Dissociation between two modes of spatial processing by a visual form agnosic

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Abstract
We report a dissociation between two aspects of visuospatial processing in a patient with a profound impairment in the visual perception of objects ('visual form agnosia'). The orientation-in-depth of a visual field ('visual pitch') was found to systematically influence the elevation at which she perceived her own eye level, just as it does in normal individuals; but at the same time, she was unable to discriminate perceptually the orientation-in-depth of the same visual field, a trivial task for individuals with normal vision. These results suggest that, in the normal brain, the processes that integrate orientation information from the visual field with extraretinal information about eye position are separable from those supporting the perception of the orientation of the visual field itself. The pattern of brain damage in D.F., in conjunction with the reported dissociation, suggests that the former 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 primary visual cortex to the inferotemporal cortex, the so-called ventral stream.

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