The purpose of this study is to examine the brain changes in people with Parkinson's Disease (PD) after they get deep brain stimulation (DBS) surgery, compared with people who do not have Parkinson's Disease. Treatment of Parkinson's disease is often difficult and challenging. Deep brain stimulation is an established surgical treatment that is effective for the treatment of PD, but the details of why it helps are not known. In order to achieve maximum benefits from this treatment, it is important to understand how it changes the brain. Specifically, the investigators will study electrophysiology, which is the study of how the brain conducts electrical messages to the rest of the body. To do this, the investigators will use transcranial magnetic stimulation (TMS), which is a painless and non-invasive procedure. They will also conduct motor physiology experiments of the upper and lower limbs to collect data about skeletal muscle movement. The data from this study will help explain whether the electrical changes in the brain have any relation to the physical benefits patients with Parkinson's Disease sometimes receive from DBS surgery.