Recognizability and timing of infant vocalizations relate to fluctuations in heart rate.

For human infants, producing recognizable speech is more than a cognitive process. It is a motor skill that requires infants to learn to coordinate multiple muscles of varying functions across their body. This coordination is directly linked to ongoing fluctuations in heart rate; a physiological process that can scaffold behavior. We investigated whether ongoing fluctuations in heart rate coincide with vocal production and word formation in 24-mo-old infants. Infants were most likely to produce a vocalization when heart rate fluctuations reached a peak (local maximum) or trough (local minimum). Vocalizations produced at the peak were longer than expected by chance. Functionally, vocalizations produced just before the trough, while heart rate is decelerating, were more likely to be recognized as a word by naive listeners. Thus, for the developing infant, heart rate fluctuations align with the timing of vocal productions and are associated with their duration and the likelihood of producing recognizable speech. Our results have broad and immediate implications for understanding normative language development, the evolutionary basis and physiological process of vocal production, and potential early indicators of speech and communication disorders.