Auditory MMN EEG in TRD in Response to Ketamine

Inclusion Criteria: 1.18 to 65 years of age 2. Meeting Diagnostic and Statistical Manual for Mental Disorders (DSM-5) criteria for MDD and in a Major Depressive Episode and ≥10 on the Montgomery-Asberg Depression Rating Scale (MADRS). 3\. Competent to consent-based on their ability to provide a spontaneous narrative description of the key elements of the study. 4\. Failure of at least two trials of antidepressant therapy during the current episode (treatment-resistant depression) and receiving ketamine treatment clinically as administered by their clinician 5. Staying on stable dosages of any concomitant psychotropic medications. Exclusion Criteria: 1. History of bipolar disorder or psychosis 2. Current substance use or history of substance use during the past year (excluding nicotine and caffeine use disorder) 3. Concomitant major and unstable medical or neurologic illness or a history of seizures. 4. Non-English-speaking individuals.