Transcranial Direct Current Stimulation Modulates Resting Brain Hemodynamics and Autonomic Function: A Multimodal fNIRS-HRV Study

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that can influence brain activity and physiological function. We conducted a sham-controlled multimodal study to examine the effects of low-intensity (0.375 mA) bifrontal tDCS at rest in healthy adult males. The tDCS, with the anode placed over the left dorsolateral prefrontal cortex (DLPFC) and the cathode over the right DLPFC, was applied for twelve minutes, and outcomes were measured with functional near-infrared spectroscopy (fNIRS) for cortical hemodynamics and connectivity, heart rate variability (HRV), photoplethysmography (PPG) for autonomic function, and subjective surveys for emotional stress. The results showed that, compared to sham, active tDCS produced a decrease in oxyhemoglobin (HbO) concentration in the left DLPFC during stimulation, indicating reduced cortical oxygenation in the stimulated region. Functional connectivity analysis of fNIRS signals further revealed altered network connectivity, including modulation of intra- and inter-frontal connections, in the tDCS condition relative to sham. Concurrently, tDCS induced an increase in HRV indices such as RMSSD, SDNN, pNN50, and Poincare plot measures SD1 and SD2, reflecting enhanced parasympathetic activity and autonomic regulation. Participants in the active tDCS group also reported selective reductions in both state and trait anger after stimulation, whereas the sham group showed minimal changes in mood. These findings demonstrate that even at a subthreshold intensity, bifrontal tDCS at rest engaged neurovascular and neurovisceral mechanisms, coupling changes in prefrontal cortical activity with autonomic outflow and mood. This study provides new evidence of cortical-autonomic coupling during neuromodulation and suggests that low-intensity frontal tDCS may promote a calm physiological and emotional state. The results have implications for future research on brain stimulation in emotion regulation and for developing neuromodulation-based interventions to improve autonomic balance and mood in both healthy individuals and clinical populations.