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Rodika Sokoliuk, Rufin VanRullen; The Flickering Wheel Illusion: When alpha rhythms make a static wheel flicker. Journal of Vision 2011;11(11):937. doi: https://doi.org/10.1167/11.11.937.
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© ARVO (1962-2015); The Authors (2016-present)
Even though neuronal oscillations pervade the brain, our sensory perception of the world does not appear to oscillate: the brain must have developed strategies to conceal the consequences of these oscillations from our perception. Could these concealment strategies be fooled by suitable stimuli, so that our brain rhythms would be revealed to us? Here we report on a novel illusion in which the center of a static wheel stimulus, made up of 30 to 40 alternating black and white spoke sectors, is experienced as flickering. The flicker occurs most strongly during small eye movements performed with the stimulus in the visual periphery (indeed, the illusion was first discovered accidentally while reading text just above a similar-looking wheel). However, we show that stimulus motion relative to the retina is not crucial to perceive the illusory flicker: comparable amounts of flicker are also experienced on the afterimage of a wheel, produced after a few seconds' exposure to the stimulus pattern followed by instant removal -yet by definition this afterimage is stationary on the retina. To address the neuronal basis of the illusion, we recorded EEG while observers (N = 20) performed smooth pursuit eye movements, following with their gaze a slowly rotating dot around a wheel pattern; all the while they reported the occurrence of perceived flicker using a button press. The illusion was maximal for certain eye positions around the wheel, and decreased for others. The only frequency band of the EEG that displayed a compatible time course was the alpha rhythm (8–14 Hz) recorded over occipital electrodes: when alpha amplitude was strong, the probability of reporting flicker increased. We propose that this new flickering illusion is a unique way to experience the alpha rhythms that constantly occur in the brain, but normally remain unnoticed.
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