 The overarching aim is to compare the effects of ventral capsuleventral striatum VCVS and subthalamic nucleus STN deep brain stimulation DBS in the same participants. Investigators will test the hypothesis, grounded in cognitive neuroscience, that DBS at both sites is better than either site alone for treating the symptom dimensions of obsessive compulsive disorder OCD. Specifically, Investigators will employ novel cognitive paradigms and neurophysiological measures of cortical synaptic function to test the hypothesis that VSVC and STN DBS have different mechanisms of action and that alleviation of OCD symptoms is mediated by improvement in moodanxiety with VSVC DBS and by directly interrupting obsessions and compulsions with STN DBS. Investigators will additionally determine whether adjunctive cognitive behavioural therapy CBT enhances the response to DBS by providing the cognitive and behavioural skills to optimise symptom management and daily function. 