FUNCTIONALLY DEFINED SMOOTH AND SACCADIC EYE-MOVEMENT SUBREGIONS IN THE FRONTAL EYE FIELD OF CEBUS MONKEYS

1. Intracortical microstimulation was used to localize and define the smooth and saccadic eye movement subregions of the frontal eye field ( FEF) and the supplementary eye field (SEF) in nine hemispheres of six Cebus apella monkeys and to map the hand/arm areas in the dorsal premo tor area and other adjacent areas in five hemispheres of three C. apel la monkeys. Monkeys were anesthetized during experiments with Telazol, a dissociative agent that has no significant effect on microstimulati on-induced eye movement parameters (current threshold, velocity, and d uration). The functional subregions were defined with the use of low t hreshold current (less than or equal to 50 mu A). Electrically elicite d eye movements were videotaped and quantified. The two types of eye m ovements were clearly distinguished by their significantly different d uration and velocity (P < 0.0001) and their different responses to lon g stimulus trains. 2. The saccadic subregion of the FEF in Cebus monke ys is in the same location as in macaque monkeys (Walker's areas 8a an d 45). Most of the functional and anatomic characteristics of the sacc adic subregion of Cebus are the same as those reported in the saccadic FEF subregion of macaque monkeys. 3. A subregion in which only smooth eye movements were evoked was found in the posterior shoulder of the superior arcuate sulcus near its medial tip. A band of inexcitable cor tex separated the SEF and this smooth eye movement subregion of the FE F. This supports the proposal that the smooth eye movement subregion i s independent of the SEF but is analogous to the saccadic subregion of the FEF. The existence of two subregions of the FEF was further confi rmed by single-unit recording results. It is proposed that the smooth eye movement subregion in Cebus monkeys may be comparable with the one described in macaque monkeys. 4. Both saccadic and smooth eye movemen ts were also reliably evoked in the SEF in each hemisphere studied. Th is result strongly indicates that the SEF is concerned with not only s accadic eye movements, as previously reported, but also with smooth (p ursuit) eye movements.