Suppression of task-related saccades by electrical stimulation in the primate's frontal eye field.

Abstract

Patients with frontal lobe damage have difficulty suppressing reflexive saccades to salient visual stimuli, indicating that frontal lobe neocortex helps to suppress saccades as well as to produce them. In the present study, a role for the frontal eye field (FEF) in suppressing saccades was demonstrated in macaque monkeys by application of intracortical microstimulation during the performance of a visually guided saccade task, a memory prosaccade task, and a memory antisaccade task. A train of low-intensity (20-50 microA) electrical pulses was applied simultaneously with the disappearance of a central fixation target, which was always the cue to initiate a saccade. Trials with and without stimulation were compared, and significantly longer saccade latencies on stimulation trials were considered evidence of suppression. Low-intensity stimulation suppressed task-related saccades at 30 of 77 sites tested. In many cases saccades were suppressed throughout the microstimulation period (usually 450 ms) and then executed shortly after the train ended. Memory-guided saccades were most dramatically suppressed and were often rendered hypometric, whereas visually guided saccades were less severely suppressed by stimulation. At 18 FEF sites, the suppression of saccades was the only observable effect of electrical stimulation. Contraversive saccades were usually more strongly suppressed than ipsiversive ones, and cells recorded at such purely suppressive sites commonly had either foveal receptive fields or postsaccadic responses. At 12 other FEF sites at which saccadic eye movements were elicited at low thresholds, task-related saccades whose vectors differed from that of the electrically elicited saccade were suppressed by electrical stimulation. Such suppression at saccade sites was observed even with currents below the threshold for eliciting saccades. Pure suppression sites tended to be located near or in the fundus, deeper in the anterior bank of the arcuate than elicited saccade sites. Stimulation in the prefrontal association cortex anterior to FEF did not suppress saccades, nor did stimulation in premotor cortex posterior to FEF. These findings indicate that the primate FEF can help orchestrate saccadic eye movements by suppressing inappropriate saccade vectors as well as by selecting, specifying, and triggering appropriate saccades. We hypothesize that saccades could be suppressed both through local FEF interactions and through FEF projections to subcortical regions involved in maintaining fixation.