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Rasa Gulbinaite, Barkin Ilhan, Rufin VanRullen; Something out of nothing: The role of alpha-frequency reverberation in the triple-flash illusion. Journal of Vision 2016;16(12):1128. doi: https://doi.org/10.1167/16.12.1128.
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© ARVO (1962-2015); The Authors (2016-present)
Two successive flashes can sometimes be perceived as three, and predominantly when the delay between flashes is ~100ms [Bowen, 1989]. This "triple-flash" illusion was proposed to result from the hypothetical superposition of two oscillatory impulse response functions (IRF), one for each stimulus flash. When the delay between flashes matches the period of the oscillation, the superposition enhances a later part of the oscillation that is normally damped; when this enhancement crosses perceptual threshold, a third flash is erroneously perceived. However, so far no electrophysiological evidence supports this theoretical account of the illusion. In Experiment 1, we systematically varied the inter-flash interval (stimulus onset asynchrony, SOA) and validated Bowen's theory: The subject-specific optimal SOA for illusory perception was strongly correlated with the period of that person's parietal "perceptual echo" – an oscillatory IRF measured in a separate EEG experiment [as described in VanRullen & Macdonald, 2012]. Although this finding lends support to Bowen's notion that the illusion reflects a superposition of two oscillatory responses, it does not explain trial-to-trial variability: At the subject-specific optimal SOA, the third flash is only perceived on average half of the time (45%). In Experiment 2, by presenting two flashes at a fixed SOA while recording EEG, we contrasted brain activity for physically identical trials on which the third-flash was either reported, or not. The findings revealed that: (1) Across subjects, the probability of third-flash perception was correlated with subject-specific alpha peak frequency at parietal but not occipital sources (measured during the baseline, prior to the first flash); (2) Significantly stronger alpha-band (7-11 Hz) inter-trial phase coherence at parietal sites 210-250ms after the first flash was related to the perception of the third illusory flash. Overall, oscillatory reverberations in the brain can create something out of nothing – a third flash where there are only two.
Meeting abstract presented at VSS 2016
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