A neurological dissociation between preserved visual and impaired spatial processing in mental imagery.

Studies on primates have shown that visual and spatial perceptual analysis depends on two separate neural pathways, associated with the processing of "what" and "where" an object is (visual characteristics and spatial coordinates, respectively). Similar dichotomies have been proposed by cognitive psychologists (e.g., the contrast between visual and spatial processing in working memory) and by neuropsychologists (e.g., the distinction between topographic agnosia and amnesia). In this paper we report the case of a patient with a severe spatial disorientation whose perceptual processing of visual and spatial information was normal, but in imagery tasks she had a dissociation between preserved visual and impaired spatial processing. While her ability to represent objects visually was intact, she failed in any task requiring mental rotation, recall of spatial position or execution of spatially based imagery operations. The case clearly demonstrates that visual and spatial imagery are functionally independent processes which must rely on different underlying neural systems. This pattern of impairment also explains the associated topographical amnesia as an inability to integrate spatial information in a mental map.