 Attentional processes are also thought to contribute to impaired behavioural control in OCD. Being able to stop or inhibit planned acts when environmental circumstances change depends on the function of the right inferior cortical-STN hyperdirect pathway and can be operationalised by measuring the stop signal reaction time SSRT. The finding of specific prolongation of SSRT in OCD patients and their unaffected first-degree relatives suggests that this cognitive abnormality is also a phenotype of OCD. Although the neural substrate of this mechanism does not involve the limbic cortico-striato-pallido-thalamic circuit, recent evidence suggests that neural systems outside this circuitry, involving the right inferior frontal cortex, are impaired in OCD. Investigators therefore hypothesise that STN DBS allows inhibition of repetitive thoughts and acts in OCD by facilitating the action of the hyperdirect pathway from the right inferior frontal cortex to STN. Investigators will test this by measuring the effectiveness of STN DBS on obsessions and compulsions and relate these to changes in SSRT and performance on other tasks sensitive to right inferior frontal cortex function which measure the ability to inhibit pre-potent tendencies. 