FIXATION CELLS IN MONKEY SUPERIOR COLLICULUS .1. CHARACTERISTICS OF CELL DISCHARGE

1. We studied the role of the superior colliculus (SC) in the control of visual fixation by recording from cells in the rostral pole of the SC in awake monkeys that were trained to perform fixation and saccade tasks. 2. We identified a subset of neurons in three monkeys that we r efer to as fixation cells. These cells increased their tonic discharge rate when the monkey actively fixated a visible target spot to obtain a reward. This sustained activity persisted when the visual stimulati on of the target spot was momentarily removed but the monkey was requi red to continue fixation. 3. The fixation cells were in the rostral po le of the SC. As the electrode descended through the SC, we encountere d visual cells with foveal and parafoveal receptive fields most superf icially, saccade-related burst cells with parafoveal movement fields b elow these visual cells, and fixation cells below the burst cells. Fro m this sequence in depth, the fixation cells appeared to be centered i n the deeper reaches of the intermediate layers, and this was confirme d by small marking lesions identified histologically. 4. During saccad es, the tonically active fixation cells showed a pause iii their rate of discharge. The duration of this pause was correlated to the duratio n of the saccade. Many cells did not decrease their discharge mte for small-amplitude contraversive saccades. 5. The saccade-related pause i n fixation cell discharge always began before the onset of the saccade . The mean time from pause onset to saccade onset for contraversive sa ccades and ipsiversive saccades was 36.2 and 33.0 ms, respectively. Mo st fixation cells were reactivated before the end of contraversive sac cades. The mean time from saccade termination to pause end was -2.6 ms for contraversive saccades and 9.9 ms for ipsiversive saccades. The e nd of the saccade-related pause in fixation cell discharge was more li ghtly correlated to saccade termination, than pause onset was to sacca de onset. 6. After the saccade-related pause in discharge, many fixati on cells showed an increased discharge rate exceeding that before the pause. This increased postsaccadic discharge rate persisted for severa l hundred milliseconds. 7. The discharge rate of fixation cells was no t consistently altered when the monkey actively fixated targets requir ing different orbital positions. 8. Fixation cells discharged during s mooth pursuit eye movements as they did during fixation. They maintain ed a steady tonic discharge during pursuit at different speeds and in different directions, provided the monkey looked at the moving target. 9. We hypothesize that the fixation cells in the rostral pole of the monkey SC provide a signal related to active visual fixation. Activati on of these cells would inhibit the activity of saccade-related cells in the rest of the SC and the saccade premotor circuitry in the brain stem. A prerequisite step in the generation of a saccade would be the reduction of activity in these collicular fixation cells.