Shape Integration for Visual Object Recognition and Its Implication in Category-Specific Visual Agnosia

A series of experiments was conducted on a patient (ELM) with bilateral inferior temporal lobe damage and category-specific visual agnosia in order to specify the nature of his functional impairment. In Experiment 1, ELM performed a task of picture/word matching that used line drawings of fruits and vegetables as stimuli. The pattern of confusions exhibited by the patient suggested a failure in processing the full range of shape features necessary for the unique specification of the target relative to other structurally related items. This hypothesis of a shape integration impairment was tested and verified by subsequent visual recognition experiments (Experiments 2-4), which used synthetic stimuli with shapes precisely defined on the dimensions of elongation, curvature, and tapering. Furthermore, it was determined (Experiment 5) that the integration deficit is specific to the retrieval of shape knowledge from memory and does not affect the encoding of the properties of visual stimuli. It is argued that these findings have critical implications for cognitive theories of visual object recognition and for an interpretation of the visual function of the inferior temporal cortex. Finally, it was shown that the patient's deficit for structural knowledge integration is modulated by the semantic properties of the objects (Experiment 6), thereby demonstrating the applicability of the present findings to an explanation of category-specific visual agnosia.