Pre-surgical Neuroimaging Before surgery, all patients underwent 3T Siemens TRIO system (Siemens, Erlangen, Germany) MRI scanning. Patients also performed an OCD symptom provocation task during fMRI scanning using modified version of the MOCSS (Maudsley Obsessive-Compulsive Stimuli Set). During scanning, patients were presented with pictures of 4 classes of provocative stimuli, 50 of each type: 1) contamination/washing, 2) checking, 3) hoarding, 4) symmetry/order. The study comprised 4 'blocks' pertaining to the 4 classes of provocative stimuli. Each block consisted of ten 20-s alternating epochs in which subjects viewed either 10 provocative or 10 neutral pictures (see Supplementary material for a full description). Post-operative computed tomography (CT) scanning was used to determine correct electrode positioning.