EFFECTS OF LESIONS OF THE OCULOMOTOR VERMIS ON EYE-MOVEMENTS IN PRIMATE - SACCADES

We studied the effects on saccades of ablation of the dorsal cerebella r vermis (lesions centered on lobules VI and Vn) in three monkeys in w hich the deep cerebellar nuclei were spared. One animal, with a symmet rical lesion, showed bilateral hypometric horizontal saccades. Two ani mals, with asymmetrical lesions, showed hypometric ipsilateral saccade s, and saccades to vertically positioned targets were misdirected, usu ally deviating away from the side to which horizontal saccades were hy pometric. Postlesion, all animals showed an increase (2- to 5-fold) in trial-to-trial variability of saccade amplitude. They also showed a c hange in the ratio of the amplitudes of centripetal to centrifugal sac cades (orbital-position effect); usually centrifugal saccades became s maller. In the two animals with asymmetrical lesions, for saccades in the hypometric direction, latencies were markedly increased (up to sim ilar to 500 ms). There was also an absence of express and anticipatory saccades in the hypometric direction. When overall saccade latency wa s increased, centrifugal saccades became relatively more delayed than centripetal saccades. The dynamic characteristics of saccades were aff ected to some extent in all monkeys with changes in peak velocity, eye acceleration, and especially eye deceleration. There was relatively l ittle effect of orbital position on saccade dynamics, however, with th e exception of one animal that showed an orbital position effect for e ye acceleration. In a double-step adaptation paradigm, animals showed an impaired ability to adaptively adjust saccade amplitude, though inc reased amplitude variability postlesion may have played a role in this deficit. During a single training session, however, the latency to co rrective saccsdes-which had been increased postlesion-gradually decrea sed and so enabled the animal to reach the final position of the targe t more quickly. Overall, both in the early postlesion period and durin g recovery, changes in saccade amplitude and latency tended to vary to gether but not with changes in saccade dynamics or adaptive capability , both of which behaved relatively independently. These findings sugge st that the cerebellum can adjust saccade amplitude and saccade dynami cs independently. Our results implicate the cerebellar vermis directly in every aspect of the on-line control of saccades: initiation (laten cy), accuracy (amplitude and direction), and dynamics (velocity and ac celeration) and also in the acquisition of adaptive ocular motor behav ior.