psychophysical or direct theory

Modern sensory physiology

Current research/future developments

Some of the more recent theoretical and research developments fall within the areas of emotion, neuropsychology, ecological psychology, and artificial intelligence .

Emotion

Neuropsychology

Neuropsychologists study changes in thinking due to brain injury, and use brain imaging techniques such as magnetic resonance imaging (MRI) and positron emission tomography (PET) scans, to examine the activity of the brain while performing high-level mental tasks such as problem-solving. A number of their findings have challenged explanations of perception based on behavioral studies. And improvements in brain imaging techniques hold the promise of shedding even more light on the neural basis of perception.

Ecological psychology

Artificial intelligence

Resources

books

Masin, S.C., ed. Foundations of Perceptual Theory. New York: Elvesier Science, Inc., 1993.

Niedenthal, P.M., and S. Kitayama, eds. The Heart's Eye: Emotional Influences in Perception and Attention. New York: Academic Press, 1994.

Ono, T., ed. Brain Mechanisms of Perception and Memory: From Neuron to Behavior. New York: Oxford University Press, 1993.

Schiffman, H.R. Sensation and Perception: An Integrated Approach. 3rd ed. New York: John Wiley & Sons, 1990.

Sekular, R., and R. Blake. Perception. 3rd ed. New York: John Wiley & Sons, 1993.

Marie Doorey

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Constancy: —A striking aspect of perception, constancy refers to how our perception of properties of objects such as size, shape, and color, remains the same despite changes in their image on the retina.
Delusions: —Fixed, false beliefs that are resistant to reason or factual disproof.
Fovea: —Tiny hollow in the retina and area of acute vision.
Gestalt theory of perception: —A theory holding that organization is basic to perception; discovered many rules of organization used in perceiving form.
Hallucination: —A sensory experience of something that does not exist outside the mind. A person can experience a hallucination in any of the five senses.
Illusions: —Naturally occurring misperceptions of stimuli that follow regular rules.
Retina: —An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.
Saccade: —Rapid directed eye movement, often used to obtain detailed information about an object or scene.

The Gale Encyclopedia of Science

perception

views updated May 21 2018

perception Our senses probe the external world, and they also tell us about ourselves as they monitor the positions of our limbs and the balance of our bodies. Through pain they signal injury and illness.

How we experience and know about external objects is a question that was discussed by the Greek philosophers and has been ever since. Planned experiments on perception, in the spirit of the physical sciences, were hardly attempted before the mid nineteenth century. They have revealed a surprising complexity of physiological and cognitive (knowledge-based) processes, of which we are normally unaware, though many can be demonstrated simply and dramatically, especially through the phenomena of illusions.

There is a long-standing tradition in philosophy that perception, especially touch and vision, gives undeniably true knowledge. For philosophers have generally sought certainty and often claimed it, whereas scientists (who are used to their theories being upset by new data) are more ready to settle for today's best bet. Many scientific instruments have been developed because of the unreliability or inadequacy of perception.

Perceptions are separate, and in several ways different, from conceptual understanding, for perception must work very fast (whereas we may take minutes or hours to ‘make up our minds’, and years to form new concepts). Also, it would be impossible for perception to draw upon all of our knowledge; and perceptions are of individual objects and events in present time, while concepts are abstract and generally timeless.

The evolution of mechanisms for the perception of objects and events at a distance (most completely through vision and hearing) freed organisms from the tyranny of reflex responses to immediate situations, and no doubt was a necessary precursor of all intelligence. It is a fairly new notion that perception itself is an intelligent activity, requiring still only partly understood problem-solving to infer the objective world from sensory signals. Earlier accounts, especially British Empiricism, portrayed sensory perception very differently, as a passive, undistorting window, through which the mind accepts sensations directly from objects. This is not consistent with physiological knowledge of the senses and the brain, nor with many phenomena, such as illusions of vision and hearing and touch. The notion of ‘direct perception’ is, however, still maintained by some followers of the American psychologist J. J. Gibson, perhaps by taking this aspect of his important writings too literally. Perception is not traditionally thought of as an intelligent activity; though the power, especially of vision, to probe distance gains the time needed for intelligent behaviour and for the intelligence of perception itself.

