Priming is a form of implicit memory, and refers to the fact that the mere processing of an item can facilitate subsequent processing of that same item. Priming effects can occur in the absence of conscious recollection of the prior study episode, and it has been suggested that priming and explicit memory are mediated by different memory systems. Evidence from memory disordered patients is critical for this hypothesis, since if it is possible to link the breakdown of explicit memory and priming to damage to different brain regions this would strengthen the independent memory systems hypothesis. Many studies have found normal priming effects in amnesic patients and some investigators have reported impaired priming in patients with Alzheimer's disease. However, many difficulties are associated with evaluating priming effects in memory impaired subjects. The priming measures may be less sensitive than explicit memory measures, and the lack of group differences on priming measures may simply reflect low measurement sensitivity. The degree of priming obtained in a given task is related to processing efficiency (i.e., baseline performance) with less efficient processing resulting in greater priming. Therefore, if patients have even mild processing deficits this may mask priming impairments. The proposed project consists of a series of experiments designed to manipulate baseline performance and sensitivity of the priming measures. Normal control subjects across the adult age range and three groups of patients will be investigated; patients with probable Alzheimer's disease, patients with Huntington's disease and amnestic patients. These patient groups differ in the severity of their explicit memory deficits and in the extent to which different component processes contributing to their baseline performance are compromised. In addition to the behavioral measures, measures of volume loss in specific brain structures will be obtained from MRI. These brain measures will be related to the behavioral indices, using a multiple regression approach, to determine the specific role of each brain region in baseline processing speed, explicit memory, and strength of priming. Specific hypotheses about the relationship between the behavioral and structural brain measures derived from the investigators' previous studies will be tested.
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