Semantic systems or system? neuropsychological evidence re-examined

In this paper we consider whether our stored conceptual knowledge about stimuli is represented within a single semantic system which is indifferent to the modality of stimulus presentation, or whether conceptual knowledge is represented in different semantic systems according to either the modality of stimulus presentation or the nature of the concept (e.g. whether the concept specifies visual or verbal knowledge about an object). Previous work suggests three areas of neuropsychological research which are relevant to this issue: modality-specific aphasias, modality-specific priming on semantic access disorders, and modality-specific aspects of semantic memory disorders. Evidence from each of these areas is reviewed and we argue that there is only equivocal support for the multiple semantic systems position. We outline an alternative account which distinguishes between a single amodal semantic system and modality-specific perceptual recognition systems, and we discuss the evidence in light of this single semantic system account.