Rapid-rate transcranial magnetic stimulation of human frontal cortex can evoke saccades under facilitating conditions

Rapid-rate transcranial magnetic stimulation (rTMS) of a localized area within premotor frontal cortex elicited short-latency, multistep eye movements during a double-step saccade task in 3 of 9 subjects. These evoked saccades occurred in 14-32% of trials when rTMS was delivered over premotor regions located 5-7 cm lateral and 2-4 cm anterior to the vertex. In trials without rTMS or when rTMS was delivered medial or posterior to these sites, multistep saccades occurred in less than 2% of trials. In two subjects, rTMS of the right premotor cortex evoked contraversive saccades. In a third subject, rTMS of the left premotor cortex evoked saccades whose trajectory depended on the direction of the double-step targets. The amplitude and likelihood of occurrence of evoked saccades varied between rTMS trials. The intervals between evoked saccades were proportional to the intervals between rTMS pulses delivered at 16, 20 or 25 Hz. rTMS did not elicit saccades during fixation on a stationary target or during straight-ahead gaze in darkness, suggesting that the double-step saccade task facilitated the activation of a discrete premotor area consistent with the human frontal eye fields.