Transcranial stimulation of the human frontal eye field by magnetic pulses

Single transcranial magnetic pulsed stimuli were applied over the cortical area of the putative right frontal eye field (FEF) in 11 healthy subjects. An especially designed figure of eight shaped twin coil was used, to focus the stimulus, the strength of which was adjusted to the individual motor threshold of the left hand muscles. Eye positions and movements were recorded by an infrared reflection technique. Three different experiments were performed: 1. Stimulation during different primary gaze position did not evoke any discernible eye movement. 2. Stimulation just prior to visually elicited horizontal saccades did not cause a significant alteration of the latency, velocity, or amplitude of the saccades. 3. Only stimulation during an antisaccade task induced a significant latency prolongation, when the stimulus was applied between 50 to 90 ms after the target flashed up. This latency prolongation was found in all subjects for the antisaccades to the right, with a statistically significant average latency difference of +66 +/- 55.5 ms. In contrast, the antisaccades to the left were prolonged in the female subjects only by an average of +98 +/- 41.8 ms (p = 0.0064), whereas in the male subjects they did not alter with stimulation (average difference: -3 +/- 41.9 ms, p = 0.753). Significant latency prolongations were only obtained when the magnetic FEF stimuli were applied within a vulnerable period, which varied from subject to subject.