Acoustic signatures of communicative dimensions in codified mother-infant interactions

Nursery rhymes, lullabies, or traditional stories are pieces of oral tradition that constitute an integral part of communication between caregivers and preverbal infants. Caregivers use a distinct acoustic style when singing or narrating to their infants. Unlike spontaneous infant-directed (ID) interactions, codified interactions benefit from highly stable acoustics due to their repetitive character. The aim of the study was to determine whether specific combinations of acoustic traits (i.e., vowel pitch, duration, spectral structure, and their variability) form characteristic “signatures” of different communicative dimensions during codified interactions, such as vocalization type, interactive stimulation, and infant-directedness. Bayesian analysis, applied to over 14 000 vowels from codified live interactions between mothers and their 6-months-old infants, showed that a few acoustic traits prominently characterize arousing vs calm interactions and sung vs spoken interactions. While pitch and duration and their variation played a prominent role in constituting these signatures, more linguistic aspects such as vowel clarity showed small or no effects. Infant-directedness was identifiable in a larger set of acoustic cues than the other dimensions. These findings provide insights into the functions of acoustic variation of ID communication and into the potential role of codified interactions for infants' learning about communicative intent and expressive forms typical of language and music.