tDCS in Parkinson's Disease With Depression

Inclusion Criteria: 1. are outpatients, 2. are voluntary and competent to consent to treatment, 3. have a Mini-International Neuropsychiatric Interview (MINI) confirmed diagnosis of major depressive disorder (MDD), 4. have a diagnosis of PD according to the UK PD brain bank criteria, 5. are 19 years of age or more, 6. have a score \> 13 on the Beck Depression Inventory-II (BDI-II), 7. still have depressive symptoms after 6 weeks or more of antidepressant (SSRI) medication treatment (and on a stabilized dose of at least 4 weeks), 8. are able to adhere to the treatment schedule, 9. are proficient in written and verbal English. Exclusion Criteria: 1. have a history of substance dependence or abuse within the last 6 months, 2. have a concomitant significant unstable medical illness, 3. have active suicidal intent, 4. have any history of seizure or medication-resistant epilepsy in the family, 5. have a lifetime Mini-International Neuropsychiatric Interview (MINI) diagnosis of bipolar I or II disorder, schizophrenia, schizoaffective disorder, schizophreniform disorder, delusional disorder, or current psychotic symptoms 6. have a MINI diagnosis of obsessive compulsive disorder, post-traumatic stress disorder (current or within the last year), anxiety disorder (generalized anxiety disorder, social anxiety disorder, panic disorder), or dysthymia, assessed by a study investigator to be primary and causing greater impairment than MDD, 7. have failed a course of ECT in the current episode or previous episode, 8. have received tDCS or other neurostimulation therapy for any previous indication due to the potential compromise of expectancy effects, 9. have any significant neurological disorder or insult including, but not limited to: any condition likely to be associated with increased intracranial pressure, space occupying brain lesion, any history of seizure except those therapeutically induced by ECT, cerebral aneurysm, Huntington's chorea, multiple sclerosis, significant head trauma with loss of consciousness for greater than or equal to 5 minutes, or developmental disorder, 10. have an intracranial implant (e.g., aneurysm clips, shunts, stimulators, cochlear implants, or electrodes) or any other metal object within or near the head, excluding the mouth, that cannot be safely removed, 11. are taking a non-SSRI antidepressant medication, 12. are pregnant (women of childbearing age only). Exclusion Criteria for PET: 1. unable to tolerate staying off anti-parkinsonian medication for 12-18 hours pre-PET scan, 2. have a history of radiation therapy treatment or other high amounts of radiation. Exclusion Criteria for MRI: * Artificial heart valve; * Brain aneurysm clip; * Electrical stimulator for nerves or bones; * Ear or eye implant; * Implanted drug infusion pump; * Coil, catheter, or filter in any blood vessel; * Orthopedic hardware (artificial joint, plate, screws); * Other metallic prostheses; * Shrapnel, bullets, or other metal fragments; * Surgery or tattoos (including tattooed eyeliner) in the last six weeks.