Largely distinct networks mediate perceptually-relevant auditory and visual speech representations

Visual speech is an integral part of communication, but it remains unclear whether information carried by lip movements is represented in the same brain regions that mediate acoustic speech comprehension. Our ability to understand acoustic speech seems independent from that to understand visual speech, yet neuroimaging studies suggest that the neural representations largely overlap. Addressing this discrepancy, we tested where the brain represents acoustically and visually conveyed word identities in a full-brain MEG study. Our analyses dissociate cerebral representations that merely reflect the physical stimulus from those that also predict comprehension, and suggest that these overlap only in specific temporal and frontal regions. Moreover, representations predictive of auditory and visual comprehension converge only in angular and inferior frontal regions. These results provide a neural explanation for the behavioural dissociation of acoustic and visual speech comprehension and suggest that cerebral representations encoding word identities may be more modality-specific than often upheld.