Infant-caregiver vocal contingency changes across varying interpersonal space.

Vocal contingency between caregivers and infants is a fundamental building block for socio-communicative development. This paper investigates vocal contingency in home settings and how it varies as a function of the physical distance between infants and their caregivers. We recorded 61 mother-infant dyads when infants were 5-14 months old during long-form home recording sessions (M = 7.2 h; SD = 2.3; Range = 1-13), using wearable audio and proximity sensors. In total, 737 h of indoor audio data were processed with machine learning-based pipelines to extract and classify vocalisations automatically. When we considered all ages and proximities, we observed that mothers vocalised contingently to their infants, but that infants did not. Unexpectedly, above-chance contingency did not increase with age for either partner. However, contingency varied with proximity in a quadratic fashion: it strengthened when dyads were closer, peaking at approximately 0.4 m, and decreasing thereafter. This profile remained stable across ages. Importantly, above-chance contingency was also observed in infants (in addition to mothers) at proximities that fell between 0.3-0.5 m. The selective emergence of infant vocal contingency at these proximities suggests that being within arm's reach may potentiate vocal contingency in infancy.