The Interaction of Object Form and Object M eaning in the Identification Performance of a Patient with Category-specific Visual Agnosia

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Abstract
Category-specific visual agnosia following bilateral inferior temporal lobe stroke was investigated in the patient ELM. Experiment 1 verified that computer-generated blobs could not be identified when members of a set varied along a single but not along multiple shape dimensions. Experiments 2 through 6 showed that for both ELM and, to a much lesser degree, healthy participants, this dimensionality effect was modulated by semantics. By pairing the exact same shapes with semantically close vs. disparate sounds or labels, the role of an object's semantics in category-specific agnosia was assessed independently from object form. For single-dimension shape sets, the semantic proximity of the concepts associated with the shapes had no impact on ELM's identification performance. For multidimensional shape sets, ELM's error rates showed a strong positive correlation with semantic proximity (r= .84, P < .01). These results were interpreted using an exemplar model of categorisation in w hich a deficit in exemplar node specificity is assumed. It is concluded that biological objects are more likely than nonbiological objects to have the combination of semantic proximity and shared values along multiple shape dimensions that pose recognition problems for patients with such specificity deficits.