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Daw-An Wu, Shinsuke Shimojo; TMS reveals the correct location of flashes in motion-mislocalization illusions. Journal of Vision 2002;2(7):26. doi: 10.1167/2.7.26.
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© ARVO (1962-2015); The Authors (2016-present)
Dual-pulse transcranial magnetic stimulation (TMS) to the occipital cortex causes a brief illusory flash (phosphene). Here we trigger phosphenes during a moving visual stimulus. This causes subjects to see an image from the past — the stimulus located where it was earlier in its trajectory (“retrieval”). When TMS is used to retrieve a flash that had been displaced by a motion illusion, the retrieved flash is seen not as a repetition of the erroneous percept, but at the position of the real flash.
TMS occurs while a “clock hand” rotates around fixation. Coincident with the phosphene, two hands are visible: one which is part of the ongoing motion, and one caused by TMS. The reported location of the first hand corresponds to the stimulus position 25–80ms after TMS; reports for the latter correspond to 25–50ms prior to TMS.
At some position in the trajectory, the hand is displayed with a different color (“flash”). The effect is similar to Cai's “asynchronous binding illusion” (VSS'01), as viewers do not detect discontinuity, but report the position of the flash to be further forward in the trajectory. Displacements cross both meridianal borders. When TMS follows the flash with a long delay (∼300ms), two color flashes are perceived sequentially. The first flash is mislocalized as before, then TMS retrieves a flash at the correct position.
The direction of motion reverses at the flash. The flash and reversal still appear continuous, but the perceived location falls short of the true position by as much as 20 degrees. The perceptual blankness around the true flash position makes observation clearer at shorter TMS delays, revealing the clearest “retrieval” at ∼200ms.
Here, TMS makes neural excitability states visible, revealing moments in a dynamic cortical process. The optimal position of the stimulating coil suggests that a correct representation of the color flash is linked to lower visual cortex, but does not reach consciousness intact.
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