Categories of knowledge: further fractionations and an attempted integration

In this study we investigated the category specificity of the comprehension impairments of Y.O.T., a patient with a severe global dysphasia. Using matching to sample techniques it was possible to demonstrate selective impairments and selective preservations not only of broad categories of semantic knowledge but also of particular subsets of such categories. Specifically, Y.O.T.'s comprehension of 'objects' was, in general, significantly more impaired than for foods or living things. Within the broad class of objects she was significantly more impaired in the comprehension of small manipulable objects than large man-made objects. Within her proper noun vocabulary there was a significant dissociation between her good comprehension of proper nouns having a unique and well-known referent (e.g., Churchill) and common proper nouns without such a referent (e.g., Jones). Her error responses were not consistent, semantic similarity and significant rate effects were observed, and it was therefore considered that her category specific comprehension deficits were primarily ones of access to a full semantic representation. We attempt to give a principled account of the increasing number of seemingly arbitrary instances of fine-grain categorical impairments of semantic knowledge. We have suggested that different weighting values from multiple sensory channels will be important in the acquisition of different categories of knowledge and that such differential weightings could be the basis of the categorical organization of systems in the brain subserving semantic knowledge.