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Yi-Chuan Chen, Yi-Syuan Peng, Da Li, Su-Ling Yeh; Audiovisual Spatial Congruency Modulates Perceptual and Metacognitive Components of Sound-induced Flash Illusion. Journal of Vision 2021;21(9):2270. doi: https://doi.org/10.1167/jov.21.9.2270.
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Sound-induced flash illusion constitutes a classical phenomenon of audiovisual integration: The fission illusion occurs when two flashes are reported when one flash is paired with two beeps; in contrast, the fusion illusion occurs when one flash is reported when two flashes are paired with a single beep. Here we investigated the spatial modulations on perceptual and introspective experience of the fission and fusion illusions by utilizing the fact that the magnitude of illusions differs in different eccentricity. The visual flashes were presented on a monitor either in the center at which the participants fixate, or in the periphery (10° to the left or right). The participants had to report the number of flashes and rate the confidence regarding the correctness of their report. When the beeps were presented from loudspeakers that is spatially congruent with the flashes, the fission illusion was larger in the periphery than in the center, whereas the fusion illusion was larger in the center than in the periphery. Interestingly, participants also reported higher confidence when either illusion was larger, suggesting that participants considered their illusory experience as genuine. When the beeps were presented from headphones that are spatially incongruent with the flashes, the fission illusion strength was similar in the center and periphery, whereas the fusion illusion remained larger in the center. However, there was no center-periphery difference in confidence rating for both illusions. Hence, when the flashes and beeps were spatially separated, center-periphery difference of the fission illusion was eliminated, and the magnitude and confidence rating of the fusion illusion can be dissociated. Taken together, spatial coincidence is a critical factor for the fission and fusion illusions as manifested in both perceptual and introspective measures. Furthermore, the fission and fusion illusions were sensitive to audiovisual spatial congruency to different extents, plausibly underpinned by different mechanisms.
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