Facilitation of smooth pursuit initiation by electrical stimulation in thesupplementary eye fields

The role of the supplementary eye fields (SEF) during smooth pursuit was in vestigated with electrical microstimulation. We found that stimulation in t he SEF increased the acceleration and velocity of the eyes in the direction of target motion during smooth pursuit initiation but not during sustained pursuit. The increase in eye velocity during initiation will be referred t o as pursuit facilitation and was observed at sites where saccades could no t be evoked with the same stimulation parameters. On average, electrical st imulation increased eye velocity by similar to 20%. At most sites, the thre shold for a significant facilitation was 50 muA with a stimulation frequenc y of 300 Hz. Facilitation of pursuit initiation depended on the timing of s timulation trains. The effect was most pronounced if the stimulation was de livered before smooth pursuit initiation. On average, eye velocity in stimu lation trials increased linearly as a function of eye velocity in control t rials, and this function had a slope greater than one, suggesting a multipl icative influence of the stimulation. Stimulation during a fixation task di d not evoke smooth eye movements. The latency of catch-up saccades was incr eased during facilitation, but their accuracy was not affected. Saccades to ward stationary targets were not affected by the stimulation. The results a re further evidence that the SEF plays a role in smooth pursuit in addition to its known role in saccade planning and suggest that this role may be to control the gain of smooth pursuit during initiation. The covariance betwe en pursuit facilitation and the timing of the catch-up saccade as a result of stimulation suggests that these different eye movements systems are coor dinated to achieve a common goal.