Visual form agnosia: Neural mechanisms and anatomical foundations

Visual form agnosia is a severe disorder of visual recognition resulting from extrastriate lesions in occipital and temporal cortex. Despite preserved visual acuity and minor visual field defects, patients with visual form agnosia are severely impaired in shape and form discrimination. The perception of colour, motion and stereoscopic depth is relatively unimpaired. This review presents four case reports of visual form agnosia attributable to a common aetiology (carbon monoxide poisoning) showing remarkable homogeneity. The basic deficit found in these patients is a failure to group single elements of a composite visual scene into a 'Gestalt', and to segregate figure from ground in static visual displays. Evidence from single-cell and lesion studies in non-human primates shows that extrastriate cortex is essential for these tasks. Correlations between anatomy and physiology further suggest that mechanisms of figure-ground segregation based on static occlusion cues are located in regions of area V2, which contribute to the form processing pathway that leads to area V4 and to the inferotemporal cortex. These findings lead to the hypothesis that visual form agnosia, as described in the present review, can be caused by selective damage to the visual form pathway.