The Triple-Flash Illusion Reveals a Driving Role of Alpha-Band Reverberations in Visual Perception

Abstract

The modulatory role of spontaneous brain oscillations on perception of threshold-level stimuli is well established. Here, we provide evidence that alpha-band (∼10 Hz) oscillations not only modulate perception of threshold-level sensory inputs but also can drive perception and generate percepts without a physical stimulus being present. We used the "triple-flash" illusion: Occasional perception of three flashes when only two spatially coincident veridical ones, separated by ∼100 ms, are presented. The illusion was proposed to result from superposition of two hypothetical oscillatory impulse response functions generated in response to each 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 (Bowen, 1989). In Experiment 1, we varied stimulus onset asynchrony and validated Bowen's theory: The optimal stimulus onset asynchrony for illusion to occur was correlated, across human subjects (both genders), with the subject-specific impulse response function period determined from a separate EEG experiment. Experiment 2 revealed that prestimulus parietal, but no occipital, alpha EEG phase and power, as well as poststimulus alpha phase-locking, together determine the occurrence of the illusion on a trial-by-trial basis. Thus, oscillatory reverberations create something out of nothing: A third flash where there are only two. We highlight a novel property of alpha-band (∼10 Hz) oscillations based on three experiments (two EEG and one psychophysics) by demonstrating that alpha-band oscillations do not merely modulate perception, but can also drive perception. We show that human participants report seeing a third flash when only two are presented (the "triple-flash" illusion) most often when the interflash delay matches the period of participant's oscillatory impulse response function reverberating in alpha. Within-subject, the phase and power of ongoing parietal, but not occipital, alpha-band oscillations at the time of the first flash determine illusory percept on a trial-by-trial basis. We revealed a physiologically plausible mechanism that validates and extends the original theoretical account of the triple-flash illusion proposed by Bowen in 1989.

Awareness is related to reduced post‐stimulus alpha power: a no‐report inattentional blindness study

“…These studies have consistently shown that lower alpha amplitude prior to stimulus onset predicts increased likelihood of the stimulus being perceived (e.g., Ergenoglu et al ., 2004; Babiloni et al ., 2006; Hanslmayr et al ., 2007; van Dijk et al ., 2008; Romei et al ., 2010; MacLean & Arnell, 2011; Limbach & Corballis, 2016; Iemi et al ., 2017). Lower alpha power also increases the likelihood of perceiving a stimulus when no stimulus is presented, both in terms of false positives (Limbach & Corballis, 2016; Iemi et al ., 2017), and visual illusions (Lange et al ., 2013; Cecere et al ., 2015; Gulbinaite et al ., 2017), consistent with alpha's role in spatial gain modulation. These studies and others have supported the conclusion that alpha oscillations are a key mechanism underlying the effects of spatial attention (Jensen & Mazaheri, 2010).…”

Single trial prestimulus oscillations predict perception of the sound-induced flash illusion

“…Previous studies investigating prestimulus oscillatory activity in double flash illusions using MEG have found that crossmodal influence is associated with increased beta band power (13–21 Hz) in left temporal areas 18 ; enhanced excitability in visual areas, as reflected by decreased alpha band power; and increased gamma band power in a more extensive cortical network including temporal areas 19 . Prestimulus alpha band power is also decreased in the triple flash illusion, where two rapidly presented flashes are occasionally perceived as three 23 . Interestingly, the optimal delay between flashes is correlated with the subject-specific impulse response frequency, pointing to a contribution of oscillatory reverberation to illusory visual perception.…”

Dynamics of Visual Perceptual Echoes Following Short-Term Visual Deprivation

“…Their results revealed a large alpha oscillation that begins after the ERP-like early response and is sustained for lags up to >1 s. This component, the perceptual echo, represents the rhythmicity that the visual system naturally selects from the random sequence. Recent studies have demonstrated that the echo response may be employed as a mechanism of active rhythmic sampling that can be modulated according to current visual demand ( Gulbinaite et al. 2017 ; Benedetto et al.…”