 The main aim of CBT is to enable OCD patients to obtain greater control over behaviour so that they can inhibit their repetitive thoughts and acts and switch to more meaningful alternatives. One abnormal mechanism contributing to such cognitive inflexibility is thought to be blunting of the value of positive and negative reinforcement normally used to guide responses and dependent on intact OFC-striatal neural circuitry. Dysfunction of this cognitive control system has been shown in first-degree clinically unaffected relatives of OCD patients thus confirming this mechanism as a neurobiological phenotype of OCD 15.It is therefore hypothesised that DBS of VSVC will improve the processing of reinforcing stimuli and that this will result not only in elevation of mood and reduction in anxiety but also in increased cognitive flexibility. Investigators will test this by measuring the effectiveness of VSVC DBS on clinical measures of mood, anxiety, obsessions and compulsions and relate these to changes in reinforcement learning and emotional processing using cognitive tasks sensitive to OCD which measure the ability to respond appropriately to reinforcing contingencies. 