Effects of fastigial stimulation upon visually-directed saccades in macaque monkeys

When the initial eye position was changed by stimulating the fastigial nucleus prior to visually-directed saccades, monkeys could not compensate for the stimulation-induced movement. The line of sight missed the target location by a distance and direction almost equal to the vector of the evoked saccade. When the stimulation was delivered 75-130 ms after the target presentation, saccades were triggered prematurely. Their initial movement reflected only the evoked saccade in some response and reflected the vector sum of the evoked and visually-directed saccades in the other. In contrast, when stimulus latencies were greater than 130 ms. saccades started toward the target location. The visuomotor processing for saccades seemed to be completed during this period, which is approximately half the latency of normal saccades. When the stimulation was applied while the eyes were already in motion, the trajectories of the saccades were strongly modified and terminated in deviated locations. These results indicate that cerebellar output impulses are projected downstream to saccade-programming circuits where visual information has already been converted into motor-command signals.