Vocalisation is Progressively Decoupled From Autonomic Arousal Over The First Two Years of Life

Infant vocal production is closely linked to autonomic arousal yet the developmental transition from reflexive vocal production to vocalisations produced independent of internal state remains unclear. Using naturalistic daylong recordings from wearable devices, we examined the relationship between vocal output and heart rate in 92 infants aged 4 to 21 months. Early vocalisations occurred predominantly at heightened levels of autonomic arousal, but this coupling weakened progressively with age. Daylong heart rate time series associated with vocalisation timing above chance (ROC AUC = 0.64) and hierarchical linear mixed effects modelling confirmed a progressive developmental decoupling. Arousal also strongly associated with the acoustic features of both cry and non-cry vocalisations, with higher arousal associated with higher and more variable pitch, more harmonicity and longer duration. Notably, the influence of arousal diminished with age for non-cry vocalisations but remained stable for cries. This differential developmental trajectory suggests the emergence of parallel vocal pathways: one remaining tied to physiological state for fixed affective signals such as cries and laughs, and another increasingly decoupled from internal states for the functionally flexible signals critical to speech communication. Importantly, infants whose mothers responded contingently to a greater proportion of non-cry vocalisations showed significantly greater decoupling, suggesting that caregiver responsivity plays a role in this developmental trajectory.