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

Previous research shows that infant vocal output varies with fluctuations in autonomic arousal. It remains unclear, however, how infants transition from reflexive vocal production to being able to produce vocalisations independent of internal state. Using naturalistic daylong recordings with wearable devices, we demonstrate that early vocalisations occur at heightened levels of autonomic arousal, measured via heart rate, in infants aged 4-21 months (n=92), but that this relationship becomes progressively decoupled over development. Daylong heart rate time series discriminated vocalisation timing above chance (ROC AUC = 0.64), and hierarchical linear mixed effects modelling demonstrated this relationship became significantly weaker over age. Arousal also strongly associated with the acoustic features of infants’ cry and non-cry vocalisations, with vocalisations produced at higher arousal being higher-pitched and more variable in pitch, more harmonic and longer in duration. The influence of arousal on non-cry acoustic features diminished significantly with age, as opposed to cry acoustics which remained tied to arousal. 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 verbal communication. We also found that mothers who contingently responded to a greater proportion of their infants’ non-cry vocalisations had infants who were significantly more decoupled, suggesting that caregiver responsivity may play a role in this progressive process.