Hand and target positions in multiple reference frames affect reach errors in unilateral parietal damaged patients

*A. Z. KHAN1,2, L. PISELLA2, G. BLOHM1, Y. ROSSETTI2, J. D. CRAWFORD1
1Ctr Vision Res, York Univ, Toronto, ON, CANADA, 2Espace et Action, INSERM 534, Bron, FRANCE.

Recent neurophysiological studies suggest that the posterior parietal cortex (PPC) encodes both the reach goal and initial hand position in gaze-centered coordinates. We tested how varying gaze (36° left to 36° right in 12° intervals), reach target (12° left, 0° and 12° right) and initial hand position (24° left, 0° and 24° right) affected reach errors in two left unilateral optic ataxia patients with different degrees of right parietal damage, along with seven controls.
Reaching errors from all subjects showed an influence of the reach target position in gaze-centered coordinates. In contrast with the controls, both patients’ mean errors across all gaze positions were offset toward the left and they showed greater errors when the reach target was in their left visual field, consistent with the damage to the right parietal cortex. Control subjects also showed a small quasi-independent shoulder-centered effect due to the reach target position. This effect was reduced in patient CF, who had widespread damage to the parietal cortex. In contrast, for patient OK, who had more focal parietal damage, the shoulder-centered effect was larger and interacted to a greater degree with the gaze-centered effect of the reach target position.
Errors from all three groups of subjects revealed an effect of initial hand position on their reach errors in shoulder-centered coordinates. Compared to the controls, both patients showed larger influences due to initial hand position. Reaching errors did not appear to depend on initial hand position in gaze centered coordinates in an independent fashion (i.e. whether it was in the left or right visual field), but both patients showed an interaction effect between the shoulder-centered and gaze-centered initial hand positions.
Taken together, these results suggest that in the normally functioning brain, the target and hand are compared at more than one level in the visuomotor pathway in multiple reference frames and these comparisons are then optimally integrated. With different degrees of damage to the parietal cortex, these comparisons are disrupted changing the relative influence of hand and target position in different reference frames on the final reaching movement.