Audio–visual congruency alters power and coherence of oscillatory activity within and between cortical areas

• We studied the influence of congruency on audio–visual speech perception.
• Oscillatory activity was modulated by congruency in the theta-, beta, and gamma band.
• Modulations were found in auditory cortex, Broca's area, and supramarginal gyrus.
• Auditory cortex and Broca's area were coupled by coherence in the theta band.
• Gamma power and theta coherence were found to be coupled for congruent stimuli.

Dynamic communication between functionally specialized, but spatially distributed areas of the brain is essential for effective brain functioning. A candidate mechanism for effective neuronal communication is oscillatory neuronal synchronization. Here, we used magnetoencephalography (MEG) to study the role of oscillatory neuronal synchronization in audio–visual speech perception. Subjects viewed congruent audio–visual stimuli of a speaker articulating the vowels /a/ or /o/. In addition, we presented modified, incongruent versions in which visual and auditory signals mismatched. We identified a left hemispheric network for processing congruent audio–visual speech as well as network interaction between areas: low frequency (4–12 Hz) power was suppressed for congruent stimuli at auditory onset around auditory cortex, while power in the high gamma (120–140 Hz)-band was enhanced in the Broca's area around auditory offset. In addition, beta-power (20–30 Hz) was suppressed in supramarginal gyrus for incongruent stimuli. Interestingly, coherence analysis revealed a functional coupling between auditory cortex and Broca's area for congruent stimuli demonstrated by an increase of coherence. In contrast, coherence decreased for incongruent stimuli, suggesting a decoupling of auditory cortex and Broca's area. In addition, the increase of coherence was positively correlated with the increase of high gamma-power. The results demonstrate that oscillatory power in several frequency bands correlates with the processing of matching audio-visual speech on a large spatio-temporal scale. The findings provide evidence that coupling of neuronal groups can be mediated by coherence in the theta/alpha band and that low frequency coherence and high frequency power modulations are correlated in audio–visual speech perception.