Early and Late Beta Band Power reflect Audiovisual Perception in 1 the McGurk Illusion

33 The McGurk illusion is a prominent example of audiovisual speech perception and the influence, visual 34 stimuli can have on auditory perception. In this illusion a visual speech stimulus influences the 35 perception of an incongruent auditory stimulus resulting in a fused novel percept. In this high-density 36 electroencephalography (EEG) study we were interested in the neural signatures of the subjective 37 percept of the McGurk illusion as a phenomenon of speech-specific multisensory integration. 38 Therefore, we examined the role of cortical oscillations and event-related responses in the perception 39 of congruent and incongruent audiovisual speech. We compared the cortical activity elicited by 40 objectively congruent syllables with incongruent audiovisual stimuli. Importantly, the latter elicited a 41 subjectively congruent percept: the McGurk illusion. We found that early, event-related responses (N1) 42 to audiovisual stimuli were reduced during the perception of the McGurk illusion compared to 43 congruent stimuli. Most interestingly, our study showed that a stronger post-stimulus suppression of 44 beta band power (13-30 Hz) at short (0-500ms) and long (500-800ms) latencies during the perception 45 of the McGurk illusion compared to congruent stimuli. Our study demonstrates that auditory perception 46 is influenced by visual context and that the subsequent formation of a McGurk illusion requires 47 stronger audiovisual integration even at early processing stages. Our results provide evidence that 48 induced beta band suppression at early stages reflects stronger stimulus processing in the McGurk 49 illusion. Moreover, stronger late beta band power suppression in McGurk illusion indicates the 50 resolution of incongruent physical audiovisual input and the formation of a coherent, illusory 51 multisensory percept. 52