EFFECT OF IBOTENIC ACID LESIONS OF THE OMNIPAUSE NEURONS ON SACCADIC EYE-MOVEMENTS IN RHESUS MACAQUES

1. Although much is known about the neurons that control saccadic eye movements, the precise manner in which they interact is still uncertai n. To test the validity of competing models of the pontine saccade gen erator. neurotoxic lesions were made in the nucleus raphe interpositus (rip), which contains one of the principal types of saccade-related n eurons, the omnipause neurons (OPNs). The correlated changes in eye mo vement were quantified in three juvenile rhesus macaques and compared with the results predicted by different models. 2. After the location of the OPNs was mapped, the rip was subjected to sequential, punctate pressure injections of ibotenic acid. The resulting progressive damage was correlated with changes in saccade metrics, including a decrease in peak saccadic velocity and an increase in saccade duration. 3. The damage to rip and presumably to the OPNs was not associated with a cha nge in the animals' ability to maintain steady fixation of a stationar y target.4. The results suggest that Robinson's original local feedbac k model of saccade generation should be modified. Either a second inte grator should be added or the concept of local feedback should be aban doned entirely. 5. The suggestion that the OPNs are primarily responsi ble for fixation is probably incorrect. OPNs may contribute to fixatio n stability along with a number of other sources.