Neural Speech Tracking Highlights the Importance of Visual Speech in Multi-speaker Situations.

Visual speech plays a powerful role in facilitating auditory speech processing and has been a publicly noticed topic with the wide usage of face masks during the COVID-19 pandemic. In a previous magnetoencephalography study, we showed that occluding the mouth area significantly impairs neural speech tracking. To rule out the possibility that this deterioration is because of degraded sound quality, in the present follow-up study, we presented participants with audiovisual (AV) and audio-only (A) speech. We further independently manipulated the trials by adding a face mask and a distractor speaker. Our results clearly show that face masks only affect speech tracking in AV conditions, not in A conditions. This shows that face masks indeed primarily impact speech processing by blocking visual speech and not by acoustic degradation. We can further highlight how the spectrogram, lip movements and lexical units are tracked on a sensor level. We can show visual benefits for tracking the spectrogram especially in the multi-speaker condition. While lip movements only show additional improvement and visual benefit over tracking of the spectrogram in clear speech conditions, lexical units (phonemes and word onsets) do not show visual enhancement at all. We hypothesize that in young normal hearing individuals, information from visual input is less used for specific feature extraction, but acts more as a general resource for guiding attention.