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D. A. Wu, Y. Kamitani, F. Maeda, S. Shimojo; Interaction of TMS-induced phosphenes and visual stimuli. Journal of Vision 2001;1(3):198. doi: 10.1167/1.3.198.
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Purpose: Whereas single-pulse transcranial magnetic stimulation (TMS) of the human visual cortex results in scotoma, dual-pulse TMS results in the transient visual percept of an area of brightness, termed a phosphene. In this study, we presented disk-shaped visual stimuli that overlapped the phosphene in the visual field. Subjects evaluated the brightness of the overlapping region when dim or bright disks were either flashed or steadily shown. Procedure: Dual magnetic pulses were delivered by a Neotonus Neopulse stimulator. The coil position over the left occipital lobe, power, and inter-pulse interval were optimized for phosphene induction. Trials began with a dark background (< 0.1 cd/m^2), then a large homogeneous disk (15 deg in diameter) was either: flashed for 93 ms and followed by TMS (40 ms after flash offset), or shown steadily for 6 s, during which (at the 4th second) TMS was triggered. The disks were set either dimmer or brighter than phosphenes perceived on a dark background. The four conditions (flashed/steady x dim/bright) were randomly repeated five times each. After each trial the disk was redisplayed and subjects adjusted the luminance of the region that overlapped with the phosphene to match their percept. Results: 1) For both dim and bright flashed disks, the region overlapping the phosphene was reported to be brighter than the rest of the disk (p < .01). However, in steady displays the region was darkened in bright disks, but brightened in dim disks (p < .01). 2) With the 40 ms delay between the flashed disk and TMS, which maximizes the brightening effect, the disappearance of the disk was perceived first, followed by the simultaneous perception of the phosphene and its brighter spatially overlapping region. Discussion: Phosphenes clearly cannot be considered a mere brightening of the visual field. Phosphenes show non-linear brightness interactions with visual stimuli that are both physically and perceptually non-concurrent.
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