The within-person association of relative left frontal activity and vagally mediated heart rate variability not moderated by history of depression

Motivated by the Neurovisceral Integration Model (NVI) of cardiac vagal control, we investigated the relationship between relative left frontal activity (rLFA) and vagally mediated heart rate variability or respiratory sinus arrhythmia (RSA) in 287 participants, half of whom had a history of depression. We hypothesized that there would be a within-person association of rLFA and RSA such that when RSA is lower rLFA would also be lower (Hypothesis I). Moreover, it was hypothesized that this within-subject association would be moderated by a history of depression (Hypothesis II). Metrics of rLFA and RSA were derived from concurrent electroencephalogram and electrocardiogram recordings. The logarithmic difference in EEG alpha power between the homologous right and left electrodes (Ln (Right/Left)) in the frontal region was used to index rLFA. A Hilbert transform was applied to the mean-centered and bandpass-filtered (0.12-.40 Hz) inter-beat interval (IBI) time series to get a fine-grained measure (in the time domain) of RSA. A linear mixed ANOVA model with rLFA as the dependent variable and RSA as the main fixed effect found that participants had less rLFA during epochs when they had lower RSA, which was consistent with the prediction from Hypothesis I. Contrary to the prediction from Hypothesis II, the within-person association of RSA and rLFA was not moderated by a history of depression. However, the association between RSA and rLFA varied across the four pairs of frontal electrodes that we examined. Thus, more research is needed to determine the spatial extent of this association, e.g., examining the relationship between source-localized rLFA and RSA.