Implicit memory in Alzheimer's disease

While patients with Huntington's disease (HD) and Korsakoff's syndrome (KS) demonstrate "preserved" performance on implicit word-stem completion tasks, patients with Alzheimer's disease (AD) are typically found to be impaired. It has been hypothesized that AD patients do poorly as a result of degenerative changes in posterior cortical association areas thought to mediate performance on this type of implicit memory task, and that the relative sparing of these areas in KS and HD results in their "preserved" performance. The present study was undertaken to examine the implicit memory performance of AD patients on this task after equating their explicit performance to that of normal controls by manipulating the number of encoding exposures. When this was accomplished, the implicit memory performance of AD patients was equivalent to that of controls. The results are discussed within the context of a critical review of the evidence for the existence of separable neural systems in mediating implicit and explicit memory.