Impaired grasping in a patient with optic ataxia: primary visuomotor deficit or secondary consequence of misreaching?
Optic ataxia is defined as a spatial impairment of visually guided reaching, but it is typically accompanied by other visuomotor difficulties, notably a failure to scale the handgrip appropriately while reaching to grasp an object. This impaired grasping might reflect a primary visuomotor deficit, or it might be a secondary effect arising from the spatial uncertainty associated with poor reaching. To distinguish between these possibilities, we used a new paradigm to tease apart the proximal and distal components of prehension movements. In the "far" condition objects were placed 30 cm from the hand so that subjects had to make a reaching movement to grasp them, whereas in the "close" condition objects were placed adjacent to the hand, thereby removing the need for a reaching movement. Stimulus eccentricity was held constant. We tested a patient with optic ataxia (M.H.), whose misreaching affects only his right hand within the right visual hemifield. M.H. showed a clear impairment in grip scaling, but only when using his right hand to grasp objects in the right visual hemifield. Critically, this grip-scaling impairment was absent in M.H. in the "close" condition. These data suggest that M.H.'s grip scaling is impaired as a secondary consequence of making inaccurate reaching movements, and not because of any intrinsic visuomotor impairment of grasping. We suggest that primary misgrasping is not a core symptom of the optic ataxia syndrome, and that patients will show a primary deficit only when their lesion extends anteriorly within the intraparietal sulcus to include area aIPS.