Are perceptions simply picked up by the senses passively, or are they created actively by the brain, or mind? This issue between passive or active perception is a long-standing debate, with significant implications, such as: what is ‘objective’ and what ‘subjective’? The philosopher John Locke (1690) suggested that there are primary characteristics, such as hardness and mass and extension of objects, in space and time — in the world before life, existing apart from mind — and secondary characteristics, created in minds or brains. Thus colours are not in the world, but are created within us, though related in complex ways to light and to the surfaces of objects. Sir Isaac Newton (Opticks, 1704) expressed clearly that red light is not itself red, but is: ‘red-making’:
…there is nothing else than a certain power and disposition to stir up the sensation of this or that colour. For as sound in a bell or musical string … is nothing but a trembling motion.Then (in Query 23 of Opticks) Newton speculates on something like a neural mechanism of vision:
Is not vision perform'd chiefly by the Vibrations of this (Eatherial) Medium, excited in the bottom of the eye of Rays of Light, and propagated through the solid, pellucid and uniform Capillamenta of the optic Nerves in the place (the ‘Sensorium’) of Sensation?The Empiricist school (of which, in their different ways, Locke and Newton were founders) also rejected the notion that minds can receive knowledge by direct intuition quite apart from sensory experience. Mind was now regarded as essentially isolated from the physical world — linked only by tenuous threads of nerve and by fallible inferences of what might be ‘out there’. Some people find this too unsettling to be true. But it is now generally accepted that perception depends on active, physiologically based, intelligent processes. This is not intuitively obvious, since perception seems so simple and easy and we know nothing of the processes in our brains by introspection. Seeing happens so fast and so effortlessly that it is hard to conceive the complexity of the processes that we now know are needed to interpret the nature of the visual world from sensory signals — processes that remain largely beyond the capabilities of the most advanced computers.

Paradoxically, this takes us to concepts familiar to engineers and useful for physiology. We may describe the organs of the senses as ‘transducers’, which accept patterns of energy from the external world, signalling them as coded messages to be read by the brain, which uses these patterns to infer the state-of-play of the surrounding world, and something of the body's own states. Another useful engineering concept is that of ‘channels’. The various senses feed specialized ‘brain modules’ through neural channels, discovered by physiological and ‘psychophysical’ (perceptual) experiments. Thus, as Thomas Young suggested in 1801, colour vision is created from information about the wavelength of light transmitted through three channels, red, green, and blue, responding to light of long, medium, and short wavelength, respectively. All the hundreds of colours we can see are interpretations by the brain of the relative activity of these three colour channels. The three colour channels correspond, initially, to three kinds of light-catching photopigment in the photoreceptors, called cones, in the retina.

There are similar neural channels representing the orientation of lines and edges, and for movement, as first shown by direct physiological recording from nerve cells in the visual cortex of cats by the physiologists D. H. Hubel and T. N. Wiesel in 1962. There are channels for many other visual characteristics: stereoscopic (3-D) depth, texture, spatial size, etc. The ear has many different frequency channels, and there are scores of channels for the sense of ‘touch’, including those for various kinds of pain, for tickle, and for monitoring the positions of the limbs and the stretch of muscles in order to control movement. We are unaware of activity in these sensory channels themselves. Somehow outputs from the many channels are combined to give consistent perceptions. Small discrepancies — such as the delay in sound between seeing a ball hit a bat and hearing the impact — are rejected or pulled into place to maintain a consistent world. Equally, whole objects are somehow assembled from the many signals in different sensory channels that define them. But how this (‘the binding problem’) is done is not understood.

The theory that perception is ‘cognitive’, depending on inferences from essentially inadequate sensory signals, was first clearly proposed by the German polymath physicist, physiologist, and psychologist, Hermann von Helmholtz (1821–94). He called perceptions ‘unconscious inferences’. We might say that they (our most intimate experiences and knowledge) are simply hypotheses, essentially like the predictive hypotheses of science — though not always agreeing in particular accounts.

More recently, attempts to program computers to see (an important component of artificial intelligence) has shown how hard it is to infer objects from sensed data. The most influential attempt, by physiologist David Marr, suggested that object shapes are derived from the retinal images via three essential stages: (i) the ‘primal sketch(es)’, describing intensity changes and locations or critical features and local geometric relations;(ii) the ‘2¹/₂-D sketch’, giving a preliminary analysis of depth, surface discontinuities, and so on, in a frame that is centred on the viewer;(iii) the ‘3-D model representation’, in an object-centred co-ordinate system, so that we see objects much as they really are in 3-D space, though they are presented from just one viewpoint. Marr supposed that this last stage is aided by restraints on the range of likely solutions to the problem of what is ‘out there’. These information-processing constraints are set by assuming typical object shapes; for example, that the shapes of many objects, such as other human beings, are modified cylinders. Interestingly, the painter Paul Cézanne came close to this notion in 1904:
Treat nature by the cylinder, the sphere, the cone, everything in proper perspective so that each side of an object or a plane is directed towards a central point … nature for us men is more depth than surface. David Marr stressed the importance of immediate, passive processing of sensory signals, over active, cognitive ‘top down’ application of knowledge gained from the past. This is a central controversy, currently moving towards greater cognitive ‘top down’ contributions, especially for vision. When computers (or the form of computing known as ‘neural nets’) can access vast amounts of knowledge appropriately, in real time, they might share our miracle of perception.

