When during horizontal saccades in monkey does cerebellar output affect movement?

The caudal part of the cerebellar fastigial nucleus (CFN) influences the horizontal component of saccades. Previous reports show that activity in the CFN contralateral to saccade direction aids saccade acceleration and that activity in the ipsilateral CFN aids saccade deceleration. Here we refine this description by characterizing how blocking CFN activity changes the distance that the eye rotates during each of 4 phases of saccades, the increasing and decreasing saccade acceleration (phases 1 and 2) and deceleration (3 and 4). We found that unilateral CFN inactivation increases total eye rotation to ∼1.8× normal. This resulted from rotation increases in all four phases of ipsiversive saccades. Rotation during phases 1 and 2 increases slightly, more during phase 3, and most during phase 4, to ∼4.4× normal. Thus, the ipsilateral CFN normally reduces eye rotation throughout a saccade but reduces it the most near saccade end. After unilateral CFN inactivation, rotation during contraversive saccades was ∼0.8× normal. This resulted from decreased rotation during phases 1-3, to ∼0.7× normal, and then normal rotation during phase 4. Thus the CFN contraversive to saccade direction normally increases eye rotation during acceleration and the first phase of deceleration. These data indicate that the influences of the CFNs on saccades overlap extensively and that there is a smooth shift from predominance of the contralateral CFN early in a saccade to the ipsilateral CFN later. The pathway from the CFN to contralateral IBNs and then to the abducens nucleus can account for these effects.