Cerebellar control of saccadic eye movements: its neural mechanisms and pathways

Microstimulation studies on monkeys have shown that the cerebellar cortex which is related to saccadic function is located in lobules VIc and VII of the vermis. This vermal area is designated as the oculomotor vermis and characterized by low thresholds (less than 10 microA) and by saccade-related neuronal activity. The saccade evoked by the vermal stimulation has been shown to be the result of activation of Purkinje-cell axons. On the other hand, an anterograde WGA-HRP transport study has indicated that the Purkinje-cell axons of the oculomotor vermis terminate almost exclusively in a fatigial region which is designated as the fastigial oculomotor region (FOR). Microstimulation of the oculomotor vermis and the ventromedial aspect of the FOR results in saccades which differ in their horizontal directions, with vermal stimulation resulting in ipsilateral and fastigial stimulation resulting in contralateral saccades. Since the ipsilateral saccades evoked from the caudal part of the FN were suppressed by bicuculline, they were the results of stimulation of the Purkinje axons. It has been also shown that stimulation of the oculomotor vermis causes inhibition of FOR neurons. Furthermore, fastigial neurons bursting with saccades can be recorded only within the anatomical confines of the FOR. These data are consistent with the concept that signals from the vermis are transmitted to the saccadic nuclei of the brainstem via the FOR. Neurons in the FOR have been shown to project to various saccade-related nuclei, including the riMLF and PPRF. Some neurons in the FOR have divergent axon collaterals which terminate in both the vertical and horizontal preoculomotor nuclei. When the initial eye position is changed by stimulating the FN prior to visually-guided saccades, monkeys cannot compensate for the stimulation-induced movement. When the stimulation is delived 75-130 ms after the target presentation, saccades are triggered prematurely. The visuomotor processing for saccades seems to be completed during this period, which is approximately half the latency of normal saccades. When saccades were triggered prematurely at an early stage of information processing, the eyes moved first in the direction of evoked saccade and then changed the direction toward the location of the target without any intervening period. The retinal error information sampled before the stimulation was not disturbed by the cerebellar stimulation. These observations suggest that cerebellar output impulses are projected downstream to saccade-programming circuits where visual information has already been converted into motor-command signals. The cerebellum is a domain for parallel processing of visuomotor information.