1. The purpose of this study was to analyze the response properties of neurons in the frontal eye fields (FEF) of rhesus monkeys (Macaca mulatta) and to compare and contrast the various functional classes with those recorded in the supplementary eye fields (SEF) of the same animals performing the same go/no-go visual tracking task. Three hundred ten cells recorded in FEF provided the data for this investigation. 2. Visual cells in FEF responded to the stimuli that guided the eye movements. The visual cells in FEF responded with a slightly shorter latency and were more consistent and phasic in their activation than their counterparts in SEF. The receptive fields tended to emphasize the contralateral hemifield to the same extent as those observed in SEF visual cells. 3. Preparatory set cells began to discharge after the presentation of the target and ceased firing before the saccade, after the go/no-go cue was given. These neurons comprised a smaller proportion in FEF than in SEF. In contrast to their counterparts in SEF, the preparatory set cells in FEF did not respond preferentially in relation to contralateral movements, even though most responded preferentially for movements in one particular direction. The time course of the discharge of the FEF set cells was similar to that of their SEF counterparts, except that they reached their peak level of activation sooner. The few preparatory set cells in FEF tested with both auditory and visual stimuli tended to respond preferentially to the visual targets, whereas, in contrast, most set cells in SEF were bimodal. 4. Sensory-movement cells represented the largest population of cells recorded in FEF, responding in relation to both the presentation of the targets and the execution of the saccade. Although some of these sensory-movement cells resembled their counterparts in SEF by exhibiting a sustained elevation of activity, most of the FEF sensory-movement cells have two discrete bursts, one after the presentation of the target and another before and during the saccade. Like their counterparts in SEF, the sensory-movement cells tended to be tuned for saccades into the contralateral hemifield, but this tendency was more pronounced in FEF than in SEF. The FEF sensory-movement cells discharged more briskly, with a shorter latency relative to the presentation of the target, than their counterparts in SEF. In addition, the FEF sensory-movement neurons reached their peak activation sooner than SEF sensory-movement neurons. Most FEF sensory-movement cells exhibited different patterns of activation in response to visual and auditory targets. 5. The pause-rebound cells that were identified in SEF were not observed as commonly in FEF. No further analysis was therefore possible. 6. Presaccadic movement neurons that discharged before goal-directed saccades were encountered in FEF. These cells comprised a similar proportion to that found in SEF. The presaccadic movement cells in FEF appeared to have smaller movement fields that were more restricted to the contralateral hemifield than were their counterparts in SEF. The temporal discharge characteristics of the presaccadic eye movement cells in FEF and SEF were not distinguishable, however. 7. Postsaccadic movement cells discharged specifically after saccades had been initiated. These comprised a significantly larger proportion than in SEF. They tended to respond best for targets in the contralateral hemifield. In addition, the onset of activity after the saccade was later in FEF than in SEF. 8. No cells were recorded in this study of FEF that were modulated according to eye position. 9. Although low-intensity (< 50-mu-A) electrical microstimulation of SEF as well as of FEF evokes saccadic eye movements, the elicited eye movements have markedly different characteristics. Saccades evoked by microstimulation of FEF do not vary with eye position, whereas those evoked from SEF do. In addition, whereas prolonged stimulation of FEF often elicits a series of saccades all of the same vector, prolonged stimulation of most sites in SEF elicits a single saccade to a particular orbital position followed by maintained fixation. 10. No cells were encountered that discharged specifically in no-go trials that required withholding the saccade. However, preparatory set cells and sensory-movement cells in FEF exhibited patterns of differential modulation in no-go trials that were not observed in SEF. Many of these neurons exhibited sustained activation after the no-go cue until the reward was delivered. In addition, the visual responsiveness of the phasic sensory-movement cells was attenuated if the no-go cue was presented simultaneously. 11. The results of this investigation indicate that, although there may be specific substantial differences between FEF and SEF, the two cortical areas also have much in common. On the one hand, it seems clear that FEF and SEF serve in parallel in generating goal-directed but not spontaneous saccades. On the other hand, both single-unit and microstimulation data suggest that SEF represents eye position in a more explicit fashion than FEF. Although there were several pieces of evidence showing that FEF responds more robustly and specifically to visual and auditory stimuli, it does not seem correct to make a rigid distinction between these two regions in terms of externally versus internally guided saccades. However, the results are consistent with the speculation that SEF may be more involved in regulating when a goal-directed saccade will occur, whereas FEF may be more involved in targeting and initiating the gaze shift.
