Older Adults Show Preserved Neural Signatures of Audiovisual Integration but Altered Links to Speech Comprehension

Older adults often experience difficulty understanding speech in noisy environments despite relatively preserved hearing. Although visual speech cues can improve speech perception, it remains unclear whether the neural mechanisms supporting audiovisual integration are preserved in healthy ageing. We addressed this question using magnetoencephalography (MEG) combined with Rapid Invisible Frequency Tagging (RIFT) during challenging audiovisual speech task. Forty younger and forty older adults attended to one of two competing speech streams while the speaker's lip movements were frequency tagged, with the visual modulation driven by either the attended or unattended speech envelope. Task-Relevant amplitude modulation of frequency tagging in visual speech improved speech comprehension in younger adults as previously reported for this cohort; this accuracy benefit was absent in older adults. Auditory and visual tagging responses were modulated by task relevance similarly across age groups, indicating largely preserved modality-specific sensory processing in older adults. Intermodulation responses, reflecting nonlinear interactions between auditory and visual speech signals, also remained sensitive to task relevance and did not differ in their condition-dependent modulation between age groups. Greater Task-Relevant versus Task-Irrelevant differences in intermodulation coherence were also associated with greater comprehension benefits in younger adults; no significant association was found in older adults, and this relationship differed significantly between age groups. These findings suggest that the absence of a comprehension benefit in older adults is linked to a change in the relationship between audiovisual intermodulation and behaviour, despite largely preserved task-sensitive brain responses.