DISSOCIATION BETWEEN 2 MODES OF SPATIAL PROCESSING BY A VISUAL FORM AGNOSIC

WE report a dissociation between two aspects of visuospatial processin g in a patient with a profound impairment in the visual perception of objects ('visual form agnosia'). The orientation-in-depth of a visual field ('visuaI pitch') was found to systematically influence the eleva tion at which she perceived her own eye level, just as it does in norm al individuals; but at the same time, she was unable to discriminate p erceptually the orientation-in-depth of the same visual field, a trivi al task for individuals with normal vision. These results suggest that , in the normal brain, the processes that integrate orientation inform ation from the visual field with extraretinal ifnormation abvout eye p osition are separable from those supporting the perception of the orie ntation of the visual field itself. The pattern of brain damage in D.F ., in conjnction with the reported dissociation, suggests that the for mer set of processes maps onto the stream of information flowing from primary visual cortex to the posterior parietal cortex, the so-called dorsal stream, whereas the latter involves the projections from primar y visual cortex to the inferotemporal cortex, the so-called ventral st ream.