Processing of eye movement signals in the flocculus of the monkey

1. 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. 2. Among a population of 108 units, eight were climbing fibres, seventy-one were mossy fibres, and twenty-nine were non-Purkinje cells. Based on their discharge patterns the latter two groups of units were categorized into one of four classes. 3. Long-lead burst units (twenty-two units) exhibited saccade-related discharges substantially before saccade onset (average: 113 msec). Most of these (twenty 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. 4. Burst units (twenty-seven units) started discharging slightly before saccades (average: 6.9 msec). Discharges were associated with saccades in all directions (sixteen units) or in preferred directions (eleven units) and were not observed during periods of fixation. 5. Burst-tonic units (twenty-two units) were characterized by saccade-related burst and position-related intersaccadic tonic activity. 6. Tonic units (thirty-seven 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.