Inclusion Criteria:

   - 18-70 years in age

   - U.S. Veteran

   - Diagnosis of MDD

   - On a stable medication regimen for at least two weeks prior to testing

   - Stable social environment and housing to enable regular attendance at clinic visits

   - Ability to undergo cognitive testing, clinical assessments, and PET/MR scans

   - Stable medical health

   - Will undergo rTMS treatment for MDD at the VA Palo Alto

   - Able to complete a PET-MR scan without the use of sedation

Exclusion Criteria:

   - Active substance use within three months of testing

   - IQ < 70

   - Major medical neurological illness or significant head trauma

   - Pregnancy or breastfeeding

   - Contraindication to MR scanning, including magnetic-resonance incompatible metal or
   hardware including pacemakers, cochlear implants, and bullets near a critical organ

   - Weight > 350 lbs or a large body habitus that MR scanner cannot accommodate

   - History of or current claustrophobia

   - Inability to comply with basic study requirements such as following directions and
   punctuality

   - Acute or unstable chronic medical illness that would affect participation or
   compliance with study procedures, e.g. unstable angina

   - Unstable psychiatric symptoms that precludes consistent participation in the study,
   e.g. active current suicidal intent or plan, severe psychosis

   - Inability to undergo PET/MR scan, e.g. claustrophobia, presence of ferromagnetic
   objects in subject's body