Context-dependent stimulation effects on saccade initiation in the presupplementary motor area of the monkey

Although evidence suggests that the contribution of the presupplementary motor area (pre-SMA) to voluntary motor control is effector-nonselective, the question of how electrical stimulation of the pre-SMA affects eye movements remains unanswered. To address this issue, stimulus effects of the pre-SMA of monkeys on saccade initiation were investigated during performance of a visually guided saccade task with an instructed delay period. This report describes two major findings. First, when stimuli with currents of < or =80 microA were applied before the presentation of a go signal, the reaction time (RT) of an upcoming saccade shortened with comparable effects on ipsi- and contraversive saccades. Second, stimuli that were delivered after the go signal lengthened the RT; this resulted in greater effects on ipsiversive saccades. In addition, the stimulation yielded a mild impairment of saccade accuracy, particularly when the stimulation was delivered after the go signal. By themselves, however, these stimuli did not directly elicit eye movements. Therefore the stimulus effects appeared only in the context of the behavioral task and were dependent on the phase of the task. These findings provide additional support for the hypothesis that the involvement of the pre-SMA in motor control can be linked to either eye or arm motor system dependent on behavioral context.