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Susanne L. Watkins, Ladan Shams, John-Dylan Haynes, Geraint Rees; Sound-induced illusory flash perception modulates V1 activity. Journal of Vision 2005;5(8):884. doi: 10.1167/5.8.884.
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The auditory-induced double flash illusion shows that conscious visual perception can be modified by irrelevant auditory stimulation. A single visual flash is perceived incorrectly as two flashes when it is accompanied by two auditory bleeps. Here, we used event-related high field functional MRI in conjunction with retinotopic mapping of early visual cortex to examine the neural basis of this illusory phenomenon. On each trial, participants were presented with either one or two successively flashed high contrast annuli around fixation, either alone or in association with one or two binaurally presented bleeps. Participants indicated on each trial by 2AFC button press whether they perceived either one or two flashes. Behaviorally, on a significant proportion of single flash trials that were accompanied by two bleeps, participants reported the perceptual experience of two flashes i.e. the illusion. We then compared brain activity evoked by physically identical but perceptually dissimilar (illusion versus no illusion) one-flash-two-bleep trials. In retinotopic area V1, there was significantly greater activation for illusion versus no illusion trials. Such activation cannot reflect a general auditory alerting effect as the trials in such a comparison are physically identical. Taken together, our findings provide strong evidence that irrelevant auditory stimulation can modulate brain activity and conscious perception at the earliest stages of “visual” cortical processing.
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