Saccade dysmetria during functional perturbation of the caudal fastigial nucleus in the monkey

The caudal fastigial nucleus (cFN) is the output nucleus by which the medioposterior cerebellum influences the brainstem saccade generator. In the monkey, inactivation of one cFN by local injection of muscimol impairs all saccades: ipsiversive saccades become hypermetric, contraversive saccades become hypometric, and saccades aimed at a target located in the upper or lower visual fields are biased horizontally toward the injected side. The pharmacological action of muscimol does not allow deficits that are presaccadic to be distinguished from those occurring during saccade execution. To determine the interval during which altered cFN activity affects saccade accuracy, we applied low-frequency electrical microstimulation (100 Hz for 100-300 ms) to the cFN of three monkeys while they were making saccades toward a flashed target. Similar to the effect of muscimol injection in cFN, low-frequency microstimulation biased all saccades toward the ipsilateral side. When the microstimulation was applied after target flash and before saccade onset, the ipsilateral bias was absent. However, when the stimulation was applied during the ongoing movement, the saccade trajectory was biased toward the stimulated side. The muscimol-like effect of the microstimulation suggests that the stimulation inhibits cFN activity, possibly by recruiting the inhibitory afferents from the cerebellar vermis (axons of Purkinje cells). Low-frequency microstimulation had to be applied during the saccade to bias its trajectory. These data suggest that the ipsilateral horizontal bias observed during muscimol inactivation results from an imbalance in the intrasaccadic activity between the two caudal fastigial nuclei.