PLASTICITY

For at least a century, the term plasticity has been used in a variety of circumstances pertaining to the scholarly study of human development. Although varying in certain conceptual aspects and practical applications, the fundamental meaning of the term may be found in its numerous appearances across several developmental literatures and historical decades. In its most enduring and generalized sense, plasticity refers to the capability of, or susceptibility to, being molded, shaped, modified, or otherwise changed. As such, the concept of plasticity has occupied important positions in theories of ontogenetic development, phylogenetic evolution, neuronal development and adaptation, and psychological aging. This article focuses on the latter two areas of scholarship.

The concept of plasticity

The term plasticity first appeared in the psychological and biological literature over a century ago. For example, in the 1901 Dictionary of Philosophy and Psychology, James Mark Baldwin and E. B. Poulton defined plasticity as "that property of living substance or of an organism whereby it alters its form under changed conditions of life" (Baldwin and Poulton, p. 302). Several well-developed characteristics of human plasticity were annotated. These included the following still-relevant points: (1) seemingly fixed organic structures may exhibit some plasticity; (2) the plasticity of the brain and nervous system may allow "newer. . . intelligent accommodations" (p. 303); (3) there are limits to plasticity in certain realms; and (4) plasticity underlies much motor and cognitive learning.

In the intervening century, plasticity has been a featured element of several perspectives on human functioning and change. Two of the broadest reviews of plasticity were published in the 1980s. In an edited volume entitled Developmental Plasticity (1981), E. S. Gollin invited scholars to discuss plasticity in the domains of biological and psychological development. Gollin's own contribution referred to plasticity in terms of the range of possible variations that can occur throughout individual development. R. M. Lerner's 1984 monograph, On the Nature of Human Plasticity, covered plasticity as it functions in development at the biological, neurological, psychological, and social levels of analysis. Here, plasticity referred to changes in either structure or function, and could occur throughout life.

Plasticity at the neuronal level

If the Gollin and Lerner volumes reflect watershed reviews of broad applications of plasticity, Bryan Kolb's Brain Plasticity and Behavior (1995) served a similar function for the rapidly expanding study of neurological plasticity. According to Kolb, ideas that had germinated a century before, and that had been propelled by D. O. Hebb (1949) at midcentury, had now become a core principle of the neurosciences. Specifically, contemporary technology and accumulating evidence had confirmed that a variety of brain structures can grow or otherwise be modified as a function of experience and in response to behavioral demands. Moreover, this growth can occur in adult brain structures that have otherwise completed the early-life neurological development phase. Indeed, the number of structural changes associated with experience is continuing to increase: Notable changes include "increases in brain size, cortical thickness, neuron size, dendritic branching, spine density, synapses per neuron, and glial numbers" (Kolb and Whishaw, 1998, p. 47).

In addition, changes in the human brain may have a beneficial effect at the behavioral level. Brain plasticity is thus linked to a variety of compensatory and other recovery mechanisms at the brain and behavioral levels—given a manageable degree of damage resulting from normal aging, an injury, or neuropathogenic disease, one or more compensatory or restitutive mechanisms operating at the neuroanatomical level may function to reduce the associated behavioral deficit. From this perspective, much basic and applied research in cognitive neurorehabilitation has examined such themes as spontaneous recovery, functional reorganization, enriched environment, and cognitive training. In a 1999 article, Robertson and Murre provided a comprehensive review of how the concept of cerebral plasticity may be used to generate guided rehabilitative processes for recovery from brain damage. Among the notable predictors of successful recovery are severity of original injury (lesion) and age at which the lesion is sustained. Briefly, the greater the severity and the older the age of onset, the lower the probability of recovery of function. All other things being equal, older adults may have more difficulty recovering function from cerebral damage than younger adults.

