Physiological Synchrony Across Modalities Within and Between Individuals: A Methodological Exploration of Crossmodality.

This methodological study aims at exploring several extensions of physiological synchrony. As yet, synchrony is commonly analyzed within the same physiological modality and between different people, whereas in the present project we were specifically interested in crossmodal synchronies between people (interpersonal), as well as within the same person (intrapersonal). In recent years, interpersonal synchrony has received increased attention as a research field and a methodology in various disciplines where social interaction is pivotal. Synchrony is often computed on a time scale of a few seconds, predominantly based on physiological measures or motor behavior. Systematic reviews and meta-analyses have shown that interpersonal synchrony is a phenomenon that characterizes people's social interaction in different contexts. Synchrony arises spontaneously in social systems, often outside the awareness of the persons interacting, and is often considered a signature of relationship quality. Synchrony was also shown to be induced by salient stimulation contexts, for example by music synchronizing concert audiences. To explore such synchronies, we used a large dataset of 703 participants who listened to classical music in a concert setting. Heart rate, heart-rate variability, electrodermal activity, breathing behavior, and respiration rate were recorded simultaneously in all participants. Using the surrogate synchrony algorithm (R package SUSY), which is based on cross-correlations, the participants' crossmodal synchronies were determined for all 10 combinations of these 5 modalities. The psychological meaning of physiological synchronies is the goal of empirical aesthetics, and findings were published previously. The present study, however, concerns methodological topics of multimodal synchrony: do crossmodal synchronies arise intra- and interpersonally? Using surrogate tests as a control condition, we found that about half of such synchronies were significantly present in the dataset at substantial effect sizes. We additionally explored the lag structure of the underlying cross-correlations to describe the leading-following dynamics between synchronized physiological modalities. We expected that crossmodal synchronies of modalities representing sympathetic activation would be significantly synchronized positively (in-phase synchrony). This was, however, not always the case, as co-activation of sympathetic and parasympathetic signals was also found. In conclusion, the present findings on crossmodal synchronies provide novel information and suggest new measures for synchrony research, which is relevant for both induced synchrony and synchrony of social interaction.