THE PERCEPTION OF SPATIAL RELATIONS IN A PATIENT WITH VISUAL FORM AGNOSIA

Abstract

It is generally believed that neuropsychological patients presenting with visual agnosia, a deficit on object perception/recognition, have suffered damage to the ventral visual cortical pathway (Milner & Goodale, 1995). Rarely has the ability of such patients to perceive the spatial location of objects been investigated-perhaps because “spatial vision” is thought by some researchers to be mediated exclusively by the dorsal visual cortical pathway. Here we present data on spatial perception in a patient DF, who has a profound visual form agnosia. DF and two control subjects were required to make a copy of the spatial arrangement of a target display of five differently coloured circular tokens using a duplicate set of the same tokens. Spatial performance was analysed in two ways: (1) relative location measured the ability to reconstruct the relative spatial relations between the tokens such as left versus right, above versus below, and nearer versus farther; (2) absolute location measured the exact displacement in millimetres of each token's copied position relative to its true location. DF was able to copy some of relative location relations between the tokens although her abilityto do so was not nearly as accurate as that of the control subjects. Nevertheless, DF's limited appreciation of relative location was enough to enable her to discriminate rather well between spatial patterns of tokens. She could not, however, reconstruct the absolute distance relations between the tokens and showed large displacements of token position compared to the control subjects. Interestingly, although Df was not “normal” in her ability to appreciate the allocentric spatial relations between the locations of the tokens relative to one another, she could accurately process token location egocentrically (i.e. relative to her own body and hand position). Thus, like controls, she was perfectly able to point to and touch all the tokens in an array. These results demonstrate deficits in the ability to perceive spatial relations between objects in a patient with visual form agnosia and suggest that the ventral steam also plays a functional role in spatial vision, particularly allocentric spatial vision.