The brain networks controlling movement are complex, involving multiple areas of the brain. Some neurological disorders, like Parkinson's disease (PD) and essential tremor (ET), cause abnormalities in these brain networks. Deep brain stimulation is a treatment that is used to treat these types of neurological diseases and is thought to help patients by modulating brain networks responsible for movement. Levodopa medication is also used to modulate this brain networks in patients with PD. The overall objective is to develop a unified theory of basal ganglia thalamocortical (BGTC) circuit dynamics that accounts for disease symptomatology, movement, and their inter-relationship. The underlying hypothesis, is that the rigidity and bradykinesia of PD are fundamentally related to excessive functional coupling across nodes in the BGTC motor circuit impeding effective information flow. In this research, the investigator will take advantage of the unique opportunity provided by awake deep brain stimulation surgery to learn more about how the brain functions in a diseased state and how deep brain stimulation changes these networks to make movement more normal. The investigator will simultaneously assess cortical and subcortical electrophysiology in relation to clinical symptoms and behavioral measures and in response to deep brain stimulation, cortical stimulation, and pharmacologic therapy in patients undergoing Deep Brain Stimulation (DBS) implantation surgery.