Effects of tcVNS and Multimodal Stimulation on Heart Rate Variability and Stress Regulation

Background Transcutaneous cervical vagus nerve stimulation (tcVNS) is a non-invasive neuromodulation technique that can enhance autonomic regulation and emotional control. Slow breathing and aromatherapy likewise promote parasympathetic activity and stress reduction. However, their combined effects on autonomic, affective, and cortical outcomes remain unclear. Methods Twenty-three healthy adults participated in four within-subject conditions: Resting baseline (5 min), tcVNS alone (25 Hz, 15 min), Breathing + Aroma (paced breathing with calming aroma, 5 min), and combined tcVNS + Breathing + Aroma (15 min). Heart rate variability (HRV) indices (SDNN, RMSSD, pNN50, SD1, SD2, LF/HF ratio, approximate entropy, and fractal α1) were derived from photoplethysmography. Prefrontal cortex (PFC) oxygenation was measured with functional near-infrared spectroscopy, and self-reported state anxiety was assessed after each condition. Data were analyzed using repeated-measures ANOVA with false discovery rate correction. Results Both tcVNS and Breathing + Aroma significantly increased HRV relative to baseline, and the combined condition produced the largest improvements. For example, SDNN rose from 76.5 ms at rest to 109.5 ms with tcVNS, 121.1 ms with Breathing + Aroma, and 157.6 ms with combined stimulation (all p < 0.05). RMSSD, pNN50, SD1, and SD2 showed similar patterns, reflecting enhanced vagal tone. Approximate entropy increased with tcVNS and combined stimulation, suggesting greater cardiac complexity. Emotional stress ratings decreased across all active conditions, with the strongest reduction in the combined condition. fNIRS revealed bilateral PFC activation during Breathing + Aroma, further amplified under combined stimulation, whereas tcVNS alone produced minimal cortical change. Conclusions Multimodal stimulation combining tcVNS with paced breathing and aroma yielded synergistic benefits, including greater vagal regulation, reduced stress, and stronger prefrontal activation compared with single modalities. These findings highlight a promising non-pharmacological strategy for enhancing stress resilience through integrated autonomic and cortical modulation.