Processing of eye movement signals in the flocculus of the monkey.

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Abstract
Extracellular recordings were made from afferents to the Purkinje cells of the flocculus of monkeys either spontaneously making saccadic eye movements (saccades) or trained to fixate a small visual target projected on a tangent screen. In the trained monkeys, saccades of known magnitude and direction were induced by changing the position of the fixation target. Among a population of 108 units, 8 were climbing fibers, 71 were mossy fibers and 29 were non-Purkinje cells. Based on their discharge patterns, the latter 2 groups of units were categorized into 1 of 4 classes. Long-lead burst units (22 units) exhibited saccade-related discharges substantially before saccade onset (average: 113 ms). Most of these (20 units) discharged for saccades in a particular direction, while the remainder exhibited discharges for saccades in all directions. All units were essentially silent between saccades. Burst units (27 units) started discharging slightly before saccades (average: 6.9 ms). Discharges were associated with saccades in all directions (16 units) or in preferred directions (11 units) and were not observed during periods of fixation. Burst-tonic units (22 units) were characterized by saccade-related burst and position-related intersaccadic tonic activity. Tonic units (37 units) exhibited position-related tonic activity. The position-related activity in both burst-tonic and tonic units was observed only for fixation points within a specified region. The units were silent for fixation positions outside this region of preferred fixation.