Discharge patterns of cerebellar output neurons in the caudal fastigial nucleus during head-free gaze shifts in primates.

Lesion studies in both human and non-human primates indicate that the cerebellum is important for accurate and stereotyped saccadic eye movements. Based on single-unit recordings and pharmacological inactivations in head-fixed monkeys, we suggested that the caudal fastigial nucleus (CFN) provides the brainstem saccade generator with a burst that helps accelerate contraversive saccades and decelerate ipsiversive ones. Here we examine this suggestion during head-free gaze shifts where there can be a 10-fold difference in saccade duration. First, the timing of the burst does not depend on whether the gaze shift has a head component. When a family of either ipsiversive or contraversive gaze shifts with a variety of saccadic durations is aligned on gaze onset, the high-frequency burst in the associated rasters occurs progressively later as saccade duration increases. Realignment of the same rasters with the end of the saccade reveals a tight timing of burst end with saccade end for all 10 CFN burst neurons studied. The delayed bursts for contraversive saccades were unexpected based on the early burst illustrated in the published head-fixed data. One hypothesis is that the late activity helps terminate contraversive as well as ipsiversive gaze shifts. An alternative explanation is that the late CFN burst could still be used as an excitatory drive to promote the late reacceleration or prolonged velocity plateau that is present during large gaze shifts.