Physiological phenomena exhibit complex behaviours arising at multiple time scales. To investigate them, techniques derived from chaos theory were applied to physiological signals, providing promising results in distinguishing between healthy and pathological states. Fractal-like properties of electrodermal activity (EDA), a well-validated tool for monitoring the autonomic nervous system state, have been reported in previous literature. This study proposes the multiscale complexity index of electrodermal activity ($MComEDA$) to discern different autonomic responses based on EDA signals. This method builds upon our previously proposed algorithm, $ComEDA$, and it is empowered with a coarse-graining procedure to provide a view at multiple time scales of the EDA response. We tested $MComEDA$’s performance on the EDA signals of two publicly available datasets, i.e., the Continuously Annotated Signals of Emotion (CASE) dataset and the Affect, Personality and Mood Research on Individuals and Groups (AMIGOS) dataset, both containing physiological data recorded from healthy participants during the view of ultra-short emotional video clips. Our results highlighted that the values of $MComEDA$ were significantly different (p-value < 0.05 after Wilcoxon signed rank test with Bonferroni’s correction) when comparing high- and low-arousal stimuli. Furthermore, $MComEDA$ outperformed the single-scale approach in discriminating among different valence levels of high-arousal stimuli, e.g., showing significantly different values for scary and amusing stimuli (p-value = 0.024). These findings suggest that a multiscale approach to the nonlinear analysis of EDA signals can improve the information gathered on task-specific autonomic response, even when ultra-short time series are considered.

Parent-infant synchrony is as a dynamic, bi-directional, multimodal process through which partners mutually adjust their physiological, neural, and behavioral states in time. Here, we systematically reviewed the available evidence on parent-infant physiological and neural synchrony during the first 12 months of life. Out of 3786 records initially identified, 42 studies met our inclusion criteria: 27 employed peripheral physiological measures (e.g., ECG, respiration) and 15 utilized central neural measures (e.g., EEG, fNIRS). Studies relying exclusively on behavioral indices, hormonal samples, or single-participant intrapersonal coupling were excluded. Findings reveal that parent-infant synchrony emerges as a dynamic, context-dependent process in which autonomic and neural signal flexibly adapt to the emotional demands of the interaction, becoming most robust during embodied, multisensory, and affectively charged episodes. Further, synchrony functions neither as uniformly protective nor risky but as a bi-directional, multimodal process that can support resilience or reflect co-dysregulation. Its magnitude depends on parental characteristics, dyadic relationship quality, and interactional context. Current parent-infant synchrony research is limited by methodological constraints, small and homogeneous samples, heterogeneous preprocessing pipelines, and difficulties aligning physiology, behavior, and neural data, highlighting the need for multi-level, longitudinal, and culturally diverse designs. Overall, this review underscores synchrony as a dyadic, bidirectional process that emerges across the first year of life and lays the foundation for socio-emotional development. It warrants further investigation through expanded research designs, greater population diversity, and multimethod approaches to better elucidate its cascading effects on the parent-child relationship and child well-being.

Preterm birth is associated with alterations in early caregiver–infant regulation, with potential consequences for socio-emotional and physiological development. However, the mechanisms through which early interactional experience shapes these processes remain unclear. Here, we tested whether a structured dyadic intervention could modify co-regulatory dynamics across physiological, behavioral, and relational levels. Fifty-four 7-month-old preterm infants and their parents were assigned to either a shared book reading intervention (n = 22) or an active control condition based on a shared building activity (n = 32) and compared with 39 full-term infants. The intervention consisted of an 8-week program of shared book reading, designed to structure parent–infant interaction. Physiological synchrony was assessed at the dyadic level, alongside infants’ autonomic regulation and cardiovascular signal complexity. Behavioral engagement and parental attachment representations were also evaluated. Results showed that mother–infant physiological synchrony emerged selectively within the interactional context trained by the intervention and only in the intervention group. This context-specific synchrony was accompanied by modulation of vagal activity and increased cardiovascular complexity in preterm infants, consistent with enhanced flexibility of autonomic control. At the behavioral and relational levels, intervention infants showed increased initiating joint attention, while parents reported higher secure attachment. These findings support a model of experience-dependent early synchrony, in which repeated dyadic interaction through shared book reading shapes the coupling between interpersonal coordination and individual physiological regulation. By linking synchrony, autonomic flexibility, and social engagement, this study identifies a mechanism through which early caregiving experience can organize developmental trajectories following prematurity.

