 OCD symptoms and cognitive impairment may be mediated by abnormal cortical excitability due to aberrant long term depression LTD synaptic plasticity andor reduced gamma-aminobutyric acid GABA mediated intracortical inhibition. Transcranial magnetic stimulation TMS can be used to measure these neurophysiological processes safely in patients undergoing DBS and both abnormalities have been shown to normalise following DBS in other disorders. Investigators will use TMS to assess these neural processes before and after DBS in OCD patients and test for whether there are differential changes from the two DBS sites and whether combined VSVC and STN DBS produces more vigorous improvements compatible with our hypothesis concerning the superior clinical effects of this condition. Investigators will measure intracortical inhibition not only at rest but also during the anticipation of reinforcement, previously shown to affect motor cortex excitability and which Investigators predict will be specifically affected by VSVC DBS. 