Saccade accuracy is maintained by adaptive mechanisms that continually modify saccade amplitude to reduce dysmetria. Previous studies suggest that adaptation occurs upstream of the caudal fastigial nucleus (CFN), the output of the oculomotor cerebellar vermis but downstream from the superior colliculus (SC). The nucleus reticularis tegmenti pontis (NRTP) is a major source of afferents to both the oculomotor vermis and the CFN and in turn receives direct input from the SC. Here we examine the activity of NRTP neurons in four rhesus monkeys during behaviorally induced changes in saccade amplitude to assess whether their discharge might reveal adaptation mechanisms that mediate changes in saccade amplitude. During amplitude decrease adaptation (average, 22%), the gradual reduction of saccade amplitude was accompanied by an increase in the number of spikes in the burst of 19/34 neurons (56%) and no change for 15 neurons (44%). For the neurons that increased their discharge, the additional spikes were added at the beginning of the saccadic burst and adaptation also delayed the peak-firing rate in some neurons. Moreover, after amplitude reduction, the movement fields changed shape in all 15 open field neurons tested. Our data show that saccadic amplitude reduction affects the number of spikes in the burst of more than half of NRTP neurons tested, primarily by increasing burst duration not frequency. Therefore adaptive changes in saccade amplitude are reflected already at a major input to the oculomotor cerebellum.
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