Semantic memory: Theory and neuropsychological evidence

This thesis is concerned with the organization of semantic memory. Potential organizing principles include organization by input modality, information type, taxonomic category and output task. The thesis examines whether any current neuropsychological model of semantic memory can account for data relevant to each of these issues simultaneously. The issue of organization by input modality was investigated through a study of 10 dementia patients. The study examined whether superior access to semantic knowledge from pictures is a necessary consequence of damage to semantic representations, as predicted by some theories. No evidence for this hypothesis was found. The relationship between organization by input modality, information type and output task was explored using reaction-time studies of normal subjects. The effect of input modality on the time taken to make semantic decisions was found to interact with the type of information sought. Furthermore, the relationship between these variables was modified by the style of stimulus presentation. However, there was no interaction between input modality and the type of decision to be made when the style of presentation and the type of information sought were held constant. Finally, the possibility that category-specific deficits are an emergent effect of organization by information type was investigated. A comparison between the performance of a patient with a deficit for having things and the performance of a computational model which only distinguishes between different modalities of information (Farah and McClelland 1991) did not produce any evidence to support this hypothesis. Two main conclusions were drawn. The first was that no current theory can account for the data relevant to each of these issues simultaneously. A new model was therefore proposed. The second was that it may be possible for different tasks to produce different patterns of performance, even if the information required to perform each task is ostensibly the same.