Cerebellar fastigial nucleus influence on ipsilateral abducens activity during saccades

To characterize the cerebellar influence on neurons in the abducens (ABD) nucleus, we recorded ABD neurons before and after we inactivated the caudal part of the ipsilateral cerebellar fastigial nucleus (cFN) with muscimol injection. cFN activity influences the horizontal component of saccades. cFN inactivation increased the activity of most ipsilateral ABD neurons (19/22 in 2 monkeys) during ipsiversive (hypermetric) saccades, primarily by increasing burst duration. During contraversive (hypometric) saccades, the off-direction pause of most (10/15) ABD neurons was shorter than normal because of the early resumption of ABD activity. Early ABD firing caused the early contraction of antagonist muscles that reduced eye rotation and made contraversive saccades hypometric. Thus the cerebellum controls ipsilateral ABD activity by truncating on-direction bursts during ipsiversive saccades and extending off-direction pauses during contraversive saccades. We conclude that cFN output keeps saccades accurate by controlling when ABD on-direction bursts and off-direction pauses end.