Intracortical microstimulation of bilateral frontal eye field.

We trained two monkeys to perform a fixation task. Intracortical microstimulation (ICMS) was applied to the monkey frontal eye field (FEF) while monkeys were fixating on one of five fixation LEDs. The ICMS was applied in two different manners. Under the single stimulation condition, ICMS was delivered to either right or left FEF. Under the paired stimulation condition, bilateral FEF were successively stimulated with an interval of 30-250 ms. The single stimulation elicited contraversive saccades. As reported previously, these saccades were not much affected by initial eye positions, maintaining the same vector. In contrast, the paired stimulation elicited double-step saccades. The first of the paired stimulation elicited constant vector saccades, but the second of the paired stimulation evoked saccades whose vector varied greatly depending on the eye position at the start of individual saccades. The second saccades, starting from various initial positions, were directed to the endpoint of saccades that were elicited from the same FEF site under the single stimulation condition. Endpoints of second saccades varied little despite variations of intervals of the stimulation pairs, ranging from 60 to 150 ms. On the basis of these observations, we propose a novel view that the FEF is involved in directing saccades to an internally referenced visual target.