Magnetic coil suppression of extrafoveal visual perception using disappearance targets

We used magnetic brain stimulation with a butterfly coil over the occipital lobes to study extrafoveal visual field effects in six subjects. The visual test pattern was a grid of asterisks around a central fixation point, and the target was the disappearance of one asterisk for a single frame of the video monitor. Using single magnetic pulses at stimulator outputs of 55-95%, we noted robust interference effects at latencies < or = 100 ms, peaking at approximately 50-90 ms. Suppression of visual perception occurred with both transverse and sagittal alignments of the coil. When the coil was moved laterally over either occipital lobe, perception of target disappearance was consistently suppressed in the contralateral visual field. Movement of the coil rostrally produced consistent suppression in the lower and middle field, but preferential suppression of the upper field could not be obtained. This altitudinal asymmetry may be correlated with the anatomy of the occipital lobe in relation to the scalp surface.