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

• Published in: The Journal of neuroscience Vol. 37; no. 30; pp. 7219 - 7230
• Main Authors: Gulbinaite, Rasa, İlhan, Barkın, VanRullen, Rufin
• Format: Journal Article
• Language: English
• Published: United States Society for Neuroscience 26.07.2017
• Subjects: Alpha Rhythm - physiology; Brain; Cognitive science; Economic models; EEG; Evoked Potentials, Visual - physiology; Experiments; Female; Frequency dependence; Humans; Illusions - physiology; Impulse response; Male; Neuroscience; Occipital Lobe - physiology; Oscillations; Parietal Lobe - physiology; Perception; Perceptual Masking - physiology; Photic Stimulation - methods; Response functions; Specific impulse; Superposition (mathematics); Visual perception; Visual Perception - physiology; Young Adult; phase; triple-flash illusion; power; EEG; alpha oscillations; individual alpha peak frequency
• ISSN: 0270-6474; 1529-2401; 1529-2401
• DOI: 10.1523/JNEUROSCI.3929-16.2017

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.