Blood Volume Pulse (BVP) Derived Vagal Tone (VT) between 5 and 7 Years of Age: A Methodological Investigation of Measurement and Longitudinal Stability ABSTRACT: The present study evaluated the possibility of collecting cardiac vagal tone data using a photoplethysmograph, and its stability and continuity in a longitudinal sample of early-school aged children. A method for the optical (i.e., blood volume pulse) estimation of heart rate was established in a pilot study. Then the longitudinal stability and continuity in photoplethysmograph- derived vagal tone was assessed in 114 children (56 girls) at three sessions between 5 and 7 years of age. Results indicate that this method possesses sub- stantial measurement reliability and individual stability, as children report low intra-individual variation over time. Children also report a mean decrease in vagal tone from 5 to 7 years of age, consistent with increased attentional capaci- ty. Overall, this suggests blood volume pulse estimation of vagal tone is both accurate and appropriate for naturalistic developmental research. ß 2012 Wiley Periodicals, Inc. Dev Psychobiol Keywords: vagal tone; children; RSA; heart rate variability INTRODUCTION Heart rate variability (HRV) is a biomarker of auto- nomic nervous system (ANS) function that has been broadly employed as a physiological correlate of be- havior, including approach behaviors (Beauchaine, Gatzke-Kopp, & Mead, 2007), developmental trajectory (DiPietro, Bornstein, Hahn, Costigan, & Achy-Brou, 2007), conduct disorders (Kibler, Prosser, & Ma, 2004), and attention (Richards & Casey, 1991). It is most com- monly operationalized as an estimation of respiratory sinus arrhythmia (RSA), a phenomenon that reflects the underlying parasympathetic dominance of the ANS at rest; here termed vagal tone (VT, Porges, 1985). In children, VT has been found to correlate with individu- al differences in attention and socio-emotional compe- tencies (e.g., Calkins & Keane, 2004; Fox, Schmidt, Henderson, & Marshall, 2007; Porges, 2011). VT is typically derived from a laboratory based electrocardio- gram (ECG), however many domains of developmental research are better pursued outside of a laboratory set- ting, such as children’s social adjustment and emotional regulation. Given the place of VT as an important ele- ment within children’s psychobiological development and regulation (e.g., Porges, Doussard-Roosevelt, Por- tales, & Greenspan, 1996), in this study we investigate the possibility of accurately and validly measuring Developmental Psychobiology James A. J. Heathers Elian Fink Rebecca-Lee Kuhnert Marc de Rosnay School of Psychology University of Sydney, Sydney New South Wales, Australia E-mail: marc.derosnay@sydney.edu.au Manuscript Received: 19 July 2012 Manuscript Accepted: 5 October 2012 Correspondence to: M. de Rosnay Contract grant sponsor: Australian Research Council’s Discovery Projects Contract grant number: DP0881855 Article first published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/dev.21088  ß 2012 Wiley Periodicals, Inc.