Margherita Barbetti

aDepartment of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parma, Italy bSchool of Social Ecology, c/o Department of Psychological Science, University of California Irvine, 4201 Social and Behavioral Sciences Gateway, Irvine, CA, 92697, USA cDepartment of Psychology and Neuroscience, Baylor University, Waco, TX, USA dDepartment of Psychology, University of California Irvine, 4201 Social and Behavioral Sciences Gateway, Irvine, CA, 92697, USA

1 INTRODUCTION Regulation of cardiac rhythm is complex and occurs at least at two major levels: the intrinsic “coupled-clock” system within the sinoatrial node, the cardiac pacemaker, and the autonomic nervous system (ANS) (Behar & Yaniv, 2016; Tsutsui et al., 2018). Mutual entrainment between intrinsic and ANS mechanisms primarily determines the average interbeat interval (IBI) for a segment of time and IBI variations from beat to beat.

The identification of reliable biomarkers of transcutaneous auricular vagus nerve stimulation (taVNS) responsiveness is a key challenge both at the clinical and preclinical level. Vagally-mediated heart rate variability (vmHRV), a surrogate measure of cardiac vagal efferent activity, is an ideal candidate.