Muscimol Inactivation of the Caudal Fastigial Nucleus and Posterior Interposed Nucleus

27 Previously, we showed that neurons in the SupraOculomotor area (SOA), known to 28 encode vergence angle in normal monkeys, encode the horizontal eye misalignment in 29 strabismic monkeys. The SOA receives afferent projections from the caudal fastigial 30 nucleus (cFN) and posterior interposed nucleus (PIN) in the cerebellum. The objectives 31 of the current study were to investigate the potential roles of the cFN and PIN in (1) 32 conjugate eye movements and (2) binocular eye alignment in strabismic monkeys. We 33 used unilateral injections of the GabaA agonist muscimol to reversibly inactivate the cFN 34 (4 injections in exotropic monkey S1 with ~4° of exotropia; 5 injections in esotropic 35 monkey S2 with ~34° of esotropia) and the PIN (3 injections in S1). cFN inactivation 36 induced horizontal saccade dysmetria in all experiments (mean 39% increase in 37 ipsilesional saccade gain and 26% decrease in contralesional gain). Also, mean 38 contralesional smooth-pursuit gain was decreased by 31%. cFN inactivation induced a 39 divergent change in eye alignment in both monkeys with exotropia increasing by an 40 average of 9.8° in S1 and esotropia decreasing by an average of 11.2° in S2 (p<0.001). 41 Unilateral PIN inactivation in S1 resulted in a mean increase in the gain of upward 42 saccades by 13% and also induced a convergent change in eye alignment, reducing 43 exotropia by an average of 2.7° (p<0.001). We conclude that cFN/PIN influences on 44 conjugate eye movements in strabismic monkeys are similar to that postulated in 45 normal monkeys and cFN/PIN play important and complementary roles in maintaining 46 the steady-state misalignment in strabismus. 47