More than (where the target) meets the eyes: disrupted visuomotor transformations in optic ataxia

Visually guided reaching entails multiple coordinate frame transformations between retina-centered target location and body-centered limb location. Reaching errors in optic ataxia (OA) may be caused by disruptions to these transformations. Consistent with this proposal, previous studies report that reaching errors in OA depend primarily on the location of a target relative to the patient's gaze regardless of its location relative to the head or body midline. We attempted to replicate this finding by testing KE, a patient with OA following bilateral parietal and left premotor lesions (as well as significant non-specific white matter disease) on a reaching task that varied the orientation of his head and torso while holding the gaze-relative position of the target constant (always foveated). In contrast to previous reports, we observed that rotating the head or body away from the midline led to decreased reaching accuracy. Further analyses showed that multiple visuomotor transformation steps might have been disrupted in KE. These results suggest that gaze-relative target position is not the sole determinant of reaching errors in all OA patients.