We will perform a pilot study to evaluate the effectiveness of human NBM DBS at improving cognitive deficits in PD patients referred and eligible for conventional DBS treatment for coexisting motor impairments. Six patients with PD with both motor fluctuations and cognitive impairments (including but not restricted to deficits in attention and working memory) will have bilateral electrodes implanted to ensure that superficial contacts lie in the conventional motor GPi target, while the deepest electrical contacts lie in the NBM- (see figure 1). We will place electrodes using our conventional image guided, stereotactic frame-based procedure currently used in patients at NHNN. Patients will be randomised into 2 groups in a crossover trial design to have 3 month periods of NBM stimulation switched on or switched off separated by a 1 month washout period, following which the patient will cross over to have the opposite condition for a further 3 months- see timeline. At the end of the crossover period, all patients will be invited for continued follow up with stimulation switched on and will have neuropsychological evaluations at 6 monthly intervals. Patients will be given the option of receiving additional conventional stimulation to the motor GPi, through the higher contacts of each electrode, at the end of the crossover period.