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Last decennia the musical characteristics of mother-infant dialogues were regularly emphasized and recent research (Van Puyvelde et al., 2010, 2013) showed that specific tonal aspects are related with crucial social-affective micro-aspects of a dyadic interaction. During mother-infant bonding moments of affect repair, the observed interrelated pitch frequency ratios of their vocals show a similar pattern as present in a harmonic and pentatonic series with a strong incidence of consonant simple frequency ratios. In contrast, during moments of prolonged disengagement or increased arousal, their vocal pitches did not show this pattern and appeared to be primarily dissonant. Moreover, the amount of simple and complex frequency ratios in relation to a shared tonal center (F0 of the harmonic series) was in line with the hierarchical structure of harmonic series (i.e., 70% high consonance, 20% medium consonance and 5% low consonance or dissonance). In this study, we wanted to know whether tonal synchrony is related with physiological events on a dyadic level. An important developmental task of an infant is to require the capacity to self-regulate. On a physiological level, self-regulation has been regularly related with cardiac vagal control or respiratory sinus arrhythmia (RSA). Moreover RSA has become a well-used measure in infant research as it can be obtained in a non-invasive way by ECG-respiratory registration during experimental research (e.g., Feldman, 2006). RSA reflects a component of the natural heart rhythm variability (HRV), that is manifest during a respiration cycle as the result of the regulation of the autonomic nervous system, mediated by the parasympathetic branch, more specific the nervus vagus (Berntson et al., 1997). During inspiration, heart rate accelerates, whereas, during expiration, heart rate decelerates. The degree of RSA thus gives an indication of the vagal control of the heart (Berntson et al., 1997). RSA as a multiply determined physiological variable. Besides vagal outflow, RSA reflects, among others, respiratory and somatomotor metabolic parameters, such as movement and talking speech (Berntson et al., 2007). It was thus not possible to study a physiological-vocal coupling with regard to mother-infant dialogues. Therefore, we presented two musical fragments to mothers and their three-month old infants (tonal fragment based on the hierarchical order of a harmonic series versus atonal fragment without tonal structure). RSA was analyzed by means of peak-valley method and respiration corrected by means of within-subject analyses. Statistical analyses were conducted with a mixed effects model that included motor activity during the experiment as a covariate. Results showed increased infant RSA from baseline to tonal music and no significant difference between baseline and atonal music. In the mothergroup, only mothers that heard the difference between the tonal and non-tonal fragment and that hold their infant close to the body showed increased RSA during tonal music. Other mothers did not show significant differences. Results will be discussed in relation to the nature-nurture debate concerning the biological versus cultural roots of consonant and dissonant music, the need to include motor and respiratory components in cardiovascular infant research and maternal-infant socio-affective development.

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 has regularly been related with infant self-regulation and has become a well-used measure in infant research as it can be obtained in a non-invasive way by ECG-respiratory registration during experimental research (e.g., Feldman, 2006). RSA reflects a component of the natural heart rhythm variability (HRV), that is manifest during a respiration cycle as the result of the regulation of the autonomic nervous system, mediated by the parasympathetic branch, more specific the nervus vagus (Berntson et al., 1997). During inspiration, heart rate accelerates, whereas, during expiration, heart rate decelerates. The degree of RSA thus gives an indication of the vagal control of the heart (Berntson et al., 1997). RSA as a multiply determined physiological variable. Besides vagal outflow, RSA reflects, among others, respiratoryand somatomotor metabolic parameters, such as movement and talking speech (Berntson et al., 2007). Therefore, inclusion of respiratory and movement components in analyses are needed. In the present study, 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. Results are discussed with regard to infants' vagal maturation and the need to include respiratory components in cardiovascular infant research.