A dissociation of shape discrimination and figure-ground perception in a patient with normal acuity

We report the case of a 21-year-old man who sustained brain damage due to anoxia. His visual acuity was normal and his colour perception was adequate if not completely normal. However, his shape discrimination was so impaired that he had great difficulty distinguishing a square from an oblong or a large square from a smaller square. At the same time, other aspects of his shape perception were remarkably intact. His performance on a shape detection task was good. He had no difficulty on tests of figure-ground (boundary) segmentation and his perception of subjective and illusory contours appeared to be normal. S.M.K.'s performance on a number of tasks is compared with that of a patient (F.G.P.) previously reported who showed the opposite pattern of deficit (Kartsounis and Warrington, Neuropsychologia 29, 969-980, 1991). This evidence of a double dissociation between two components of shape perception is discussed in the context of a neurophysiological model of the organization of the visual system.