in neural representation. Further research is needed to clarify the relationships between particular kinds and durations of life experience and particular changes in the brain and thus to clarify which aspects of "experience" might be protective against cognitive decline.
The intellectual creations of human culture also leave imprints on the brain. For example, letters and digits are culturally created symbols that can be used to represent the same concepts (e.g., "four" and "4"). Yet letter and digit recognition depend on different neural regions in literate subjects (Polk and Farah, 1998). Handwriting can be selectively impaired by brain damage that does not affect other sensory-motor functions of the hand (Alexander et al., 1992). And bilinguals show regional segregation of their different languages (Ojemann and Whitaker, 1978), illustrating that the products of specific cultures can sometimes be recognized in the brain.
There is evidence that the aging brain continues to change. For example, dendritic growth increases, possibly in compensation for cell loss (Kolb and Whishaw, 1998). New research presents evidence of neurogenesis in adulthood: new neurons continually appear in areas of the adult primate brain associated with higher cognitive functions (Gould et al., 1999b). The role of life experiences in such processes has hardly begun to be investigated.
These lines of evidence that experience changes the brain in lasting ways support the idea that many outcomes of brain development from infancy through old age are the expression of experiential-cultural factors and suggest that simple reductionistic and deterministic models in which cognitive capabilities flow exclusively from brain development are inappropriate. They suggest a program of research to examine in detail the proposition that although evolution-based brain development lays the basic foundation of brain architecture, subsequent differentiation and development of the brain is importantly influenced by how societies are organized and by how individuals live their lives. This research program would aim to clarify the relationships between particular kinds and durations of life experience and particular changes in the brain.
Older people often continue to perform well the cognitive tasks of living despite declines in some of the underlying cognitive capabilities. For cognitive tasks that require new learning or that depend on speed of responding, performance diminishes with age (Burke and MacKay, 1997); such performance is also related to the ability to enact certain tasks of everyday life, such as such as paying bills or filling out tax forms (Diehl, 1998; Willis and Marsiske, 1991). However, performance on laboratory tasks that measure cognitive processes does not map perfectly onto performance in many important life domains, such as performance in the workplace and the exercise of