Topography of the oculomotor area of the cerebellar vermis in macaques as determined by microstimulation.

1. Oculomotor responses to microstimulation of the cerebellar vermis of macaque monkeys were investigated by using a magnetic search-coil method. 2. The oculomotor responses were conjugate eye movements with an ipsilateral horizontal component. Analyses of amplitude-velocity and amplitude-duration relationships revealed that the peak eye velocities and the durations of the responses were comparable to those of saccadic eye movements. 3. Systematic mapping with microstimulation disclosed that the region in the cerebellar vermis that yielded saccades with weak stimulus currents was confined to lobule VII in five monkeys but included a part of folium VIc in the other four monkeys. This region coincided with the distribution of the saccade-related neural activity observed in the present study and also corresponded to the vermal folia from which we recorded the burst mossy-fiber units and the oculomotor Purkinje cell activity. 4. The oculomotor vermis was defined as that region of the cerebellar vermis that met the following criteria: 1) saccades were evoked with low-intensity microstimulation (with currents less than 10 microA); 2) vigorous saccade-related neural activity was present; and 3) Purkinje cell discharges were modulated with eye movements. The oculomotor vermis was more circumscribed and located more posteriorly than the vermal cortex explored in previous microstimulation experiments on monkeys. 5. Microstimulation of the oculomotor vermis evoked more or less curved saccades in oblique directions. The horizontal and vertical components were not simultaneous in some saccades: the shorter component started later or ended earlier than the other component and their peak velocities were not always synchronous. 6. The amplitude of the saccade depended on stimulus parameters; microstimulation with 10-12 pulses within a period of approximately 20 ms (500-600 Hz) was shown to be optimal. When the pulses were applied to the white matter or to the granular layer, a stimulus current of 10 microA was sufficient to evoke saccades. When the molecular layer was stimulated, evoked saccades were smaller and frequently curved, and an increase in the stimulus current changed either the initial direction or the trajectory of the saccade. 7. When the stimulus current was carefully controlled and maintained near the threshold, the direction of the saccade evoked from the oculomotor vermis was topographically organized.(ABSTRACT TRUNCATED AT 400 WORDS)