Neurocognitive Aging of Memory and Executive Processes

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Working memory consists of storage mechanisms that hold information on-line for brief intervals and executive processes that can manipulate the stored contents in service of higher cognitive demands. Working memory declines dramatically with age, however the cognitive and neural mechanisms underlying this decline have yet to be identified. We propose a program of research that combines behavioral, functional, and structural neuroimaging analyses to investigate age differences in working memory storage and executive functions. Comparisons of good and poor performers in each age group will be used to distinguish between a continuum of neurocognitive processes, with continuity across the lifespan, versus qualitative changes unique to aging. The working memory circuitry includes several parietal and frontal regions, with left hemisphere prominence for verbal working memory and right hemisphere prominence for spatial working memory. Evidence from positron emission tomography studies indicates that aging (1) alters the laterality of regions recruited by verbal and spatial working memory tasks, and (2) leads to the recruitment of prefrontal sites under conditions that do not necessarily engage these regions in the younger brain. The proposed research has 2 major goals. First, we want to identify the functional significance of the laterality and prefrontal activation differences between younger and older adults. We will use event-related fMRI to identify the cognitive operations associated with these age differences comparing verbal and spatial working memory tasks that emphasize storage. These studies constitute Aim 1 and provide the foundation for investigating executive processes in Aims 2-3. Considerable evidence indicates that executive processes and the frontal regions that mediate them are particularly vulnerable to aging. Therefore, our second major goal is to examine age effects on two executive processes in working memory: (1) the coding of temporal context and (2) the resolution of interference from competing events. For each process our goals are to identify the underlying neural mechanisms, to determine whether the executive mechanisms contribute to encoding, maintenance, and retrieval processes, to determine the extent to which these mechanisms differ for verbal and spatial materials, and to identify whether aging exerts selective effects on these mechanisms that are compensatory in nature or lead to impaired cognitive performance. By obtaining structural measurements of brain regions implicated in working memory, we intend to determine the contribution of age-related atrophy to cognitive performance and to alterations in the neural substrates as revealed through fMRI.