The role of the flocculus of the monkey in fixation and smooth pursuit eye movements.

1. Purkinje cell discharges were recorded from the flocculus of monkeys trained to fixate a small visual target and to track the target when moved slowly. 2. A striking feature of Purkinje cell activity during the steady fixation was a high rate of tonic discharges with regular interspike intervals. The average discharge rate in the whole population of Purkinje cells ranged from 37 to 145 spikes/sec. The coefficient of variation of the interspike intervals was typically smaller than 0.5 in most units. 3. In 43.9% of the Purkinje cells, tonic levels of activity changed by more than 20% of the average background activity with shifts of gaze. 4. In some Purkinje cells, especially in most burst-pause units, discharge rates during steady fixation were proportional to eye positions in one plane, implicating these cells as sources of eye position information to their target cells. 5. When the monkey tracked a sinusoidally oscillating target, the activity of some Purkinje cells was clearly modulated in phase with the eye velocity. 6. In the other Purkinje cells exhibiting smooth pursuit modulation, the activity curve appeared with a phase shift. When these cells were tested with sinusoidal target movements at different frequencies, but with a constant magnitude, the peak firing rates were proportional to the frequencies of the excursions, i.e. the velocities. 7. The flocculus thus provides the oculomotor system with eye position information during fixation and with velocity information during smooth pursuit and participates in the control of oculomotor functions stabilizing retinal images.