The purpose of this study is to evaluate the safety and effectiveness of deep brain stimulation of the nucleus basalis of Meynert (also called the "nbM") at improving memory in Parkinson's disease patients with mild cognitive impairments and memory difficulties. Patients with Parkinson's disease (PD) that are eligible for Deep Brain Stimulation (DBS) therapy for improvement of their motor symptoms and with evidence of mild cognitive impairments and memory difficulties will be enrolled.