Discharge properties of saccade-related neurons in the primate fastigial oculomotor region.

To clarify the mechanisms by which the cerebellar fastigial oculomotor region (FOR) contributes to the control of saccadic eye movements, we recorded saccade-related FOR units in alert monkeys that made horizontal saccades between neighboring points of a three-by-three grid of target positions (16 degrees amplitude). As in previous studies, FOR units exhibited saccade-related bursts that occurred earlier for contralateral than for ipsilateral saccades. In addition, many FOR units reflected variations in the kinematic profiles of the saccades by exhibiting bursts with earlier onset and shorter peak latencies and higher peak discharge rates for fast as compared with slow saccades of the same amplitude. Moreover, reflecting systematic differences in saccade velocity rather than an influence of eye position itself, FOR bursts showed subtle but recurrent and, at the population level, statistically significant differences between centripetal and centrifugal saccades that closely paralleled the eye position dependency of saccadic dysmetria seen after FOR lesions. We conclude that the FOR output signal is not, as previously proposed, specifically related to the temporal properties of the saccade, but also contains information about saccade velocity. Moreover, the FOR output signal appears to change systematically depending on the actual kinematic properties of the saccade, in a way that would help to maintain saccadic accuracy.