Left Motor delta Oscillations Reflect Asynchrony Detection in Multisensory Speech Perception

During multisensory speech perception, slow delta oscillations (similar to 1-3 Hz) in the listener's brain synchronize with the speech signal, likely engaging in speech signal decomposition. Notable fluctuations in the speech amplitude envelope, resounding speaker prosody, temporally align with articulatory and body gestures and both provide complementary sensations that temporally structure speech. Further, delta oscillations in the left motor cortex seem to align with speech and musical beats, suggesting their possible role in the temporal structuring of (quasi)-rhythmic stimulation. We extended the role of delta oscillations to audiovisual asynchrony detection as a test case of the temporal analysis of multisensory prosody fluctuations in speech. We recorded Electroencephalograph (EEG) responses in an audiovisual asynchrony detection task while participants watched videos of a speaker. We filtered the speech signal to remove verbal content and examined how visual and auditory prosodic fea- tures temporally (mis-)align. Results confirm (1) that participants accurately detected audiovisual asynchrony, and (2) increased delta power in the left motor cortex in response to audiovisual asynchrony. The difference of delta power between asyn- chronous and synchronous conditions predicted behavioral performance, and (3) decreased delta-beta coupling in the left motor cortex when listeners could not accurately map visual and auditory prosodies. Finally, both behavioral and neurophysiological evidence was altered when a speaker's face was degraded by a visual mask. Together, these findings suggest that motor delta oscillations support asynchrony detection of multisensory prosodic fluctuation in speech.