Title : Involvement of the occipito-temporal pathway in delayed reaching: Data from a case of optic ataxia
Two functionally specialized cortical pathways of visual processing have been hypothesized: a dorsal (occipito-parietal) action-related and a ventral (occipito-temporal) perception-related pathway (Milner and Goodale, 1993, 2008). However, the ventral pathway would also contribute to actions directed to remembered target positions (Milner et al. 2003). In this theoretical framework, the present experiment aimed to study the performance of a patient, G.P., with bilateral lesion of the dorsal pathway. G.P. (33 years-old, right-handed woman) exhibited optic ataxia, bilateral concentricvisual field defect and neglect for the proximal and inferior hemispaces. MRI showed a bilateral lesion involving occipital lobes with extension to the adjacent posterior parietal lobes. Initially, G.P. performed a visual reaching task, in which she was asked to reach a target under visual control. Subsequently, G.P. and eight healthy controls performed a delayed reaching task. In this task, G.P. and controls saw the target, closed their eyes, and then, when the retention interval (2 s) had elapsed, reached the remembered target position. The target could be localized in the left or right hemispace, and the participants used their right or left hand. In visual reaching task, G.P. undershot visual targets in all conditions. In the delayed reaching task, G.P.’s distance errors differed from those of controls in all conditions, since she overshot whereas controls undershot target positions. The data of the present study are consistent with the hypothesis that the ventral pathway is involved in reaching of memorized target locations. We suggest that the damage of the dorsal pathway, associated with integrity of the ventral, produced a forward shifting of attention, as shown by G.P.’s proximal neglect. In turns, the forward attentional shifting, displacing in the same direction memorized target locations, produced G.P.’s overshot errors in delayed reaching.