Whose clock makes yours tick? Maternal-infant physiological adaptations during the first months of life.

In this longitudinal study, we examined the existence of a direct maternal-infant physiological relationship in the synchrony of respiratory sinus arrhythmia (RSA) between mothers and infants. RSA is considered as an essential physiological index of the capacity to self-regulate (e.g., Moore et al., 2009) and is a required developmental factor for later social participation (Friedman, 2007). We conducted paced breathing experiments with 11 mother-infant dyads when the infant was 1, 2, 4, 8 and 12 weeks old. We instructed mothers to breathe at 5 different rates (6-12-15-20-6 cycles per minute) with their infant lying on their body. Maternal-infant ECG and respiration were registered with simultaneous video recording. RR-interval (RRI), respiratory sinus arrhythmia (RSA) and respiration frequency (Fresp) were calculated. We analyzed mother-infant RSA response profiles, using an ANOVA approach followed by post-hoc testing of interaction terms. Polynomial and Helmert contrasts were used to evaluate respectively maternal-infant paced-breathing response differences and developmental differences. Results showed that a maternal-infant physiological relationship is manifest during the first two months of life. During this period, an infant is more susceptible to fluctuations in the RSA curves of their mother than on a later age. At the age of 3 months, maternal-infant differences in RSA were too large to determine a relationship. These results are in line with the 2-months developmental shift of social orientation.