Involvement of Purkinje cells in evoking saccadic eye movements by microstimulation of the posterior cerebellar vermis of monkeys

Abstract

Neural mechanisms for evoking saccadic eye movements by microstimulation of the posterior vermis were investigated in monkeys trained to fixate a visual target. The low-threshold region from which saccadic eye movements could be evoked with currents less than 10 microA was confined to lobule VII in two monkeys and it included a posterior part of lobule VI (lobule VIc) in another monkey. The region from which saccade-related neural activity was recordable coincided with the low-threshold region. This region corresponded to the vermal lobules from which eye position and saccade-related Purkinje cells were recorded. Kainic acid (kainate) injected in the white matter of lobule VII resulted in severe losses of Purkinje cells within a radius of 1-2 mm of the injection site. The lesion tended to be larger toward the peripheral cerebellar cortices, which were connected to the injection site by natural courses of the afferent and efferent fibers. After the kainate administration, the distribution of saccade-related neural activity did not differ significantly from that of the preoperative mapping, in spite of the severe losses of cortical neurons. Burst discharges of mossy fibers were recordable in the white matter near the injection site, indicating that afferent fibers were relatively unaffected by kainate. After kainate administration, the saccadic eye movements could no longer be evoked by microstimulation applied to the posterior vermis. The stimulus sites from which saccades could be evoked after kainate administration were always associated with the presence of intact Purkinje cells. In such cases, the minimum current necessary to evoke saccades depended on the percentages of intact Purkinje cells spared. In the folia with normal Purkinje cell layers, the amplitude and direction of evoked saccades and the thresholds for evoking such eye movements were almost comparable to the preoperative data. Saccadic eye movements in response to microstimulation of the posterior vermis were caused by orthodromic impulses conveyed through the axons of the Purkinje cells. Insofar as the saccades elicited from lobule VII with currents less than 10 microA are concerned, antidromic activation of the afferent fibers is not the neural mechanisms subserving the oculomotor responses.