| Visual search is a dynamic process whereby we explore the environment in a purposeful manner to achieve specific goals. The present studies investigated the overlap and separability of the neural networks that subserve visual search in younger and older adults. Specifically, the functional engagement of these networks during the preparation stage of search was examined by utilizing an event-related fMRI paradigm that separated the maintenance, cueing, and execution periods. Neural activations associated with working memory-dependent planning, oculomotor programming of search, and spatially-directed search were contrasts of interest. Behavioral results in younger adults indicated that search task reaction times were most efficient when subjects had additional top-down guidance based on working memory representations of possible target locations (spatially-directed search). Functional imaging results supported the theory that there is a mosaic of areas that allows the brain to maintain a spatial map of relevant item locations and to program a strategy for making these locations the targets of attentional behaviors. The frontal eye fields seemed to be a pivotal area demonstrating overlap of these processes.; Normal aging detrimentally affects visual search behavior by increasing search time and decreasing accuracy. A second aim of this dissertation was to investigate how the patterns of activation during cueing might differ in an older adult population. Older adults had slower reaction times, decreased accuracy, and did not demonstrate a behavioral benefit of directed cueing of spatial attention to speed search. Thus, interpretation of functional data in older subjects was limited. In general, activations were less circumscribed than in young adults and were observed outside of the anticipated canonical networks.; In conclusion, by focusing analyses on the preparation period, we were able to match perceptual input and the preceding working memory requirements in a situation where no overt search was required. The manipulation of spatial information in working memory, the programming of visual search, and the planning of a spatially-informed search displayed overlapping and unique activations. This research illustrated the dynamic recruitment of neural network components as the brain prepared to allocate attention in an efficient and task-specific manner. |
