Neuropsychological functioning was assessed in patients with Parkinson 's disease ( PD ) at baseline and after 6 months in a prospective , randomised , controlled study comparing best medical therapy ( BMT , n = 116 ) and bilateral deep brain stimulation ( DBS , n = 164 ) at either the subthalamic nucleus ( STN , n = 84 ) or globus pallidus interna ( GPi , n = 80 ) , using standardised neuropsychological tests .