This is a prospective, double-blind (patient and neuropsychologist), randomized, cross-over study to evaluate the safety of deep brain stimulation of the nucleus basalis of Meynert (nbM) in Parkinson's disease (PD) patients with mild cognitive impairment (PD-MCI) who are undergoing globus pallidus internus (GPi) stimulation for their motor symptoms. Six patients with PD who are eligible for GPi DBS for improvement of their motor symptoms and who have a clinical diagnosis of PD-MCI (amnestic, single or multiple domain) per Movement Disorder Society (MDS) criteria will be enrolled and implanted with bilateral electrodes in in the nbM in addition to the globus pallidus internus (GPi). Conventional stereotactic implantation of the electrodes will be guided by intraoperative computerized tomography (CT) and magnetic resonance tomography (MRT).