Abstract: Semantic dementia (SD) is a syndrome marked by progressive and profound deterioration of semantic knowledge, consequent upon bilateral atrophy of the antero-lateral temporal lobes. This deficit is rarely characterized by significant category- or modality-specific effects: the great majority of SD patients are impaired for all semantic categories and all varieties of both input to and output from the semantic system, suggesting disruption to a unitary and amodal system of conceptual knowledge. Such patients do, however, show interesting differences in the kinds of errors they make in different semantic domains. We have attempted to explain these differences with reference to a computational model in which central semantic representations emerge in a system that must learn the mappings among structured perceptual representations in different modalities. Domain effects arise as a consequence of differences in the degree to which objects in a given domain share structure in different modalities. We will consider the implications of this idea for theories of category-specific deficits, and will propose an extension of the unitary-semantics model that may account for different patterns of impairment in generalised semantic dementia vs category-specific cases.
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