THE DORSOMEDIAL FRONTAL-CORTEX OF THE RHESUS-MONKEY - TOPOGRAPHIC REPRESENTATION OF SACCADES EVOKED BY ELECTRICAL-STIMULATION

The dorsomedial frontal cortex (DMFC) of monkeys has been implicated i n mediating visually guided saccadic eye movements. The purpose of thi s study was to determine whether the DMFC has a topographic map coding final eye position, and to ascertain whether this region subserves th e maintenance of eye position. The DMFC was stimulated electrically wh ile monkeys fixated a target presented somewhere in visual space. A se ries of parametric tests was conducted to ascertain the best stimulati on parameters to evoke saccades. Electrical stimulation typically prod uced contraversive saccades that converged onto a region of space, the termination zone. For some stimulation sites, however, stimulation pr oduced ipsiversive saccades. This occurred when the termination zone w as located straight ahead of the monkey. Convergence onto an orbital p osition was never observed during stimulation of the frontal eye field s (FEF), stimulation of which evoked fixed-vector saccades. The latenc y to evoke a saccade from the DMFC varied with fixation position, such that it increased monotonically the closer the fix spot was to the te rmination zone. Moreover, the probability of evoking a saccade from th e DMFC decreased the closer the fix spot was to the termination zone. The latency for evoking a saccade and the probability of evoking a sac cade from the FEF did not vary with fixation position. Horizontal head movements were not evoked from the DMFC while a monkey fixated target s presented in different positions of visual space. Moveover, changing the position of the head with respect to the body did not change the location of a termination zone with respect to the head. The DMFC was found to contain a topographic coding of termination zones, with rostr al sites representing zones in extreme contralateral visual space, and caudal sites representing zones straight ahead or ipsilaterally. Furt hermore, lateral sites represented zones in upper visual space, wherea s medial sites represented zones in lower visual space. Once the eyes were positioned within a termination zone, further stimulation fixed t he gaze and inhibited visually evoked saccades. Following release from inhibition, which occurred shortly after the end of stimulation, the saccades reached the visual target accurately. This shows that the sti mulation delayed the execution of the saccades without actually aborti ng their execution. We conclude that the DMFC contains a map represent ing eye position in craniotopic coordinates, and we argue that this ma p is utilized to maintain eye position.