Audio-visual cues in language development: Do visual speech cues enhance infants’ cortical speech tracking?
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2022-03-17
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en
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Infants are sensitive to audio-visual speech cues from a very young age. In this study, we investigated the role of visual speech cues, such as a speaker’s rhythmic movements of the mouth, lips and jaw, on infants’ cortical speech tracking. In adults, research has shown that seeing congruent audio-visual speech enhances neuronal tracking of the speech envelope. This is found specifically between 2-7 Hz which corresponds to the rate in which syllables appear in adult-directed speech and thus the frequencies in which mouth movements are tightly locked to the speech envelope. Using electroencephalography (EEG), we investigated whether 10-month-old Dutch infants (N=7) also show enhanced cortical speech tracking when a speaker’s visual speech cues are visible compared to a condition in which the speaker’s mouth and jaw movements are occluded with a block. We predicted that visual speech cues would enhance infants’ cortical speech tracking, specifically at the syllable rate (2.5–3.5 Hz for our stimuli) and stressed syllable rate (1–1.75 Hz). First, our results show that infants looked significantly longer towards audio-visual stimuli compared to stimuli in which the speaker’s visual speech cues were occluded. Furthermore, our findings suggest that visual speech cues indeed enhanced infants’ cortical speech tracking. However, this was not found at the syllable or the stressed syllable rate, but instead at 3.75–4 Hz, a frequency range that corresponds to the theta band in infants. Spectral power of the EEG signal was also found to be enhanced by the presence of visual speech cues at the theta range (3.75–4 Hz). Our results suggest that theta-band oscillations may play a role in merging multi-modal information, such as the visual speech signal and the speech envelope. Furthermore, our findings provide further evidence for infants’ sensitivity to audio-visual cues and highlight their influence on infants’ language processing.
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Faculteit der Sociale Wetenschappen