Vocal Dynamics of Caregiver-Child Interactions During Shared Book Reading Sessions*

There is an established body of research showing that both the quality and quantity of caregiver input plays an integral role in infants’ language development. Equally, infants play an active role in eliciting that input, with their interests also guiding what they learn. In this study, we examined monolingual German children to see whether caregivers’ perception of their child’s interest predicts the quality of caregiver speech as well as infant vocalisations during a shared book-reading interaction at two timepoints, 18 months and 24 months. We also aimed to examine longitudinal changes in prosodic qualities of infant-directed speech over infant development. Caregivers read two books to their child, one of which they rated to be of high interest to their child, while the other was rated to be of low interest to the child. We measured the pitch range, mean pitch and duration of caregiver utterances as well as the number of infant vocalisations during the book reading sessions. We found that the duration of utterance varied significantly, with caregivers producing shorter utterances when reading high interest books to their child at $\mathbf{1 8}$ months and producing longer utterances reading high interest books to their older, $\mathbf{2 4}$-month-old infant. Infants also vocalised more during high interest books, which we interpret in terms of their active elicitation of information during reading. Longitudinally, we observed that pitch range increased and mean pitch decreased (approaching adultdirected speech), aligning with previous research on infantdirected speech. These findings help underscore the dynamic feedback loops of caregiver-child interactions which play a pivotal role in early development, with children seeking information and caregivers responding contingently.