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 th e accuracy of visuo-motor tracking was studied in 6 patients with cere bellar syndromes and 6 healthy subjects. Subjects made extension and f lexion movements of the wrist to superimpose a cursor displaying their actual movement (movement cursor) upon one indicating the target (tar get cursor) on a VDU screen. The target trajectory consisted of a sawt ooth pattern of slow (4 deg/sec) ramp extension and instantaneous flex ion return phases. Following practice, the tracking of cerebellar pati ents was significantly less accurate than that of healthy subjects for each phase (P = 0.02). Temporary suppression of the movement cursor d uring 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 e rrors in the patients whereas suppression of the target cursor did not alter their performance. Suppression of neither cursor altered the tr acking accuracy of healthy subjects during the ramp extensions whilst suppression of either caused reduced (P = 0.02) performance for the re versal phase. We interpret the increased dependence of patients upon v isual information of their movements during slow trajectories as indic ating an impairment of proprioceptive guidance.