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