Vagal Nerve Stimulation for Treatment Resistant Major Depression

Inclusion Criteria: 1. Age 18-75 years old 2. Sufficient fluency in English to understand testing procedures and provide written informed consent 3. A Hamilton Depression Rating Scale total score greater than or equal 18 4. A DSM 5 diagnosis of MDD based on the MINI Exclusion Criteria: 1. Evidence of alcohol or other substance use disorder in the past 3 months 2. For females: current pregnancy or lactation (women of reproductive potential must have a negative urine pregnancy test at screening). 3. Depressed patients who have failed at least one adequate antidepressant trial during the current depressive episode based on the ATRQ. 4. Diagnosis of other primary psychiatric disorder (defined in this case as being the main focus of treatment) as determined by the MINI, such as: bipolar disorder, personality disorders, psychotic disorders, post-traumatic stress disorder, obsessive-compulsive disorder, dissociative disorders, eating disorder, or cognitive task due to neurological conditions 5. Systolic blood pressure \< 150 and/or diastolic blood pressure \< 90 at screening 6. Post-partum state (being within 2 months of delivery or miscarriage) 7. Imminent suicide or homicide risk as determined by the investigator 8. Being treated with one of the following medications: benzodiazepines or other CNS depressants. 9. No clinically significant neurological disease based on medical history (e.g., epilepsy) or significant head injury. 10. Any of the following disorders are exclusionary: Rheumatoid arthritis; Lupus erythematosus; Autoimmune hepatitis; Autoimmune peripheral neuropathy; Autoimmune pancreatitis; Behcet's disease; Crohn's disease; Autoimmune glomerulonephritis; Grave's disease; Guillain-Barre syndrome; Hashimoto's thyroiditis; Autoimmune polymyositis or polymyalgia; Myasthenia gravis; Narcolepsy; Polyarteritis nodosa; Scleroderma; Sjogren's syndrome; Transverse myelitis; Wegener's granulomatosis; History of seizures (only childhood febrile seizures are allowed) 11. The presence of clinically significant laboratory findings in the opinion of the investigator including, but not limited to, clinically significant anemia or transaminase elevation is considered exclusionary. 12. If the UDS is positive, the subject would be excluded if, in the opinion of the investigator, the positive UDS meant the subject has an active substance use disorder. 13. Patients with prior exposure to VNS therapy whether using an implantable or external device will be excluded. 14. An active implantable medical device, such as a pacemaker, hearing aid implant, or any implanted electronic device 15. A metallic device, such as a stent, bone plate, or bone screw, implanted at or near the neck 16. An open wound, rash, infection, swelling, cut, sore, drug patch, or surgical scar(s) on the neck at the treatment location 17. Patients diagnosed with narrowing of the arteries (carotid atherosclerosis) 18. Patients who have had surgery to cut the vagus nerve in the neck (cervical vagotomy) 19. Patients with clinically significant hypertension, hypotension, bradycardia, or tachycardia