Investigating the Transcutaneous vs. Transcranial Mechanisms of Trigeminal Nerve Stimulation (eTNS) Using fMRI

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. Known allergy, hypersensitivity, or adverse reactions to Lidocaine or other amide-type local anesthetics. (Crucial for this specific study design) 2. Contraindications to MRI scanning (e.g., claustrophobia, cardiac pacemakers, artificial cochlea, metallic braces, or any other ferromagnetic implants). 3. Female participants who are pregnant, lactating, or suspect they might be pregnant. 4. Contraindications to transcranial or transcutaneous electrical stimulation (e.g., personal or family history of epilepsy/seizures, implanted brain stimulators). 5. Any active skin disease, inflammation, lesions, cuts, or abrasions on the forehead (specifically over the supraorbital area), which could alter electrical impedance or anesthetic absorption. 6. History of trigeminal neuralgia, facial nerve palsy, or chronic facial pain. Current or recent (within the past month) use of any medications known to affect the central nervous system, autonomic nervous system, or pain perception (e.g., analgesics, antidepressants, beta-blockers, or sedatives). 7. History of substance abuse, heavy smoking, or excessive daily consumption of alcohol or caffeine. 8. Irregular sleep patterns, shift work, or severe sleep deprivation within 24 hours prior to the scanning sessions.