F. Ostroskysolis et al., COGNITIVE BRAIN ACTIVITY IN ALZHEIMERS-DISEASE - ELECTROPHYSIOLOGICALRESPONSE DURING PICTURE SEMANTIC CATEGORIZATION, Journal of the International Neuropsychological Society, 4(5), 1998, pp. 415-425
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.