Synchrony is a key mechanism that builds up the foundations of human interactions. Quantifying the level of physiological synchronization that occurs during dyadic exchanges is essential to fully comprehend social phenomena. We present a new index to characterize the coupling of complex physiological dynamics: the optimized Multichannel Complexity Index ( op MCI). We validated this approach using synthetic time series of two coupled Hénon Maps, with four different coupling levels in unidirectional and bidirectional manners. We demonstrated that the op MCI method allows to effectively discern between all coupling levels. Then, we applied the op MCI metric on heart rate variability data collected from 37 parent-infant dyads, during shared reading and playing activities, in the framework of the Shared Emotional Reading (SHER) project, with the aim of assessing the effects of early intervention in preterm babies. Two groups presented preterm infants: an intervention group, who participated in a two-month shared reading program, and a control group, who practiced shared play activities. A full-term group provided additional control data. The op MCI values were significantly higher for the intervention dyads with respect to the other groups during the shared reading task, showing that an early reading intervention program could increase parent-infant synchrony in preterm babies. Graphical Highlights: Interpersonal physiological synchrony is a fundamental mechanism for the evolutionary path An ad-hoc methodological framework to assess nonlinear cardiac coupling is lacking The op MCI has been shown to be a reliable tool for assessing the degree of synchrony Early shared book reading may increase parent-infant synchrony in a clinical setting

Recent advances in physiological sensing technologies enable continuous acquisition of multichannel biosignals for digital health applications. Because physiological interactions arise from nonlinear mechanisms, their characterization requires analytical approaches capable of capturing both individual signal dynamics and cross-signal dependencies, often from short recordings. This study proposes a multilayer visibility graph (VG) framework free of phase-space embedding parameters for analyzing physiological interactions from multichannel biosignals. The approach was evaluated in a simulation study based on unidirectionally coupled Rössler oscillators and in two physiological datasets comprising synchronized cardiovascular recordings from healthy adults and wearable electrodermal activity (EDA) recordings acquired during a Mindfulness-Based Stress Reduction (MBSR) intervention. Average Degree (DA) was used to characterize the topological organization of individual and multiplex visibility graphs, while inter-layer mutual information (I) between degree sequences was introduced to quantify cross-layer dependence. In the simulation study, coupling-related changes in the response dynamics were already detectable from single-layer VGs at short window lengths, whereas inter-layer mutual information captured coupling-dependent changes in cross-layer organization. Feature estimates began to stabilize at approximately 480 samples and approached a plateau at longer window lengths. In the cardiovascular dataset, sex-related differences were primarily observed in configurations involving systolic blood pressure: DA differed in single-layer and multichannel representations, while inter-layer mutual information showed significant sex differences, specifically in the interbeat interval-systolic blood pressure duplex. In the EDA dataset, the proposed descriptors identified changes in trainer–trainee physiological organization across the MBSR intervention. These findings indicate that VG-based descriptors provide complementary information on within-signal topology and inter-layer dependence and can be applied to short multichannel physiological recordings without phase-space reconstruction.

Singing to infants is a universal human practice that has beneficial effects on infant’s cognitive and affective development. Children born preterm have impaired brain development, and their perception of maternal speech is known to be affected by the atypical hospital auditory environment. Understanding how preterm infants perceive maternal singing is of critical importance, yet it remains largely unexplored. Using high-density EEG, we examined neural responses to the same melody presented through maternal singing, stranger singing, and instrument, and compared the responses in 12 preterm infants. Moreover, to examine their processing of spatialisation, auditory stimuli were presented under monaural and binaural conditions. Preterm newborns are able to discriminate the same melody when sung by their mother, a stranger, or played by an instrument. When presented monaurally, the mother’s singing voice enhances widespread brain synchrony across the entire scalp. In contrast, this synchrony diminishes with binaural spatialization. These findings suggest that maternal singing constitutes a highly salient auditory stimulus for preterm newborns, eliciting a distinct neural signature. Given that brain synchrony is a critical component of healthy brain function and development, harnessing maternal singing may offer a promising, natural intervention to support neurodevelopment—particularly in vulnerable populations such as preterm infants.