A paradoxical improvement of misreaching in optic ataxia: new evidence fortwo separate neural systems for visual localization

We tested a patient (A. T) with bilateral brain damage to the parietal lobe s, whose resulting 'optic ataxia' causes her to make large pointing errors when asked to locate single light emitting diodes presented in her visual f ield. We report here that, unlike normal individuals, A. T's pointing accur acy improved when she was required to wait for 5s before responding. This c ounter-intuitive result is interpreted as reflecting the very brief time-sc ale on which visuomotor control systems in the superior parietal lobe opera te. When an immediate response was required, A. T's damaged visuomotor syst em caused her to make large errors; but when a delay was required, a differ ent, more flexible, visuospatial coding system-presumably relatively intact in her brain-came into play, resulting in much more accurate responses. Th e data are consistent with a dual processing theory whereby motor responses made directly to visual stimuli are guided by a dedicated system in the su perior parietal and premotor cortices, while responses to remembered stimul i depend on perceptual processing and may thus crucially involve processing within the temporal neocortex.