The rhythms of mother-infant interactions and language

Caregiver-infant interactions rely on various rhythms (Markova et al., 2019), following repeated and predictable patterns, which may facilitate the development of regulatory functions and language. Co-regulation—a social process in which caregivers and infants adapt their behaviors to each other’s ongoing and anticipated actions (Fogel, 1993)—sets the ground for a shared understanding, providing an optimal foundation for language learning. Co-regulation can occur at different levels—behavioral, physiological, and affective—but the connections between these levels remain unclear. Research has shown that young children (ages 2-7) with enhanced self- regulatory skills have a larger expressive vocabulary compared to those with lower degrees of self- regulation (Montroy et al., 2016; Vallotton & Ayoub, 2011). Co-regulation is a precursor of self- regulation (Aureli et al., 2022), and word segmentation ability is a precursor to vocabulary development (Junge et al., 2012). However, the interplay between these two mechanisms during infancy remains unexplored. The present work has two primary objectives. First, it aims to examine whether there is an association between mother-infant co-regulation and infants’ word segmentation performance at nine months of age. Rhythmicity might be the linking element between social and language development, specifically between co-regulation and word segmentation. Second, given the connection between co-regulation and language at this age, the research investigates whether this connection is modality-specific or amodal. Specifically, it aims to explore whether the association varies based on the levels of co-regulation examined—namely behavioral, cardiac, or brain-to-gaze synchrony. The studies in the dissertation employ a multilevel methodological approach. An eye-tracking experiment assesses infants’ word segmentation performance using the well-established central- fixation procedure (Cooper & Aslin, 1990). In addition, infants and their mothers engage in a 5- minutes free-play interaction. In this context, on the one hand, videos are recorded and coded for overt processes (i.e., facial expressions and gaze behaviors). On the other hand, covert processes are measured using dual-electrocardiography (ECG) and functional near-infrared spectroscopy (fNIRS). This combination of methods allows for measurements of multilevel co-regulation between mother and infant, indexed by behavioral (i.e., emotional) synchrony (Study 1, Chapter 6.1), cardiac synchrony (Study 2, Chapter 6.2), and brain-to-gaze synchrony (Study 3, Chapter 6.3). To analyze rhythmic aspects of co-regulation, this research employs Recurrence Quantification Analysis, a mathematical tool that captures coordination dynamics during dyadic interactions across dimensions and timescales (Coco & Dale, 2014). Particularly, two metrics are used: a) recurrence rate (i.e., RR), indicating the level of co-regulatory processes (i.e., coordination) within interactions, and b) entropy (i.e., ENTR), reflecting the unpredictability of these co-regulatory processes. Drawing from the findings of the three empirical studies, the following theses are derived: 1) There is a significant connection between co-regulation and language processing already in infancy. Specifically, the ways in which mothers and their infants co-regulate are linked with the infant’s performance to segment words. Notably, rhythm detection may serve as a precursor for engaging in bodily co-regulation and for understanding linguistic structures, particularly in terms of speech processing and word segmentation. 2) The connection between co-regulation and word segmentation is evident across all modalities examined in this research. However, the inconsistent correlations across different metrics (RR and ENTR) and levels (overt and/or covert) of co-regulation suggest that this connection may be modality-specific. This variability implies that perceptual information—possibly related to the differing relevance of emotional, cardiac, and gaze signals—affects co-regulation at various developmental stages. In conclusion, all three studies in the dissertation find a connection between mother-infant co- regulation and infants’ language development, emphasizing their mutual interaction. This research paves the way for future investigations into interactions across domains (i.e., social and linguistic). Continuously expanding the areas of contact will enhance our understanding of cognitive development. Furthermore, this research suggests that advancements in language acquisition depend on considering socio-developmental factors, including early social experiences and caregiver-infant co-regulation.