Visual control of arm movement in Parkinson's disease

Patients with Parkinson's disease (PD) are more dependent on visual information during movements than normals. To investigate the mechanisms underlying deterioration of movement under nonvisual conditions, we studied two-dimensional pointing movements to randomly occurring targets. The experimental design allowed us to systematically manipulate visual feedback during the movement by removing vision of the target, of the moving hand, or of both. Execution of pointing movements in PD deviated most severely from that of normals when PD patients moved without vision of their own moving hand. Under this condition, undershooting of the target appeared, and movements were particularly slow. In contrast, with complete vision or when only vision of the target was occluded, pointing movements of PD patients were accurate and faster. PD patients had no difficulties selecting the correct movement direction. Reaction times were longer in PD patients irrespective of the availability of visual feedback. Our findings suggest that the ability of PD patients to use nonvisual feedback during execution of arm movements is impaired.