Categoryspecific Visual Agnosia

For some patients with associative agnosia, recognition is not equally impaired for all categories of objects. Category-specific visual agnosia (CSVA) is a deficit in which the boundary between impaired and intact recognition can be approximately defined along the semantic criterion of biological vs nonbiological objects. In other words, these patients can show severely impaired visual recognition of objects from biological categories while recognition of most other categories is largely spared. For example, a patient may be able to recognize all manners of tools or artifacts but show marked difficulties in recognizing even the most common fruits or vegetables. CSVA is believed to be a semantic disorder, in which patients have problems associating the view of an object, in a specific category, with stored knowledge ofits identity. The mechanisms underlying visual perception do not appear to have access to the semantic knowledge of certain categories of objects.

It has also been proposed that this dissociation may be the result of the recognition of living things depending on some specialized neural mechanisms that are not needed for the recognition of nonliving things. Evidence for this derives from the findings that CSVA for biological objects usually follows inferiortemporal damage. Moreover, recent studies have found that defective recognition of persons was associated with damage to right temporal regions, defective recognition of animals was associated with damage to the right mesial occipital/ventral temporal region and left mesial occipital region, and defective recognition of tools was associated with damage in the occipital-temporal-parietal junction of the left hemisphere. As we discuss later, these studies have helped to reveal the extent to which there is modular organization in the visual system.

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