We studied the effects on smooth pursuit eye movements of ablation of the dorsal cerebellar vermis (lesions centered on lobules VI and VII) in three monkeys in which the cerebellar nuclei were spared. Following the lesion the latencies to pursuit initiation were unchanged. Monkeys showed a small decrease (up to 15%) in gain during triangular-wave tracking. More striking were changes in the dynamic properties of pursuit as determined in the open-loop period (the 1st 100 ms) of smooth tracking. Changes included a decrease in peak eye acceleration (e.g., in one monkey from ~650°/s2, prelesion to ~220380°/s2, postlesion) and a decrease in the velocity at the end of the open-loop period [e.g., in another monkey from a gain (eye velocity/target velocity at 100 ms of tracking) of 0.93, prelesion to 0.53, postlesion]. In individual monkeys, the pattern of deficits in the open-loop period of pursuit was usually comparable to that of saccades, especially when comparing the changes in the acceleration of pursuit to the changes in the velocity of saccades. These findings support the hypothesis that saccades and the open-loop period of pursuit are controlled by the cerebellar vermis in an analogous way. Saccades could be generated by eye velocity commands to bring the eyes to a certain position and pursuit by eye acceleration commands to bring the eyes toward a certain velocity. On the other hand, changes in gain during triangular-wave tracking did not correlate with either the saccade or the open-loop pursuit deficits, implying different contributions of the oculomotor vermis to the open loop and to the sustained portions of pursuit tracking. Finally, in a pursuit adaptation paradigm (X0.5 or X2, calling for a halving or doubling of eye velocity, respectively) intact animals could adaptively adjust eye acceleration in the open-loop period. The main pattern of change was a decrease in peak acceleration for X0.5 training and an increase in the duration of peak acceleration for X2 training. Following the lesion in the oculomotor vermis, this adaptive capability was impaired. In conclusion, as for saccades, the oculomotor vermis plays a critical role both in the immediate on-line and in the short-term adaptive control of pursuit.
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