Artists and scientists can teach each other secrets of perception (as by Gombrich, 1960), though such cross-cultural communication is not easy for most of us.

Richard L. Gregory

Bibliography

Gibson, J. J. (1950). Perception of the visual world. Houghton Mifflin, Boston MA.
Gibson, J. J. (1966). The senses considered as perceptual systems. Houghton Mifflin, Boston MA.
Gombrich, E. (1960). Art and illusion. Phaidon, London.
Gregory, R. L. (1966, fifth edn 1998). Eye and brain. Oxford University Press.
Hubel, D. and and Wiesel, T. N. (1962). Receptive fields, binocular interaction and functional architecture in the cat's visual cortex. Journal of Physiology, 160, 106–54.
Marr, D. (1982). Vision. W. H. Freeman, San Francisco.
Zeki, S. (1993). A vision of the brain. Blackwell, Oxford.

The Oxford Companion to the Body COLIN BLAKEMORE and SHELIA JENNETT

Perception

views updated Jun 27 2018

Perception

The area of psychology associated with the functioning of sensory systems and how information from the external world is interpreted.

Psychologists have identified two general ways in which humans perceive their environment . One involves what is called "top-down" processing. In this mode , what is perceived depends on such factors as expectations and knowledge. That is, sensory events are interpreted based on a combination of what occurs in the external world and on existing thoughts, experience, and expectations. When a perception is based on what is expected, it is called a perceptual set, a predisposition to experience an event in a particular way. One example of such a predisposition involves hearing potentially disturbing words or phrases when rock music is played backwards. Although most people will not detect such words or phrases when they first listen to the backward sounds (when they do not have a perceptual set), these same people will hear them quite clearly if they are then told what to listen for. Psychologists regard this process as involving a perceptual set because perception of the distressing message does not occur until the individual is primed to hear it.

Motivation can also influence the way an event is perceived. At sporting events, the same episode can be interpreted in exactly opposite ways by fans of two different teams. In this instance, people are interpreting the episode with what they regard as an open mind, but their subjectivity colors their perceptions. The alternate approach is "bottom-up" processing that relies less on what is already known or expected and more on the nature of the external stimulus. If there are no preconceived notions of what to expect, cues present in the stimulus are used to a greater extent. One part of this process is called feature analysis, which involves taking the elementary cues in a situation and attempting to put them together to create a meaningful stimulus. When children listen to an initially unfamiliar set of sounds, like the "Pledge of Allegiance," they often hear words and phrases that adults (who use top-down processing) do not hear. Thus, the phrase "one nation indivisible," may be heard by a child as "one naked individual." The child has heard the correct number of syllables, some key sounds, and the rhythm of the phrase, but too many features are unclear, resulting in an inaccurate perception. In general, many psychologists have concluded that perceptual abilities rely both on external stimuli and on expectation and knowledge.

Much of the research in perception has involved vision for two general reasons. First, psychologists recognize that these this sense dominates much of human perception and, second, it is easier to study than audition (hearing) or the minor senses like taste , smell , touch , and balance. Other perceptual research has investigated the way people pay attention to the world around them and learn to ignore information that is irrelevant to their needs at any given moment.

Within the realm of vision, several areas have especially captured the attention of psychologists: depth perception , form perception, perceptual constancy, and perceptual organization. When a visual scene contains information that includes conflicting information about depth, form, and organization, the result is a visual illusion, commonly referred to as an optical illusion. Such illusions can occur when there is too little information available to generate an accurate interpretation of the stimulus; when experience leads to the formulation of a specific interpretation; or when the sensory systems process information in a consistent, but inaccurate, fashion. Illusions are completely normal, unlike delusions that may reflect abnormal psychological processes.

