COMPARISON OF THE DISCHARGE CHARACTERISTICS OF BRAIN-STEM OMNIPAUSE NEURONS AND SUPERIOR COLLICULUS FIXATION NEURONS IN MONKEY - IMPLICATIONS FOR CONTROL OF FIXATION AND SACCADE BEHAVIOR

Fixation neurons (SCFNs) in the rostral pole of the superior colliculu s (SC) and omnipause neurons (OPNs) in the nucleus raphe interpositus (rip) in the pens share similar discharge properties. Both types of ne urons discharge tonically during periods of visual fixation and pause for saccadic eye movements, and their activation by electrical stimula tion suppresses saccade generation. On the basis of these similarities and the projection from the rostral SC to the rip, it was hypothesize d that SCFNs provide a major excitatory input to OPNs. We investigated the role and relationship of SCFNs and OPNs with respect to both fixa tion behavior and saccade generation by comparing their activity recor ded in the same monkeys performing a gap saccade task. In this task, t he central fixation point was extinguished 200 ms before the presentat ion of an eccentric saccadic target, and the discharges of OPNs and SC FNs were contrasted during visual fixation, nonvisual(gap) fixation, a nd saccade generation. During visual fixation, the mean discharge rate of OPNs was higher and more regular than that of SCFNs. During the ga p period, SCFNs decreased their discharge rate before target appearanc e, whereas no change in discharge rate was observed in OPNs. For both SCFNs and OPNs, the activity level before target appearance was not co rrelated to saccadic reaction time. In contrast to SCFNs, several OPNs responded with a transient phasic increase in discharge immediately a fter the target presentation. Before their saccade-related pause, ther e was a gradual reduction in the activity of SCFNs, whereas OPNs had a n abrupt cessation of discharge. SCFNs paused earlier than OPNs, but t he OPN pause onset was better synchronized to saccade onset than the S CFN pause onset. OPNs resumed firing after their pause in activity ear lier than SCFNs, and the OPN pause end was better synchronized to sacc ade end than the SCFN pause end. These physiological data reveal diffe rences in the discharge properties of SCFNs and OPNs that are irreconc ilable with the hypothesis that the discharge pattern of OPNs reflects simply the excitatory input from SCFNs. It is most likely that additi onal inputs to OPNs compensate for the reduction in discharge of SCFNs during these periods.