Transcutaneous Vagus Nerve Stimulation, Tolerability, and the Alpha / Theta EEG Ratio: A Controlled Pilot Study Relevant to Emotion Regulation

Background: Emotional dysregulation is a transdiagnostic feature of anxiety and depressive disorders, sustained by altered cortical oscillatory dynamics that pharmacological treatments do not directly target. The Alpha/Theta ratio (8–12 Hz / 4–7 Hz) indexes emotion regulation, reflecting parasympathetic dominance. Transcutaneous vagus nerve stimulation (tVNS) engages this axis, yet studies linking stimulation intensity to both tolerability and EEG effects remain limited.Methods: In this controlled pilot study, ten healthy female adults (21.1 ± 1.8 years) were randomized to tVNS (n = 5) or waitlist control (n = 5). The tVNS group completed eight 30-minute left tragus sessions over four weeks (20 Hz, 200 μs, 11–25 mA, individually titrated). Resting-state EEG at Cz derived Alpha/Theta and Theta/Beta ratios. Adverse events were systematically monitored, and False Discovery Rate (FDR) correction was applied across exploratory comparisons.Results: Protocol completion was 100% across 40 sessions. Adverse events were predominantly mild (92.3% Grade 1), with a 2.9-fold lower incidence at ≤20 mA. The tVNS group showed a 25.8% increase in Alpha/Theta ratio (d = 1.47, p = .042 uncorrected); controls remained stable. Theta/Beta also increased (d = 1.18, p = .062). Stimulation intensity correlated positively with Alpha/Theta change (rₛ = 0.60), suggesting a possible dose-response. Findings did not survive FDR correction and are exploratory.Conclusion: tVNS was feasible and well-tolerated at ≤20 mA, with preliminary evidence of intensity-related neurophysiological effects on an EEG biomarker of emotion regulation. The effect sizes provide a quantitative basis for powering sham-controlled trials that optimize stimulation intensity in clinical emotional dysregulation.