The purpose of this study is to explore whether stimulating different outer parts of the ear using a non-invasive form of ear stimulation called transcutaneous auricular vagus nerve stimulation (taVNS) will create different brain activation images. Investigators will recruit 24 healthy participants. The ear stimulation will be delivered while taking brain images using a magnetic resonance imaging (MRI) machine.

Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulation technique that delivers electric stimulation to the auricular branch of the vagus nerve (ABVN). This technique has been demonstrated effective in the treatment of depression and addiction in previous studies.

Most previous applications of taVNS targeted the ABVN via the cymba conchae or tragus sites. The mastoid process (mastoid site) is an important dermatome innervated by the ABVN, but there is currently a lack of knowledge on how taVNS on the mastoid site affects brain activity and whether it differs from the effect of stimulating the cymba site.

In this proposed study, the investigators plan to explore whether delivering taVNS to the mastoid site and the cymba site can induce different patterns of brain activity. Ultimately, if successful, this project will not only deepen our knowledge in basic neuroscience but also have great value in clinical applications since the mastoid site is a much more accessible ABVN target compared to other sites.

Inclusion Criteria:

• Age 18-65
• Have the capacity and ability to provide one's own consent in English and sign the informed consent document.

Exclusion Criteria:

• Unable to speak in English.
• Contraindicated for MRI.
• Any current or recent untreated medical, neurological, or psychiatric conditions
• Metal implant devices in the head, heart or neck.
• History of brain surgery.
• History of myocardial infarction or arrhythmia, bradycardia.
• Personal or family history of seizure or epilepsy or personal use of medications that substantially reduce seizure threshold (e.g., olanzapine, chlorpromazine, lithium).
• Personal history of head injury, concussion, or self-report of moderate to severe traumatic brain injury.
• Participant has abnormal ear anatomy, skin irritation, or ear infection present
• Individuals suffering from frequent/severe headaches.
• Individuals with a reported history of psychosis or mania, or individuals who are actively manic or psychotic.
• Moderate to severe alcohol or substance use disorder.
• Pregnancy