INTEGRATING SEMANTIC INFORMATION DESPITE SYNAPTIC DEPRESSION: A CONNECTIONIST ACCOUNT OF "ACCESS" AND "DEGRADED-STORE" SEMANTIC IMPAIRMENTS

David C. Plaut and Stephen J. Gotts
Department of Psychology, Carnegie Mellon University, and the Center for the Neural Basis of Cognition, Pittsburgh, PA.


Click here to view an electronic version of the poster.


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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This page last updated 22 April 1999.