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Abstract:
Neuropsychological studies of patients with acquired semantic
impairments have yielded two distinct and contrasting patterns of
performance in a spoken-word/picture matching task (Warrington
& Cipolotti, 1996, Brain). "Access/refractory" patients are
strongly influenced by presentation rate, semantic relatedness of
distractors, and repetition, but are relatively unaffected by
lexical frequency. "Degraded-store" patients, by contrast, are
strongly affected by lexical frequency but not by presentation
rate, semantic relatedness, or repetition. Our account of these
patterns of performance is based on the role of language processes
in maintaining and integrating semantic information over time in
the face of synaptic depression---the tendency of cortical neurons
to exhibit a response decrement under repeated stimulation. We
present a connectionist network which learns to compensate for
synaptic depression in posterior semantic areas by building up
strong connections between these and more frontal regions of the
network responsible for semantic maintenance. Damage to these
frontal regions leaves semantic representations vulnerable to
synaptic depression, producing the pattern of data associated with
semantic "access/refractory" impairments. Damage to the semantic
representations themselves, on the other hand, spares the ability
to maintain residual semantic information, producing the pattern of
data associated with "degraded-store" semantic impairments. The
instantiation of synaptic depression in the model also accounts for
the occurrence of semantic satiation in normal subjects.
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