Effects of Respiratory-Gated Transcutaneous Vagal Nerve Stimulation in Major Depression (Phase 1)

Inclusion Criteria: * Diagnosis of recurrent Major Depressive Disorder with a current active depressive episode * Currently not taking psychiatric medications or on a stable therapeutic dose of psychiatric medication for at least 30 days prior to entering the study Exclusion Criteria: * History of cardio-, cerebro-, or peripheral vascular disease, diabetes mellitus, morbid obesity (BMI \> 40 kg/m2), kidney or liver failure, history of unexplained fainting spells * Any psychiatric disorder involving a history of psychosis (e.g. schizophrenia, bipolar disorders, severe personality disorders) * Any chronic condition affecting movement, speech and/or ability to read or follow written instructions * Substance use disorder, either mild, moderate, or severe within the past 12 months (excludes nicotine) * History of suicide attempt within the last year or current active suicidal ideation * History of a clinically defined neurological disorder including, but not limited to: Any condition likely to be associated with increased intracranial pressure; space occupying brain lesion; History of cerebrovascular accident; Transient ischemic attack within two years; Cerebral aneurysm; Dementia; Parkinson's Disease; Huntington's chorea; or Multiple sclerosis. * Pregnant or nursing * Metallic implants or devices contraindicating tVNS