The purpose of this study is to test the effects of non-invasive neuromodulation, transcranial magnetic stimulation on brain function in individuals with obsessive-compulsive disorder (OCD). This study is focused on the mechanism(s) by which brain stimulation might change the functioning of regions implicated in OCD, and thereby inform possible future therapeutic uses.

Inclusion Criteria:

• Current primary OCD diagnosis and current Y-BOCS total score of ≥16
• 18-70 years of age
• Ability to speak, read, write, and understand English sufficiently well to complete study procedures and provide informed consent
• No use of psychiatric medications or stable psychiatric medication use for a minimum of 6 week prior to study entry
• Ongoing psychotherapy allowed if already established for three months or more before study entry

Exclusion Criteria:

• History of primary psychotic disorder or bipolar disorder
• Present acute suicidality
• History of head injury, epilepsy, or other clinically significant neurological illness except tic disorders
• Active systemic medical (metabolic, endocrine, chronic inflammatory, vascular, autoimmune) disease
• Premorbid intelligence quotient (IQ) estimate < 80
• Visual disturbance (<20/40 Snellen visual acuity, corrected)
• Current, or alcohol or illicit substance abuse/dependence in the last 3 months
• Contraindications to TMS or MRI: metallic foreign objects, e.g., aneurysm clips/pacemakers, or questionable history of metal fragments, claustrophobia
• Women who are pregnant or breastfeeding. All women participants of reproductive age are required to have a negative pregnancy test prior to enrollment and use medically acceptable birth control throughout the study (barrier and/or oral contraceptives)
• Current psychotic symptoms
• An increased risk of seizure, determined by history
• Medications judged to notably affect cortical excitability e.g., anticonvulsants or high-dose benzodiazepines (> 4 mg/day of clonazepam or equivalent)
• Predominant hoarding symptoms