Cortical control of visually guided reaching: evidence from patients with optic ataxia.
The dorsal stream of visual information processing connecting V1 to the parietal cortex is thought to provide a fast control of visually guided reaching. Important for this assumption was the observation that in both the monkey and the human, parietal lesions may provoke disturbance of visually goal-directed hand movements. In the human, severe misreaching termed 'optic ataxia' has been ascribed to lesions of the superior parietal lobule (SPL) and/or the intraparietal sulcus. Using new tools for lesion analysis, here we re-evaluated this view investigating the typical lesion location in a large group of unilateral stroke patients with optic ataxia, collected over a time period of 15 years. We found no evidence for the assumption that disruption of visually guided reaching in humans is associated with a lesion typically centering on the SPL on the convexity. In both left and right hemispheres, we found optic ataxia associated with a lesion overlap that affected the lateral cortical convexity at the occipito-parietal junction, i.e. the junction between the inferior parietal lobule (IPL) and superior occipital cortex and--in the left hemisphere even more posteriorly--also the junction between occipital cortex and the SPL. Via the underlying parietal white matter, the lesion overlap extended in both hemispheres to the medial cortical aspect, where it affected the precuneus close to the occipito-parietal junction. These lateral and medial structures seem to be integral to the fast control of visually guided reaching in humans.