Visual apperceptive agnosia: a clinico-anatomical study of three cases.

The visuo-perceptual abilities of three cases with unilateral lesions of the right hemisphere selected on the basis of the co-occurrence of impaired performance on a test of visual object perception and normal performance on a test of shape discrimination are reported. Their performance was also impaired on other tests of visual object recognition in which the perceptual difficulty of the task was manipulated by obscuring the salient features of the representation. At the same time it was found that on a variety of other tests of visual-sensory processing their performance was entirely normal, as was their performance on tests of visual semantic knowledge, tests that were failed by a patient with a visual associative agnosia. It is argued that this syndrome has all the hallmarks of an apperceptive agnosia, a failure of perceptual categorisation in which the physical identity of the object is specified. The two discontinuities between visual-sensory processing, perceptual categorisation and visual-semantic processing are discussed in terms of a 2 categorical stage model of object recognition.