Left motor delta oscillations reflect asynchrony detection in multisensory speech perception

During multisensory speech perception, slow delta oscillations (∼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 audio-visual asynchrony detection as a test case of the temporal analysis of multisensory prosody fluctuations in speech. We recorded EEG responses in an audio-visual 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 features temporally (mis-)align. Results confirm (i) that participants accurately detected audio-visual asynchrony, and (ii) increased delta power in the left motor cortex in response to audio-visual asynchrony. The difference of delta power between asynchronous and synchronous conditions predicted behavioural performance, and (iii) decreased delta-beta coupling in the left motor cortex when listeners could not accurately map visual and auditory prosodies. Finally, both behavioural 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.SIGNIFICANCE STATEMENTSpeech perception is facilitated by regular prosodic fluctuations that temporally structure the auditory signal. Auditory speech processing involves the left motor cortex and associated delta oscillations. However, visual prosody (i.e., a speaker's body movements) complements auditory prosody, and it is unclear how the brain temporally analyses different prosodic features in multisensory speech perception. We combined an audio-visual asynchrony detection task with electroencephalographic recordings to investigate how delta oscillations support the temporal analysis of multisensory speech. Results confirmed that asynchrony detection of visual and auditory prosodies leads to increased delta power in left motor cortex and correlates with performance. We conclude that delta oscillations are invoked in an effort to resolve denoted temporal asynchrony in multisensory speech perception.