Effects of Stress on Physiological Synchrony

Abstract

A need for connectedness is balanced with a striving for individuality when navigating the social environment. Feelings of belongingness are shaped by interpersonal synchronization, i.e., the temporal correspondence of processes between individuals. On a physiological level, physiological synchrony (PS) quantifies the interrelation of autonomic or endocrinological processes between interaction partners. However, research gaps remain regarding PS and its emergence and consequences. Stress, a ubiquitous part of human experiences, could influence the degree to which people synchronize physiologically, thus providing insights into when and how PS arises. Stressors elicit a multifaceted stress response with far-reaching effects on the organism. Stress effects reach beyond the individuals and may extend to physiological synchronization, potentially influencing social connections. A prerequisite for analyzing the effects of stress on PS is choosing an appropriate methodology for quantifying synchrony. Various approaches are used in the literature, but it is unclear which approach is suited for what situation. The first project of this thesis provides an introduction to selected methods and their implementation. The overview reveals the assumptions and areas of application for different approaches, including guidance on which method is suitable for which type of research question and data. The second and third projects of the thesis investigated whether acute stress influences PS using different biomarkers (cortisol and heart rate). The second project focused on synchrony of the endocrine system, i.e., cortisol data, to examine the effect of a laboratory stressor on cortisol PS in groups of strangers. N = 44 groups (138 participants; mean age = 23.48 ± 3.99, 47.1% female) underwent a stress induction or nonstressful control paradigm. Cortisol PS, determined by stability-and-influence modeling, was attenuated in the stress condition, showing that stress reduces group-level physiological connectedness. An exploratory analysis revealed no significant sympathetic nervous system synchrony within groups. The third project investigated the effect of one-sided stress on post-stress autonomic PS (i.e., PS in parasympathetic and sympathetic components of heart rate) in romantic couples (N = 75 dyads; mean age = 22.66 ±2.99, 51% female). Partners separately underwent either a laboratory stressor or a nonstressful control paradigm. When reunited, they were not informed about each other’s stress state while completing three nonstressful tasks together. Heart rate synchrony was measured via a time-frequency analysis of PS, i.e., cross-wavelet power. Couples with a stressed partner showed lower synchrony than nonstressed couples, indicating that stress reduces couples’ physiological connection even after an acute stressor. Across projects, stress reduced PS, implying an interplay between the two phenomena. The mechanisms behind this effect are not yet identified, but multiple explanations are possible. Stress effects on PS may be related to attentional or physiological processes that could hinder synchronization. However, reduced PS in a stressful context may also be protective to avoid the spread of stress in the collective. While a short-term reduction in PS may be adaptive, chronic stress might lead to a long-term attenuation of synchrony with potential detrimental consequences. Stress can be described as an interpersonal phenomenon that may affect the fundamental building blocks of belonging and individuation.