Differential roles of delta and theta oscillations in understanding semantic gist during natural audiovisual speech perception: functional and anatomical evidence

Understanding the main topic of naturalistic speech in a multi-speaker environment is demanding though the availability of visual speech can be beneficial for speech comprehension. Recent studies provided evidence that low-frequency brain rhythms play an important role in the processing of acoustic speech features. However, at present, the neural dynamics of brain rhythms implementing a higher-order semantic system during naturalistic audiovisual speech perception is unknown. Here we investigated information processing carried by low-frequency oscillations in delta and theta bands for audiovisual speech integration for high-level semantic gist processing using a representational interaction approach. By manipulating the degree of high-level semantic content (speech chunks with high versus low topic probability) using Latent Dirichlet Allocation (LDA) topic modelling algorithm and complexity of speaker environment (single versus multi-speaker), we first found that delta and theta phase exert distinctive roles in high-level semantic processing where delta phase represents auditory and visual inputs synergistically whereas theta band does so redundantly. Next, we show both forms of representational interaction are observed to be greater for speech with low semantic gist, supported by speech comprehension and white matter tractography. Furthermore, we show that the delta phase-specific synergistic interaction in the right auditory, temporal, and inferior frontal areas is sensitive to the speaker environment, whereas theta band activity showing redundant representations is sensitive to semantic content. Our results shed new light on dynamic neural mechanisms of implementing higher-order semantic systems through representational interactions between audiovisual speech information and differential roles of delta and theta bands depending on the speaker environment.