The neurovisceral integration model proposes that information flows bidirectionally between the brain and the heart via the vagus nerves and vagally-mediated heart rate variability (vmHRV) can be used to index heart-brain interaction. Recent research has shown that voluntary reduction of breathing rate (slow-paced breathing, SPB) can enhance cardiac vagal control. Additionally, prefrontal transcranial direct current stimulation (tDCS) can modulate the excitability of the prefrontal region and influence the vagus nerve. However, fundamental research on the combination of SPB and prefrontal tDCS to increase vmHRV and other physiological indices of the autonomic nervous system is scarce. Therefore, 200 healthy participants were assigned to four experimental groups. Each group received either 20 min of active or sham tDCS combined with 5.5 breath per minute (BPM) or 15 BPM breathing. Regardless of the tDCS condition, the SPB group showed a significant increase in vmHRV over 20 minutes, suggesting an increase in parasympathetic activity. In addition, a significant decrease in HR at the first and second 5-minute epochs of the intervention. Regardless of breathing condition, the active tDCS group exhibited higher HR at the fourth 5-minute epoch of the intervention compared to the sham tDCS group, suggesting more sympathetic arousal. However, there was no combined effect on vmHRV, HR, skin conductance, or blood pressure. SPB is a robust technique for increasing vmHRV, whereas prefrontal tDCS may produce effects that counteract those of SPB. More research is necessary to test whether and how top-down and bottom-up approaches can be combined to improve vagal control.

The tendency to ruminate (i.e., repetitive, self-referential, negative thoughts) is a maladaptive form of emotional regulation and represents a transdiagnostic vulnerability factor for stress-related psychopathology. Vagally-mediated heart rate variability (HRV), reflecting parasympathetic nervous system activity, is commonly used as a physiological marker of stress regulation. Past research has suggested a link between trait rumination and resting HRV at baseline; however, inconsistent results exist in healthy individuals. In this study, we investigated the association between the tendency to ruminate and resting HRV measured at baseline in a healthy population using a large cross-sectional dataset (N = 1189, 88% women; mean age = 21.55, ranging from 17 to 48 years old), which was obtained by combining samples of healthy individuals from different studies from our laboratory. The results showed no cross-sectional correlation between resting baseline HRV and trait rumination (confirmed by Bayesian analyses), even after controlling for important confounders such as gender, age, and depressive symptoms. Also, a nonlinear relationship was rejected. In summary, based on our results in a large sample of healthy individuals, baseline resting HRV is not a trait marker of the tendency to ruminate.