Plasticity in human aging

Plasticity is a principal theoretical issue in life-span developmental psychology. A rationale for its relevance to life-span theory was offered by Baltes, Staudinger, and Lindenberger (1999, p. 480), who concluded that an emphasis on plasticity "highlights the search for the potentialities of development, including the upper and lower boundary conditions. Implied in the idea of plasticity is that any given developmental outcome is but one of numerous possible outcomes, and that the search for the conditions and range of plasticity. . .is fundamental to the study of development." Implications of these ideas are especially pertinent to the study of decremental processes in aging, for they suggest that the decline may be reversed if enhancing experience (e.g., training, practice) is provided.

Accordingly, plasticity has been identified as a core theoretical issue in the study of adult cognitive development. Plasticity implies that some normal aging-related cognitive decline may be reversible. Indeed, research providing experience-enhancing interventions to older adults has produced results linking specific experience to particular behaviors and skills, ranging from intelligence and memory to leisure or professional expertise. Older adults who are provided with task-related experience (e.g., practice, strategies) in some domains may maintain or develop higher levels of performance than would have otherwise seemed possible. Phenomena of plasticity are relevant to theories of cognitive aging. Similarly, empirical evidence for plasticity provides fertile soil from which everyday interventions for normal aging-related losses may be generated.

Plasticity is a concept of considerable importance to understanding how human beings change as they become older—and how they could change given certain experiences. Applied to both biological and psychological aging, it offers a cautiously optimistic perspective, for it illustrates that there is both a neurological and psychological basis for guided interventions designed to enhance adaptation in late life.

Roger A. Dixon Anna-Lisa Cohen Stuart W. S. MacDonald

See also Life Span Development; Memory Training; Neurotransmitters.

BIBLIOGRAPHY

Baldwin, J. M. Dictionary of Philosophy and Psychology. New York: The Macmillan Company, 1901.

Baltes, P., B. "Theoretical Propositions of Life-Span Developmental Psychology: On the Dynamics Between Growth and Decline." Developmental Psychology 23 (1987): 611–628.

Baltes, P. B.; Staudinger, U. M.; and Lindenberger, U. "Lifespan Psychology: Theory and Application to Intellectual Functioning." Annual Review of Psychology 50 (1999): 471–507.

Dixon, R. A., and BÄckman, L. "Concepts of Compensation: Integrated, Differentiated, and Janus-Faced." In Compensating for Psychological Deficits and Declines: Managing Losses and Promoting Gains. Edited by R. A. Dixon and L. Bäckman. Mahwah, N.J.: Lawrence Erlbaum Associates, 1995. Pages 3–19.

Dixon, R. A., and Hertzog, C. "Theoretical Issues in Cognition and Aging." In Perspectives on Cognitive Change in Adulthood and Aging. Edited by F. Blanchard Fields and T. M Hess. New York: McGraw-Hill, 1996. Pages 25–65.

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Gollin, E. S. Developmental Plasticity: Behavioral and Biological Aspects of Variations in Development. New York: Academic Press, 1981.

Hebb, D. O. The Organization of Behavior. New York: Wiley, 1949.

Kolb, B. Brain Plasticity and Behavior. Mahwah, N.J.: Lawrence Erlbaum Associates, 1995. Kolb, B., and WHISHAW, I. Q. "Brain Plasticity and Behavior." Annual Review of Psychology 49 (1998): 43–64.

Lerner, R. M. On the Nature of Human Plasticity. Cambridge, U.K.: Cambridge University Press, 1984.

Luria, A. R. Restoration of Function After Brain Injury. New York: Macmillan, 1963.

Robertson, I. H., and Murre, J. M. J. "Rehabilitation of Brain Damage: Brain Plasticity and Principles of Guided Recovery." Psychological Bulletin 125 (1999): 544–575.

Stuss, D. T.; Winocur, G.; and Robertson, I. H., eds. Cognitive Neurorehabilitation. Cambridge, U.K.: Cambridge University Press, 1999.

Verhaeghen, P.; Marcoen, A.; and Goosens, L. "Improving Memory Performance in the Aged Through Mnemonic Training: A Meta-Analytic Study." Psychology and Aging 7 (1992): 242–251.