Another aspect of perception that psychologists have studied intensively is attention. Often, people can selectively attend to different aspects of their world and tune others out. In a loud, crowded room, for example, a person can understand a single speaker by turning his or her attention to the location of the speaker and concentrating on the frequency (pitch) of the speaker's voice; the individual can also use the meaning of the conversation to help in concentration and to ignore irrelevant speech. In some cases, however, we seem incapable of ignoring information. One common example is the "cocktail party phenomenon." If something is holding our attention but an individual within earshot speaks our name, our attention is quickly diverted to that individual. When we perceive a stimulus that is important to us (like our name), our attention switches. One famous example that involves an inability to ignore information is the Stroop effect. If words are printed in colored ink, it is normally an easy task to name the color of the ink. If the words are color names, however, (e.g., "RED") that appear in a different ink color (e.g., the word "RED" in green ink), we have difficulty naming the ink color because we tend to read the word instead of paying attention to the ink color. This process seems entirely automatic in proficient readers.

Research on the perceptual capabilities of young children is more difficult because of insufficient communication skills. At birth, infants can see objects clearly only when those objects are about eight inches (20 cm) from the eye, but distance vision improves within the first month. Infants also exhibit depth perception and appear to have some color vision . Similarly, infants can detect speech sounds shortly after birth and can locate the origin of sounds in the environment, as is smell and taste. Within a few days following birth, breast-fed babies can differentiate their own mother's milk from that of another mother, and also prefer odors that adults like and respond more negatively to the types of odors adults do not like.

Perception

views updated May 21 2018

Perception

Perception is the quality of being aware of the conditions in one's environment. For example, visual perception refers to the ability of an organism to see objects in the world around it. Other forms of perception involve the senses of touch, smell, taste, and sound.

Perception is not a passive activity. That is, one way to think of visual perception is to say that light rays bounce off an object an enter one's eyes. Those light rays then create a "sight" message that travels to the person's brain where the object is recorded. But that explanation is incomplete. The brain also acts on the message received from the eyes in ways that are not totally understood by scientists. The important point is that the real world is not necessarily the world that one perceives.

A classic experiment illustrates this point. A group of young boys are all asked to look carefully at a nickel. The nickel is then taken away, and the boys are asked to draw a picture of the nickel. Boys in this experiment who come from wealthy families tend to draw the nickel smaller in size than boys from poor families. Each boy's perception of the nickel is affected to some extent by how important a nickel is in his life.

The biology of perception

All systems of perception have a common structure. They consist of cells designed specifically to detect some aspect of the surrounding environment, a series of neurons (nerve cells) that transmit the perceived information to the brain, and a specific segment of the brain for receiving and analyzing that information.

Receptor cells can be classified according to the kind of stimuli to which they respond. Chemoreceptors, for example, are cells that detect certain kinds of chemical substances. Receptor cells in the nose and mouth are examples of chemoreceptors. Photoreceptors are another kind of receptor cell. Photoreceptors detect the presence of light. Mechanoreceptors detect changes in mechanical energy, changes that occur during touch and hearing and in maintaining the body's equilibrium (balance).

Messages received by any kind of receptor cell are passed through a network of neurons into the spine and on to the brain. There, messages are received and analyzed. Visual messages are analyzed in the visual cortex, for example, and messages from the ears in the auditory cortex.

Puzzling questions

Even with our fairly complete understanding of the biology of perception, some intriguing questions remain. Those questions cannot, as yet, be answered strictly in terms of the physical make-up of an organism's body. One of these questions has to deal with constancy. The term constancy refers to the fact that our perception of objects tends to remain the same despite real changes that occur in their image on the retina of the eye.

For example, suppose that you walk down a street looking at the tallest building on the street. As you approach the building, its image on the retina of your eye gets larger and larger. Certain proportions of the building change also. Yet, your brain does not interpret these changes as real changes in the building itself. It continues to "see" the building as the same size and shape no matter how close or how far you are from it.

Another perception puzzle involves the perception of motion. The mystery lies in how perceived movement cannot be accounted for by the movement of an object's image across the retina. If that were so, movement of the observer, or even eye movement, would lead to perceived object movement. For example, when riding a bike, the rest of the world would be perceived as moving.

Depth perception. One of the puzzles that has interested scientists for centuries is depth perception. Depth perception refers to the fact that our eyes see the world in three dimensions, the way it is actually laid out. The problem is that images that enter the eye strike the retina, an essentially flat surface, at the back of the eye. How can a flat image on the retina be "read" by the brain as a three-dimensional image?

