This study will evaluate the decision-making ability and therapeutic effects of accelerated Transcranial Direct Current Stimulation (tDCS) as adjunct to pharmacotherapy in obsessive-compulsive disorder (OCD) patients, and the underlying neural mechanism by EEG and MRI.

The purpose of this study is to examine the decision-making ability and clinical efficacy of accelerated tDCS over orbitofrontal cortex (OFC) as adjunct to pharmacotherapy in treatment of OCD patients.60 OCD patients on stable medication will be randomized into two groups (i.e. active or sham stimulation). Accelerated tDCS stimulation will be performed four times a day, five times a week, for one week. Iowa Gambling Task will be conducted at the baseline, after the one week tDCS and at the 1-month follow-up to evaluate the decision-making ability of OCD patients.The investigators will assess symptom severity before and after one week of tDCS. Through the study, Yale-Brown Obsessive Compulsive Scale（Y-BOCS）, Clinical Global Impression(CGI), the Obsessive Compulsive Inventory-Revised (OCI-R), the Beck Depression Inventory-II (BDI-II), the Beck Anxiety Inventory (BAI), Childhood Trauma Questionnair(CTQ), side-effect questionnaire and other scales will be obtained by a trained investigator.The patients will also receive magnetic resonance imaging (MRI) scan, electroencephalography (EEG).

Inclusion Criteria:

1. Age 18-60 years old, Han ethnicity, right-handed
2. DSM-5 criteria for OCD;
3. Y-BOCS total score ≥16, receiving stable medication for at least 4 weeks before tDCS stimulation.
4. Years of education ≥9.

Exclusion Criteria:

1. Any axis I psychiatric disorder comorbidity
2. Severe obsessive-compulsive symptoms that render the patient unable to complete required assessment procedures.
3. Previous treatment with electroconvulsive therapy (ECT), repetitive transcranial magnetic stimulation (rTMS), or transcranial direct current stimulation (tDCS).
4. Severe somatic diseases or any physical conditions that may induce epilepsy or intracranial hypertension, including cardiovascular and respiratory diseases, etc.
5. History of neurological disorders (e.g., epilepsy, cerebrovascular accidents) or traumatic brain injury/brain surgery.
6. Implantation of intracranial stents, cardiac pacemakers, coronary stents, cochlear implants, or other internal medical devices.
7. Women who are pregnant or planning to become pregnant in the near future.
8. Serious suicide risks.