Frequency Effects of Transcutaneous Trigeminal Nerve Stimulation (eTNS) on Brain and Autonomic Function

Inclusion Criteria: 1. Healthy volunteers, aged 18 to 40 years old. 2. Right-handed. 3. Generally healthy with no history of neurological, psychiatric, or severe cardiovascular diseases. 4. Normal physical and neurological examinations. 5. Capable of understanding the study procedures and voluntarily signing the written informed consent form. Exclusion Criteria: 1. Contraindications to MRI scanning (e.g., claustrophobia, cardiac pacemakers, artificial cochlea, metallic braces, or any other ferromagnetic implants). 2. Female participants who are pregnant, lactating, or suspect they might be pregnant. 3. Contraindications to transcutaneous electrical stimulation (e.g., personal or family history of epilepsy/seizures, trigeminal neuralgia, facial skin lesions, or active skin diseases on the forehead). 4. Current or recent (within the past month) use of any medications known to affect the central nervous system or the autonomic nervous system (e.g., antidepressants, beta-blockers, sedatives, or sympathomimetics). 5. History of substance abuse, heavy smoking, or excessive daily consumption of alcohol or caffeine. 6. Irregular sleep patterns, shift work, or severe sleep deprivation within 24 hours prior to the scanning session.