The effect of attentive fixation on eye movements evoked by electrical stimulation of the frontal eye fields

Electrical stimulation of the frontal eye fields of the rhesus monkey evokes saccadic eye movements. Both the amplitude of electrically elicited saccades and the threshold current for eliciting them are primarily determined by the location of the stimulating electrode within the frontal eye fields; however, threshold and amplitude also are systematically affected by the monkey's behavioral state when the stimulation is applied. If the monkey is alert, but not performing a task, saccade amplitudes are largest and thresholds are lowest. Conversely, if the monkey actively fixates a visual target, elicited saccades are smaller and threshold currents are higher. Saccades evoked during fixation have slower velocities appropriate for their reduced amplitude. Phase plane plots of eye velocity versus eye position indicate that these saccades are originally programmed to be smaller and slower, and hence are not large saccades voluntarily braked in mid-flight. As opposed to their amplitude and threshold, the direction of electrically evoked saccades is unaffected by the state of fixation. The state of attentive fixation, but not the visual fixation target itself, is the responsible factor for these effects. These results suggest that there is a difference between the state of active fixation and the state of having the eye still in the orbit without active fixation. The oculomotor system in the latter case is relatively more susceptible to signals from the cerebral cortex.