Response: Commentary: Heart rate variability and self-control–A meta-analysis

Citation: Zahn D, Wenzel M and Kubiak T (2016) Response: Commentary: Heart rate variability and self-control–A meta-analysis. We read the commentary on our meta-analysis with great interest and we greatly appreciate that the authors developed suggestions for further research on heart rate variability (HRV) and self-control. While we mostly support these suggestions, we believe that it is necessary to clarify some points raised with respect to our meta-analysis. The first point brought up by the authors of the commentary is the focus of our meta-analysis on HRV instead of vagal tone and the inclusion of studies using the standard deviation of RR intervals (SDNN). They argue that only vagal tone and HRV metrics reflecting vagal tone are expected to be linked to self-control according to both the Neurovisceral Integration model (Thayer and Lane, 2000; Thayer et al., 2009) and Polyvagal theory (Porges, 2007). We agree with the authors that parasympathetic or vagal tone should be more strongly related to self-control according to both theories. However, we would like to emphasize that we only included studies using SDNN, if they also reported HF or RMSSD, two metrics reflecting vagal tone. We also performed a moderator analysis with respect to HRV metrics, which did not reveal a moderating effect: In fact, all three metrics revealed similar effect sizes (Zahn et al., 2016, see Table 4), but for SDNN the effect size was not significant, probably due to the small number of studies and effects referring to SDNN. In sum, while the inclusion of studies using SDNN could be criticized from a theoretical point of view, we carefully evaluated the impact of the inclusion of SDNN on the results of our meta-analysis and did not find any evidence for an impact on the overall relationship between HRV and self-control. The second issue raised in the commentary refers to the fact that we only included studies on HRV at rest, thereby excluding studies on HRV/vagal reactivity. The authors of the commentary reason (1) that this is not clearly stated throughout our manuscript, which might lead to an overgeneralization of our findings, and (2) that HRV reactivity might be associated more strongly with self-control. However, we believe that we made the focus of the current meta-analysis on HRV at rest quite clear because we emphasized HRV at rest in the abstract and also explicitly mentioned the exclusion of studies on HRV reactivity as a limitation …