Inclusion Criteria:  Adult age 18-70 years  Exclusion Criteria:  No ferrous metal implanted in head or body No history of major head trauma No neurological or psychiatric disorder Not using medication that affects brain function No implanted electronic devices (e.g. pacemaker) No implant that poses an MR contraindication Not pregnant, suspect they are pregnant, or seeking to become pregnant Not claustrophobic No piercings or jewelry that cannot be removed Does not weigh more than 250 pounds Normal or contact-corrected normal vision  Exclusion for tVNS arm:  Conditions of skin or anatomy that affect left auricle or forehead skin and could impact placement of electrodes for tVNS or forehead stimulation Diagnosis of significant cardiovascular or cerebrovascular disease [e.g. congestive heart failure, stroke, cardiac conduction disorders (including: bundle branch block, heart block, long Q-T syndrome), history of asystole or non-sustained ventricular tachycardia. Bradycardia (defined as resting heart rate <50 bpm) Hypotension defined as blood pressure <90/60 mmHg