Cortical tracking of unheard formant modulations derived from silently presented lip movements and its decline with age

The integration of visual and auditory cues is crucial for successful processing of speech, especially under adverse conditions. Recent reports have shown that when participants watch muted videos of speakers, the phonological information about the acoustic speech envelope is tracked by the visual cortex. However, the speech signal also carries much richer acoustic details, e.g. about the fundamental frequency and the resonant frequencies, whose visuo-phonological transformation could aid speech processing. Here, we investigated the neural basis of the visuo-phonological transformation processes of these more fine-grained acoustic details and assessed how they change with ageing. We recorded whole-head magnetoencephalography (MEG) data while participants watched silent intelligible and unintelligible videos of a speaker. We found that the visual cortex is able to track the unheard intelligible modulations of resonant frequencies and the pitch linked to lip movements. Importantly, only the processing of intelligible unheard formants decreases significantly with age in the visual and also in the cingulate cortex. This is not the case for the processing of the unheard speech envelope, the fundamental frequency or the purely visual information carried by lip movements. These results show that unheard spectral fine-details (along with the unheard acoustic envelope) are transformed from a mere visual to a phonological representation. Aging affects especially the ability to derive spectral dynamics at formant frequencies. Since listening in noisy environments should capitalize on the ability to track spectral fine-details, our results provide a novel focus on compensatory processes in such challenging situations.