Spatial deficits in visuomotor control along the body midline in a patient with optic ataxia

Abstract

Patients with optic ataxia resulting from damage to the posterior parietal cortex display gross pointing deficits to targets in peripheral vision. Although performance in central vision is better when compared with peripheral vision, it may be possible to detect subtle deficits in movement kinematics. We analyzed visually guided pointing movements in patient ME, who suffered a right superior parietal lesion leading to optic ataxia. ME and a group of elderly controls made pointing movements with both hands to targets that varied in size on a trial-to-trial basis (i.e., a speed-accuracy trade-off task). Two axes of movement direction were contrasted, with subjects pointing to targets in a right/left sequence perpendicular to the body midline or in a near/far sequence aligned with the body midline. While ME spent more time than controls decelerating prior to contacting targets for all directions of movement with each hand, her pattern of performance differed for each axis of movement. For right-left movements of either hand she showed a similar pattern to controls: in short, all participants spent more time decelerating prior to target contact for movements made towards contralateral space (e.g., right hand pointing leftwards). In contrast, for near-far movements, ME spent much more time decelerating for both near and far movements of either hand while controls showed an asymmetry in performance such that more time was spent decelerating for near than for far movements of either hand. In addition, ME spent more time overall in the deceleration phase of her movements made with left hand and in general, movements made along the near-far axis had longer deceleration phases than movements made along the right-left axis. For our patient then, optic ataxia has produced deficits of visually guided movement control throughout space and surprisingly, more so in near-far movements than in right-left movements.