Characteristics of saccadic dysmetria in monkeys during reversible lesions of medial cerebellar nuclei

Abstract

1. The accuracy of saccadic eye movements made by trained Cebus monkeys was studied during reversible lesions produced by cooling through probes implanted between the interpositus and fastigial nuclei (medial probe) or lateral to the dentate nucleus (lateral probe). 2. Cooling through the lateral probe did not impair the accuracy of vertical or horizontal saccades, However, cooling through the medial probe produced a dysmetria whose magnitude was dependent on the position of the eye and on the direction of the saccade. 3. The amplitude/duration relation of dysmetria saccades was not significantly different from that of normal saccades. 4. The trajectories of the horizontal and vertical components of oblique saccades remained essentially straight during medial probe cooling in spite of unequal dysmetria in the two components. This suggests that the mechanism that produces a dysmetria in one component must interact with the gaze center that determines the duration of the other component. 5. Cerebellar nuclear cooling through either lateral of medial probes did not alter the saccadic reaction time to a randomly timed step change in target position. This result differs from that found for limb movements where cerebellar dysmetria was associated with increased reaction times. 6. These results provide evidence that the cerebellum through the medial nuclei normally plays a role in terminating, but not in initiating, saccades.