The purpose of this study is to investigate the brain activity associated with non-motor symptoms of movement disorders, including Parkinson's disease and essential tremor. These movement disorders commonly have significant non-motor features also, including depression, cognitive impairment, decreased attention, and slower processing speeds. The investigators are interested in the brain activity associated with these symptoms, and perform recordings of the surface of the brain, in addition to the typical recordings the investigators perform, during routine deep brain stimulation (DBS) surgery.