Saccadic eye movements evoked by microstimulation of lobule VII of the cerebellar vermis of macaque monkeys.

1. Oculomotor responses to microstimulation of the cerebellar vermis were studied on macaque monkeys by measuring eye position with a magnetic search-coil method. 2. Vermal microstimulation resulted in conjugate eye movements. Analyses of amplitude-velocity and amplitude-duration relationships revealed that although the peak velocities were slightly faster, the durations of these responses were comparable to those of spontaneous and visually guided saccadic eye movements (saccades). 3. Systematic mapping with microstimulation revealed that the low-threshold sites were localized in the white matter of a limited number of folia in the posterior vermis. The low-threshold region from which saccades could be evoked with stimulus intensities less than 10 microA was confined to lobule VII in seven monkeys; in the other five monkeys it included a posterior part of lobule VI (folium VIc). This region coincided with the distribution of saccade-related neural activity observed in the present study and corresponded to the vermal folia from which we previously recorded the burst mossy fibre units and the oculomotor Purkinje cell activity (Kase, Miller & Noda, 1980). 4. Microstimulation of most sites in the oculomotor vermis evoked saccades in oblique directions and the horizontal component of the saccade was always ipsilateral to the stimulation side. In penetrations through the paramedian vermis, the direction of the saccade changed gradually from upward oblique to downward oblique as the electrode was advanced vertically across lobule VII. The vertical component was dominant in these saccades. When the stimulation track was systematically shifted from the medial to the lateral part of the vermis, the horizontal component of the saccade increased and the vertical component decreased gradually until the saccade became almost horizontal. 5. The direction of saccades changed with the stimulus intensity; thus the topographical change in the direction of a saccade was consistent only when the stimulus intensity was adjusted to the threshold for every stimulus site. When stimulated with near threshold intensities, the evoked saccades were very seldom straight: the trajectories were curved in the responses observed at most vermal sites. The horizontal and vertical components of these saccades showed different onset latencies, durations, and peak-velocity latencies. Increasing the stimulus current produced differing effects on the two components of the saccades.(ABSTRACT TRUNCATED AT 400 WORDS)