When speech is interrupted by noise, listeners often perceptually "fill-in" the degraded signal, giving an illusion of continuity and improving intelligibility. This phenomenon involves a neural process in which the auditory cortex (AC) response to onsets and offsets of acoustic interruptions is suppressed. Since meaningful visual cues behaviorally enhance this illusory filling-in, we hypothesized that during the illusion, lip movements congruent with acoustic speech should elicit a weaker AC response to interruptions relative to static (no movements) or incongruent visual speech. AC response to interruptions was measured as the power and inter-trial phase consistency of the auditory evoked theta band (4-8. Hz) activity of the electroencephalogram (EEG) and the N1 and P2 auditory evoked potentials (AEPs). A reduction in the N1 and P2 amplitudes and in theta phase-consistency reflected the perceptual illusion at the onset and/or offset of interruptions regardless of visual condition. These results suggest that the brain engages filling-in mechanisms throughout the interruption, which repairs degraded speech lasting up to ~ 250 ms following the onset of the degradation. Behaviorally, participants perceived speech continuity over longer interruptions for congruent compared to incongruent or static audiovisual streams. However, this specific behavioral profile was not mirrored in the neural markers of interest. We conclude that lip-reading enhances illusory perception of degraded speech not by altering the quality of the AC response, but by delaying it during degradations so that longer interruptions can be tolerated. © 2011 Elsevier Inc.
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