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Daw-An Wu, Shinsuke Shimojo; Transcranial magnetic stimulation (TMS) reveals the content of post-perceputal visual processing.. Journal of Vision 2004;4(8):47. doi: 10.1167/4.8.47.
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After one has seen a flashed visual object, administration of dual-pulse TMS to the occipital cortex causes portions of the object to be seen again. Furthermore, after one watches a flash-lag display, perceiving the position of the flash erroneously, subsequent TMS causes the flash to be seen again, but in the correct position (Wu & Shimojo, VSS ′02). We now explore spatio-temporal properties of this “retrieval” effect. Exp 1) We flashed full-screen colored spatial frequency gratings (1–2.5 cpd; 53ms) followed by TMS after a delay; varying grating orientation, delay, and stimulator coil position. Subjects outlined the region in which they saw the grating re-appear, and then adjusted the display′s brightness and color saturation to match their percept; verbal comments were tape recorded. Results: As the delay between visual stimulus and TMS increases, the elicited percept declines both in brightness and size. However, the effect is still present beyond 2 s. Regions drawn were elongated parallel to the gratings. Moving the coil caused the regions to expand or shrink, but did not shift their centers systematically. Sometimes at longer intervals (∼2 s), the grating would appear bright, but monochromatic. Exp 2) We flashed full-screen monochromatic gratings followed by full-screen homogeneous color flashes, followed by TMS; varying orientation and delays. Subjects reported that patches of solid color and monochrome grating would be elicited by TMS. Where they overlapped, they would appear overlaid or transparent. Under certain conditions, the luminance grating and color percepts would merge, and a colored grating would be seen. Neural activity continues well after the conscious percept of a visual flash has ended. Here TMS was used to reveal the visual content of that activity, building a spatio-temporal map of post-perceptual visual processing. Signals in the intensity and color channels have different rates of decay, and can be artificially segregated and merged.
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