Participants will have fMRI, which uses a strong magnetic field and radio waves to create images of the brain. The subject lies on a table in a tunnel-like cylinder (the scanner) for 1 to 2 hours, lying still for 5 to 15 minutes at a time. He or she can communicate with the technician or researcher at all times during the test through an intercom system. Scans will be done while the subject is at rest and while he or she is performing finger movements. The movements involve pushing five buttons on a box-each button every 3 seconds on average in a specific order. Patients with Parkinson's disease will be studied off- and then on- medications that restore the levels of levodopa in the brain.