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Abstract:
Abstract: Cognitive deficits in Alzheimer's Disease (AD)
include semantic memory impairment. Semantic processing involves
the left temporal lobe, and neuroimaging and anatomical studies
suggest temporal damage in AD. Functional neuroimaging studies
suggest reduced temporal activation, compensated for by activation
peripheral to regions recruited by healthy adults or recruitment of
contralateral hemisphere homologs. Such activation could reflect
either altered semantic representation or task demands. Our fMRI
study examined cortical activation during semantic processing of
nouns, including animals, with a pseudoword baseline. Ten AD
patients and 16 controls judged the "pleasantness" of written words
presented every four seconds in 40 second word-category blocks, a
minimally-demanding task ensuring a common cognitive process for
all semantic categories. AD patients showed left parietal
activation (BA40) in the combined noun conditions; controls showed
left superior and middle temporal activation (BA22, 21, 37). In the
animals condition, AD patients showed left parietal (BA40) and
right superior temporal (BA38) activation; controls showed left
superior temporal (BA39) and occipital (BA18) activation. These
results suggest that AD patients recruit regions adjacent to
"normal" regions plus right hemisphere regions, providing a neural
account for AD patients' reported difficulty with nouns,
particularly natural kinds. Prefrontal activation observed in other
AD activation studies may reflect executive processes or task
difficulty rather than cortical reallocation of semantic
representation.
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