Any coexisting psychiatric illness other than generalized anxiety disorder, panic disorder, or social/specific phobias
Any history of substance dependence
Substance abuse within the past 6 months
Significant risk of suicide, as defined by score \>4 on item 10 of the MADRS or in the clinical judgment of the study physician
Any significant medical or neurological condition which is likely to impact the central nervous system and/or affect the results of MRS or PET imaging
For the subset of unmedicated MDD patients, any psychotropic medications within 4 weeks prior to scanning. For the subgroup of medicated patients, they may be taking a stable dose (i.e., same dose for at least 4 weeks at the time of scanning) of standard antidepressant medications, but may not be taking any other psychotropic medication.
Inability to give informed consent
Contraindication to MRI (e.g., pacemaker, ferromagnetic implants in the body)
