Effects of lesions of the oculomotor cerebellar vermis on eye movements inprimate: Smooth pursuit

We studied the effects on smooth pursuit eye movements of ablation of the d orsal cerebellar vermis (lesions centered on lobules VI and VII) in three m onkeys in which the cerebellar nuclei were spared. Following the lesion the latencies to pursuit initiation were unchanged. Monkeys showed a small dec rease (up to 15%) in gain during triangular-wave tracking. More striking we re changes in the dynamic properties of pursuit as determined in the open-l oop period (the Ist 100 ms) of smooth tracking. Changes included a decrease in peak eye acceleration (e.g., in one monkey from similar to 650 degrees/ s(2), prelesion to similar to 220-380 degrees/s(2), postlesion) and a decre ase in the velocity at the end of the open-loop period [e.g., in another mo nkey from a gain (eye velocity/target velocity at 100 Ins 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 p ursuit to the changes in the velocity of saccades. These findings support t he hypothesis that saccades and the open-loop period of pursuit are control led by the cerebellar vermis in an analogous way. Saccades could be generat ed by eye velocity commands to bring the eyes to a certain position and pur suit by eye acceleration commands to bring the eyes toward a certain veloci ty, On the other hand, changes in gain during triangular-wave tracking did not correlate with either the saccade or the open-loop pursuit deficits, im plying different contributions of the oculomotor vermis to the open loop an d to the sustained portions of pursuit tracking. Finally, in a pursuit adap tation paradigm (X0.5 or X2, calling for a halving or doubling of eye veloc ity, 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 acce leration for X2 training. Following the lesion in the oculomotor vermis, th is adaptive capability was impaired. In conclusion, as for saccades, the oc ulomotor vermis plays a critical role both in the immediate on-line acid in the short-term adaptive control of pursuit.