AGING AND IMPLICIT MEMORY--EVIDENCE FROM LESION STUDIES

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A five-year program of research is proposed that aims to (1) further characterize the functional neural architecture of implicit memory for words and objects by linking specific implicit memory processes to specific neural systems through studies of patients with focal neurological damage, and (2) examine how processing in these systems changes as a function of time and progressive multi-focal disease through studies of normal aged and patients with Alzheimer's disease (AD). Recent findings with focal lesion and AD patients indicate that there may be separable neural systems that mediate three forms of implicit memory and that there may be two visual implicit memory systems. In Study 1a, repetition priming tests, thought to selectively invoke processing in these implicit memory systems, will be administered to patients with single brain lesions in circumscribed regions of either the occipital, temporal, parietal, or frontal neocortex. In patients who exhibit informative dissociations, an MRI protocol will be used that, through three-dimensional reconstruction and rendering analyses, will permit specification of the lesions in terms of gyral anatomy and Brodmann areas. In Study 1b, patients with focal occipital lobe lesions will be used to test the hypotheses that the right occipital lobe mediates and apperceptive kind of visual implicit memory that retains form-specific information about words and pictures and the left occipital lobe mediates an associative kind of visual implicit memory that retains categorical information about words and pictures. In Study 2, a longitudinal study of implicit memory in AD will be conducted. For this study, groups of AD patients and normal aged subjects will be selected from a single, well- characterized cohort to test the broad hypothesis that the implicit processing that occurs in dissociable memory systems will have differential rates of change across of the course of AD.