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Abstract:
Semantic memory deterioration is a major component of the
cognitive decline seen in patients with Dementia of the Alzheimers
Type (DAT). However, the exact nature of this deficit remains
unclear. Some research data support a procedural deficit, while
other data point to a degraded semantic store. Additional
information can be obtained through the use of the N400 component
of the Event Related Potentials (ERPs). In the present study, ERPs
were recorded from 10 young control subjects, 10 elderly control
subjects and 10 DAT patients in a picture-semantic matching task.
Stimuli were presented sequentially as prime-target pairs with one
half of the targets
matching
the primes via semantic relationships and the other half
mismatching the prime. In the young and elderly control subjects,
ERPs generated a larger N400 for unrelated than related target
pictures, with longer latencies and largest activity over the right
centroparietal region for the elderly group. These results indicate
that the time required to access a representation in memory is
delayed with increasing age and the scalp distribution could be
related to the neural changes in brain structure accompanying
normal aging. The amplitude of the N400 was significantly reduced
in DAT due to a diminution of the negativity for the unrelated
target pictures. These data seem to indicate that the semantic
activation created by the context is no used efficiently in
processing stimuli, which affects access to specific concepts and
gradually leads to a breakdown in the structure and organization of
semantic memory in DAT. Reasearch partially supported by DGAPA
(IN1994), UNAM.
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