Characterising caregivers' contingent responses: Short, slow, and clear speech facilitates infant turn-taking

Bidirectional temporal contingency in infant-caregiver interaction is believed to be instrumental in shaping the development of turn-taking and speech. Likewise, caregivers are known to modulate their voices in a distinctive register when speaking to infants. Yet, less is known about whether these modulations are features of caregivers' timely responses to infant vocalisations, and whether such features support infant participation in turn-taking bouts. We analysed 231 daylong home recordings of 111 mother-infant dyads (4-34 months). Mother and infant vocalisations were automatically identified and infant vocalisations classified as cries or non-cries. Contingent responses were vocalisations which followed the vocal offset of the preceding speaker within 2 s, allowing overlapping responses. Features of infant-directed speech (pitch, pitch variability, intensity, voice quality, duration and speech rate) were extracted from maternal vocalisations. Furthermore, alignment in pitch, intensity and voice quality of mother responses to infant vocalisations was calculated. Maternal responses were also automatically classified as single words, questions, declaratives, or imperatives. Mothers' contingent responses showed a simplified and acoustically salient profile compared to non-contingent speech: they were shorter and slower, with higher pitch variability, intensity and voice quality, and they were more likely to be single words and questions. Contingent responses that were shorter, slower and clearer (higher intensity and voice quality) were more often followed by infant vocalisation. These findings suggest that beyond temporal contingency, caregivers' brevity, acoustic clarity and question-asking may also support the turn-taking exchanges through which early communicative competence develops.