The triple-flash illusion reveals a driving role of alpha-band reverberations in visual perception

Gulbinaite, Rasa; İlhan, Barkin; VanRullen, Rufin · 2016 · Crossref

DOI: 10.1101/093500

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Summary

This study investigates whether alpha-band (7–14 Hz) brain oscillations merely modulate visual perception or can actively drive it to generate percepts in the absence of corresponding physical stimuli. The authors utilize the "triple-flash" illusion, where two brief light pulses separated by approximately 100 ms are occasionally perceived as three flashes. Building on Bowen’s (1989) theoretical model, which posits that the illusion arises from the superposition of oscillatory impulse response functions (IRFs), the research aims to provide empirical evidence for the role of endogenous alpha oscillations in creating this illusory percept. The study employed two experiments. Experiment 1 combined psychophysics and EEG to validate Bowen’s theory. Thirty participants performed a task identifying the number of flashes while stimulus onset asynchrony (SOA) varied. Concurrently, subject-specific IRFs were derived by cross-correlating EEG data with white noise flicker stimulation. The researchers correlated each participant’s optimal SOA for illusion perception with the period of their individual IRF. Experiment 2 focused on the neural mechanisms underlying the illusion using EEG data from 35 participants. With SOA fixed at 87.5 ms to maximize illusions, the authors analyzed pre-stimulus and post-stimulus alpha-band activity. They separated parietal and occipital alpha sources using independent component analysis (ICA) and examined the influence of alpha power, phase, and frequency on trial-by-trial illusion occurrence. The results confirmed Bowen’s theoretical account: the subject-specific optimal SOA for the triple-flash illusion significantly correlated with the period of the subject-specific IRF ($r = 0.51, p = .003$). In Experiment 2, the occurrence of the illusory third flash was determined by pre-stimulus parietal alpha power and phase, as well as post-stimulus alpha phase-locking. Specifically, trials resulting in an illusion were associated with distinct pre-stimulus alpha states and stronger phase consistency in the alpha band following the stimuli. The findings demonstrate that individual differences in alpha frequency and the specific phase/power state of alpha oscillations at stimulus onset critically determine whether the oscillatory reverberations from the first flash constructively interfere with the second flash to cross the perceptual threshold. These findings provide direct evidence that alpha-band oscillations can drive perception, generating a perceptual experience ("something out of nothing") rather than simply modulating the visibility of existing stimuli. The study supports the concept of perceptual cycles, suggesting that cortical excitability and stimulus-evoked responses reverberate across alpha cycles. This implies that visual perception is inherently rhythmic and that endogenous brain dynamics play a causal role in constructing visual reality, particularly in temporal processing tasks.

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