Brain activation during immediate and delayed reaching in optic ataxia

Patients with optic ataxia after lesions of the occipito-parietal cortex demonstrate gross deviations of movements to visual targets in their peripheral visual field. When the same patients point to remembered target locations their accuracy improves considerably. Taking into account opposite findings in a single patient suffering from visual form agnosia due to bilateral occipito-temporal lesions (D.F.), this paradoxical improvement was attributed to brain structures outside the dorsal stream, and supposed to be specifically associated with delayed movement execution. This conclusion was based on the still unverified assumption that the dorsal system is almost completely lacking any localization function in patients with optic ataxia who demonstrate the paradoxical delay effect. We thus investigated brain activity associated with immediately executed and delayed movements in a patient with optic ataxia due to extensive bilateral lesions (I.G.) and in 16 healthy subjects using functional magnetic resonance imaging. Our analysis revealed robust and indistinguishable activation of intact dorsal occipital and parietal areas adjacent to the patient's lesions for both types of movements. In healthy subjects, we found the same visuomotor network activated during immediate and delayed movements as well as additionally higher signal increases for movements to visible targets than for delayed movements in bilateral occipito-parietal and occipito-temporal areas. Our results suggest that in healthy subjects as well as in the optic ataxia patient I.G. dorsal areas are not only involved in immediate but also in delayed reaching. This observation questions the hypothesis that residual visuospatial abilities in patients with optic ataxia could only be mediated by a system outside of the dorsal stream.