INCREASED DEPENDENCE UPON VISUAL INFORMATION OF MOVEMENT PERFORMANCE DURING VISUO-MOTOR TRACKING IN CEREBELLAR DISORDERS

The effect of temporarily suppressing the visual display of either the target (desired) trajectory or the actual movement trajectory upon the accuracy of visuo-motor tracking was studied in 6 patients with cerebellar syndromes and 6 healthy subjects. Subjects made extension and flexion movements of the wrist to superimpose a cursor displaying their actual movement (movement cursor) upon one indicating the target (target cursor) on a VDU screen. The target trajectory consisted of a sawtooth pattern of slow (4 deg/sec) ramp extension and instantaneous flexion return phases. Following practice, the tracking of cerebellar patients was significantly less accurate than that of healthy subjects for each phase (P = 0.02). Temporary suppression of the movement cursor during both the mid-section of the ramp phase (P = 0.05) and around the reversal phase (P = 0.04) caused a significant increase in tracking errors in the patients whereas suppression of the target cursor did not alter their performance. Suppression of neither cursor altered the tracking accuracy of healthy subjects during the ramp extensions whilst suppression of either caused reduced (P = 0.02) performance for the reversal phase. We interpret the increased dependence of patients upon visual information of their movements during slow trajectories as indicating an impairment of proprioceptive guidance.