Inclusion Criteria:
1. OCD, diagnosed by Structured Clinical Interview for DSM-IV (SCID-IV), judged of disabling severity with a Yale-Brown Obsessive Compulsive Scale (YBOCS) score of at least 30 and a Global Assessment of Functioning (GAF) score of 45 or less.
2. Persistence of severe symptoms and impairment for five or more years despite at least three adequate (≥3 months at the maximum tolerated dose) serotonin transporter inhibitor trials (fluoxetine, sertraline, fluvoxamine, paroxetine, citalopram, escitalopram, or clomipramine) alone or in combination with ii. Adequate behavior therapy (≥20 sessions of expert exposure and response prevention), and iii. Augmentation of one of the selective SRIs with clomipramine, a neuroleptic, and clonazepam.
3. Age between 21 and 65 years.
4. Able to understand and comply with instructions.
5. Able to give fully informed, written consent in the judgment of the site Consent Monitor.
6. Either drug free or on a stable drug regimen for at least 6 weeks.
7. Good general health.
8. A family member or significant other, in contact with the patient every 1-3 days, is available and willing to communicate with the research team if the patient's clinical status worsens, and if necessary to accompany patients to study visits.
9. The local referring psychiatrist is willing to provide ongoing care during and after the trial
Exclusion Criteria:
1. personal/family history (1st/2nd degree relatives) of schizophrenia or schizoaffective disorder, other primary psychotic disorder, bipolar disorder;
2. present PTS;
3. present acute suicidality or suicidal ideation;
4. personal history of head injury, epilepsy, tic or other neurological disorders, neurodevelopmental (e.g., autism), systemic medical (metabolic, endocrine, chronic inflammatory, vascular, autoimmune) disease from medical records and self-report (all of which may confound interpretation of neuroimaging measures);
5. MMSE score \< 24;
6. premorbid IQ estimate \< 85;
7. visual disturbance (\<20/40 Snellen visual acuity, corrected);
8. left/mixed handedness;
9. current, or alcohol or illicit substance abuse/dependence in the last 3 months, determined by clinical assessment and urine toxicology;
10. contraindications to MRI,: metallic foreign objects, e.g., aneurysm clips/pacemakers, or questionable history of metal fragments, claustrophobia raising the risk of panicking in enclosed spaces;
11. positive pregnancy test for women of reproductive age, or women not using medically acceptable birth control throughout the study (barrier and/or oral contraceptives);
12. current psychotic symptoms (potential confounding effect on neuroimaging measures; see above);
13. an increased risk of seizure, determined by history;
4\) potentially proconvulsant medications (e.g., bupropion, tricyclic antidepressants, first-generation antipsychotics, lithium), and medications reducing cortical excitability (e.g., anticonvulsants, benzodiazepines, atypical antipsychotics).
