Functional neuroimaging of memory: implications for cognitive aging.

Our understanding of the ways in which changes in specific neural systems mediate adult age differences in memory is rapidly increasing, due in no small part to the advent of functional neuroimaging techniques. This article reviews age-related changes in memory performance obtained with behavioral measures, describes models of the neural mechanisms of memory, and derives predictions from these models regarding age-related changes in brain activation patterns. The neuroimaging findings obtained to date support models emphasizing the role of prefrontal cortex in age-related changes in memory functioning, especially for episodic memory retrieval. In general, neural activation associated with episodic memory encoding is regionally similar for younger and older adults but relatively lower in magnitude for older adults. During retrieval, activation that is restricted to the right prefrontal cortex for younger adults is more likely to be bilateral for older adults. Prefrontal activation exhibits an age-related increase when working memory tasks require simple storage and an age-related decrease when working memory requires higher-level executive processes. Although the evidence is limited, behavioral performance and activation patterns appear to be similar among younger and older adults on tests of semantic (context-independent) and implicit memory. We conclude that several methodological issues, such as defining the relation between brain structure and function, and determining the relationship between performance and activation, are particularly important for understanding age-related changes. Future directions for aging research include further investigation of the relation between encoding and retrieval and the identification of both spared and impaired neural systems.