Transcutaneous Auricular Vagus Nerve Stimulation during Movement Selectively Activates Motor Circuitry without Additional Cortical or Autonomic Effects

Abstract

Transcutaneous auricular vagus nerve stimulation (taVNS) is a promising noninvasive neuromodulation technique with growing therapeutic relevance. Although increasingly combined with physical therapy in neurorehabilitation, its mechanistic effects during active movement remain poorly understood, as most physiology studies examine taVNS at rest, overlooking the dynamic neural activity engaged during movement. This study aimed to determine the neurophysiological basis for pairing taVNS bursts with movement. Thirty-six healthy adults (10 females, 26 males) completed two experiments where 2 s taVNS bursts were delivered. The first experiment assessed autonomic [heart rate (HR), galvanic skin response (GSR)], neuromodulatory (pupil diameter), and cortical [electroencephalography (EEG) spectral slope] responses during a randomized trial design involving three stimulation conditions (taVNS, earlobe sham, no stimulation) and two behavioral contexts [movement (go) vs still (no-go)]. The second experiment evaluated corticospinal excitability by measuring transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) during taVNS. taVNS increased TMS-induced MEP amplitudes, indicating transient facilitation of corticospinal output when stimulation coincides with an engaged motor system. Concordantly, EEG sensorimotor activity was enhanced by taVNS during movement but not during stillness. In contrast, pupil diameter showed a clear phasic response to stimulation in both movement and still conditions, consistent with state-independent neuromodulatory engagement. Autonomic indices were not additionally modulated by phasic taVNS beyond movement-related changes. These findings identify a state-dependent window in which taVNS preferentially boosts task-engaged motor circuitry rather than producing nonspecific autonomic activation, providing mechanistic support for movement-paired stimulation protocols and highlighting pupil, EEG, and MEPs as sensitive biomarkers of phasic taVNS effects.