COGNITIVE BRAIN ACTIVITY IN ALZHEIMERS-DISEASE - ELECTROPHYSIOLOGICALRESPONSE DURING PICTURE SEMANTIC CATEGORIZATION

Semantic memory deterioration is a major component of the cognitive de cline seen in patients with dementia of the Alzheimer's type (DAT); ho wever, the exact nature of this deficit remains unclear. Some research data support a procedural deficit where there is an inability to acce ss or retrieve the contents of semantic memory, while other data point to a degraded semantic store where the actual content of semantic mem ory is degraded. Additional information about semantic processing in D AT can be obtained through the use of an event-related potential (ERP) component known as N400. In the present study, ERPs were recorded fro m 10 young control participants, 10 elderly control participants, and 10 DAT patients in a picture-semantic matching task. Stimuli were pres ented sequentially as prime-target pairs, with one-half of the targets matching the primes via semantic relationships (e.g., piano-violin) a nd the other half mismatching the prime (e.g., helmet-violin). The tas k was to discriminate between semantically related and unrelated pairs of pictures. In the young and elderly control groups, ERPs generated a lager N400 for unrelated than related target pictures, with a maximu m amplitude around 380 ms in the young group and around 480 ms in the elderly group, The amplitude of the N400 was significantly reduced in the DAT patients. However, a separate analysis of congruent and incong ruent ERPs trials revealed significant differences only with the incon gruent trials. The amplitude of incongruent recordings was larger for the elderly control group than for the DAT patients, while the amplitu de for congruent recordings was similar in both groups. These findings are consistent with the neuropathological evidence that Alzheimer's d isease is a neocortical disconnection syndrome in which there is a los s of structural and functional integrity of long corticocortical tract s. The semantic activation created by the context is not used efficien tly in processing stimuli, which affects access to specific concepts a nd gradually leads to a breakdown in the structure and organization of semantic memory.