Abstract
Audiovisual speech perception relies, among other things, 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. Although non-exclusive, 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 may 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 directs 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 filtered the sensitivity of the auditory cortex via theta phase synchronization.
Highlights
When hearing gets difficult, people often visually focus on their interlocutors' mouth to match lip movements with sounds and to improve speech perception
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
We first established that visual entrainment to theta lip phase modulated auditory detection, even if information from silent movies was irrelevant to perform the task
Summary
People often visually focus on their interlocutors' mouth to match lip movements with sounds and to improve speech perception. The present study focuses on theta activity conveyed by moving lips because the speaker's mouth provides a direct source of visual speech information matching sounds. A study used intracranial recordings in epileptic patients to investigate the neural responses evoked by the perception of lip movements in the auditory cortex during the presentation of syllables in uni- or multimodal conditions (Besle et al, 2008). They reported activations in response to silent lip movements in the visual cortex followed by similar responses in the secondary auditory cortex, suggesting crossmodal activation via direct feedforward processes. Do purely visually induced theta speech rhythms impose time windows that render the auditory cortex more sensitive to inputs in a phasic manner? If the answer to this question is yes, visually focusing on your interlocutor's mouth when you have trouble understanding them would be an effective filter modulator to increase auditory sensitivity
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