Localization (Brain Function)
Localization (brain function)
Refers to the concept that different areas of the brain control different aspects of behavior.
Theories of localization first gained scientific credence in the 1860s with Paul Broca's discovery that damage to a specific part of the brain—the left frontal lobe—was associated with speech impairment. Other discoveries followed: in 1874, Carl Wernicke identified the part of the brain responsible for receptive speech (the upper rear part of the left temporal lobe, known as Wernicke's area), and in 1870 Gustav Fritsch and J. L. Hitzig found that stimulating different parts of the cerebral cortex produced movement in different areas of the body. By the beginning of the twentieth century, detailed maps were available showing the functions of the different areas of the brain.
Not all researchers have agreed with theories of localization, however. An influential conflicting view is the equipotential theory, which asserts that all areas of the brain are equally active in overall mental functioning. According to this theory, the effects of damage to the brain are determined by the extent rather than the location of the damage. Early exponents of this view—including Goldstein and Lashley—believed that basic motor and sensory functions are localized, but that higher mental functions are not. There is still controversy between adherents of the localization and equipotential theories of brain function. Some experts advocate a combination of the two theories, while others search for new alternatives, such as that proposed by J. Hughlings Jackson in 1973. Jackson claimed that the most basic skills were localized but that most complex mental functions combined these so extensively that the whole brain was actually involved in most types of behavior.
Further Reading
Corballis, Michael C. The Lopsided Ape: Evolution of the Generative Mind. New York: Oxford University Press, 1991.
Edwards, Betty. Drawing on the Right Side of the Brain. Los Angeles: J. P. Tarcher, 1979.
Hampden-Turner, Charles. Maps of the Mind. New York: Collier Books, 1981.
Localization (Sensory)
Localization (sensory)
The ability of animals and humans to determine the origin of a sensory input.
One of the highly developed abilities that humans and other animals possess is the ability to determine where a sensory input originates.
The capacity to localize a sound, for example, depends on two general mechanisms. The first is relevant for low frequency (i.e., low pitch) sounds and involves the fact that sound coming from a given source arrives at our ears at slightly different times. The second mechanisms applies to high frequency (i.e., high pitch) sounds; if such a sound comes from one side, one ear hears it more loudly than the other and we can detect location based on differences in the loudness of the sound at each ear.
Low frequency sounds that come from the noisemaking source will enter the nearer ear first; these sound waves will then bend around our head and arrive at the far ear a short time later. If the sound is almost directly in front of us, the sound arrives at one ear an extremely short time ahead of its arrival at the other ear. Humans can detect differences of perhaps 10 millionths of a second in arrival time. If the sound comes from the side, the difference in time of arrival at the two ears is longer. In either case, our brain executes quick computations to inform us about the location of the sound. Other animals, like nocturnal owls, have shown greater sensitivity to differences in time of arrival.
The second mechanism involves intensity differences in sound waves traveling to the ears. High frequency sound waves do not bend around the head like low frequency waves. Instead, high frequency sound waves tend to reflect off the surface of the head. As a consequence, a sound coming from one side of the head will show greater intensity in one ear; that is, it will be slightly louder in one ear. The brain uses this intensity difference to tell us where a sound originates.
In general, we locate sounds below about 1500 Hz (i.e., 1500 cycles per second) by analyzing differences in time of arrival at each ear; above 1500 Hz, we use intensity differences. Sounds that are right around 1500 Hz are hardest to localize. Further, we are likely to confuse sounds that are directly in front of us, above us, and behind us because their positions are such that we cannot use time of arrival and intensity differences.
Finally, sometimes we ignore the cues for sound localization if logic tells us that the sound should be coming from another direction. For example, when we listen to somebody on a stage, we may hear the sounds they produce from a loudspeaker that is above us. Nonetheless, we localize the sound as coming from the person on the stage because it seems more logical. Psychologists refer to this phenomenon as "visual capture."
Further Reading
Corballis, Michael C. The Lopsided Ape: Evolution of the Generative Mind. New York: Oxford University Press, 1991.
Hampden-Turner, Charles. Maps of the Mind. New York: Collier Books, 1981