This project uses structural and diffusion MRI imaging (obtained at 7 T) and biophysical models to the model the effects of deep brain stimulation (DBS) of the globus pallidus on neural activity. Data was obtained in participants with Parkinson's disease with implanted DBS stimulators in the globus pallidus. Each stimulator has four contacts. The models will be used to estimate the structures and pathways activated at each contact. This information will be used to better understand the effects of stimulation at each contact location on movement (assessed in the laboratory).