An important element in the answer to this puzzle appears to be binocular vision. The term binocular vision refers to the fact that humans and most other organisms detect visual signals with two eyes. The two eyes, set slightly apart from each other, receive two slightly different images of the environment. By methods that are still not entirely understood, the brain is able to combine those two images to produce a binocular version of an image, a three-dimensional view of the environment.

[See also Ear; Eye; Smell; Taste; Touch ]

UXL Encyclopedia of Science

Perception

views updated May 21 2018

309. Perception

See also 9. ALERTNESS ; 198. HEARING ; 397. TOUCH ; 405. UNDERSTANDING .

Berkeleianism, Berkeleyanism: the system of philosophical idealism developed by George Berkeley (1685?-1753), especially his tenet that the physical world does not have an independent reality but exists as a perception of the divine mind and the flnite mind of man. Also Berkeleyism. —Berkeleian, Berkeleyan, n., adj.
chromesthesia: Medicine. the association of imaginary sensations of color with actual perceptions of hearing, taste, or smell. Also called photism, color hearing . Cf. synesthesia.
coenesthesia, coenesthesis, cenesthesia, cenesthesis: the combination of organic sensations that comprise an individual’s awareness of bodily existence. —coenesthetic, cenesthetic, adj.
dysesthesia: an impaired condition of any of the senses.
kinesthesia: Medicine. the sense by which movement, weight, position, etc. are perceived. —kinesthetic, adj.
oxygeusia: extreme acuteness or sensitivity of the sense of taste.
oxyopia, oxyopy: an extremely heightened acuteness of the eyesight, resulting from increased sensibility of the retina.
oxyosphresia: heightened acuteness of the sense of smell.
panesthesia, panaesthesia: the total or collective experience of all sensations or all the senses. —panesthetic, panaesthetic, adj.
paresthesia, paraesthesia: any abnormal physical sensation, as itching, a tickling feeling, etc. —paresthetic, paraesthetic, adj.
phantasm: a vision or other perception of something that has no physical or objective reality, as a ghost or other supernatural apparition. Also phantasma . See also 218. IMAGES ; 312. PHILOSOPHY .
phonism: a sound or a sensation of hearing produced by stimulus of another sense, as taste, smell, etc.
photism: chromesthesia.
sensorium: the sensory apparatus of the body as a whole; the seat of physical sensation, imagined to be in the gray matter of the brain.
synesthesia, synaesthesia: Medicine. a secondary sensation accompanying an actual perception, as the perceiving of sound as a color or the sensation of being touched in a place at some distance from the actual place of touching. Cf. chromesthesia. —synesthetic, synaesthetic, adj.
telesthesia, telaesthesia: a form of extrasensory perception, working over a distance and enabling the so gifted observer to perceive events, objects, etc., far away. —telesthetic, telaesthetic, adj.

-Ologies and -Isms

perception

views updated May 29 2018

per·cep·tion / pərˈsepshən/ • n. the ability to see, hear, or become aware of something through the senses: the normal limits to human perception. ∎ the state of being or process of becoming aware of something in such a way: the perception of pain. ∎ a way of regarding, understanding, or interpreting something; a mental impression: Hollywood's perception of the tastes of the American public | we need to challenge many popular perceptions of old age. ∎ intuitive understanding and insight: “He wouldn't have accepted,” said my mother with unusual perception. ∎ Psychol. & Zool. the neurophysiological processes, including memory, by which an organism becomes aware of and interprets external stimuli.DERIVATIVES: per·cep·tion·al / -shənl; -shnəl/ adj.

The Oxford Pocket Dictionary of Current English

perception

views updated May 14 2018

perception The faculty of acquiring sensory experience. Study of the processes by which we gather and interpret visual information is largely the province of social psychologists, who have identified several general principles (‘laws’) of perception, and also some effects upon it of (among other things) motivation and attention. The former includes the phenomenon of the ‘figure-ground contrast’; that is, how we perceive objects distinctly from their surroundings. This can be studied via so-called projective tests. ‘Constancy’ is also a principle of perception; that is, objects maintain perceptual stability through transformations of various types, such as alterations in size and proportion. The most systematic attempt to study the organization of perceptual phenomena is probably that of the Gestalt (‘form’, ‘figure’, or ‘holistic’) psychologists, who emphasize the role of innate patterning in visual perception, although behaviourist approaches have also been influential, notably in America. See also ATTITUDES; ATTRIBUTION THEORY; COGNITIVE THEORY.

A Dictionary of Sociology GORDON MARSHALL