Audiovisual speech perception relies on our expertise to map a speaker’s lip movements with speech sounds. This multimodal matching is facilitated by salient syllable features that align lip movements and acoustic envelope signals in the 4 – 8 Hz theta band. The predominance of theta rhythms in speech processing has been firmly established by studies showing that neural oscillations track the acoustic envelope in the primary auditory cortex. Equivalently, theta oscillations in the visual cortex entrain to lip movements and the auditory cortex is recruited during silent speech perception. These findings suggest that neuronal theta oscillations play a functional role in organising information flow across visual and auditory sensory areas. We presented silent speech movies while participants performed a pure tone detection task to test whether entrainment to lip movements enslaves the auditory system and drives behavioural outcomes. We showed that auditory detection varied depending on the ongoing theta phase conveyed by lip movements in the movies. In a complementary experiment presenting the same movies while recording participants’ electro-encephalogram (EEG), we found that silent lip movements entrained neural oscillations in the visual and auditory cortices with the visual phase leading the auditory phase. These results support the idea that the visual cortex entrained by lip movements increases the sensitivity of the auditory cortex at relevant time-windows for speech comprehension as a filtering modulator relying on theta phase synchronisation.
bioRxiv Subject Collection: Neuroscience