This randomized, participant-blinded, sham-controlled crossover study aims to investigate the acute effects of different frequency applications of transcutaneous auricular vagus nerve stimulation (taVNS) on autonomic physiological responses in healthy adults. Participants will receive sham (0 Hz), 10 Hz, 25 Hz, 50 Hz, and 100 Hz taVNS in separate sessions with washout periods between visits. Heart rate variability, blood pressure, pulse rate, respiratory rate, and subjective discomfort will be assessed before and after each intervention session.

The vagus nerve is a major regulator of the parasympathetic branch of the autonomic nervous system. Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulation technique applied to auricular regions innervated by the auricular branch of the vagus nerve. Although taVNS has been reported to modulate autonomic function, the acute physiological effects of different stimulation frequencies remain insufficiently characterized.

This study is designed as a randomized, participant-blinded, sham-controlled crossover trial in healthy adults aged 18 to 40 years. Each participant will undergo five intervention conditions in randomized order: sham (0 Hz), 10 Hz, 25 Hz, 50 Hz, and 100 Hz taVNS. Sessions will be separated by 48 to 72 hours to minimize carryover effects. Each stimulation session will last 10 minutes.

Primary autonomic outcomes will be derived from heart rate variability parameters obtained using a Polar H10 chest strap and analyzed with Kubios HRV software. Secondary physiological outcomes will include systolic and diastolic blood pressure, pulse rate, respiratory rate, and subjective discomfort measured using a visual analog scale. The study aims to identify frequency-specific autonomic response profiles and to evaluate whether respiratory rate influences heart rate variability changes associated with taVNS.

Inclusion Criteria:

• Healthy adults aged 18 to 40 years
• No known neurological, cardiovascular, respiratory, or metabolic disease
• Able to understand study procedures and provide written informed consent
• No active infection, open wound, or dermatological lesion in the ear region that would interfere with taVNS application
• Willing and able to attend all study visits

Exclusion Criteria:

• History of cardiac arrhythmia, pacemaker, or any implanted electronic medical device History of hypertension, diabetes, chronic pulmonary disease, epilepsy, or systemic disease affecting autonomic nervous system function
• Regular use of medications that may affect autonomic function (e.g., beta-blockers, antidepressants, antiarrhythmics)
• Intense physical exercise within the previous 24 hours
• High caffeine or alcohol intake within 12 hours prior to assessment
• Active smoking on the assessment day
• Pregnancy or suspected pregnancy
• Severe discomfort during stimulation (VAS >= 7), dizziness, nausea, or unwillingness to continue
• Any condition judged by the investigators to make participation unsuitable'

Participants will be blinded to whether the intervention session involves active taVNS or sham stimulation. The care provider/investigator cannot be blinded because of the technical requirements of device application and condition setup.