Patients with Parkinson's disease have difficulty performing precise finger movements, mainly because of a dramatic decrease of a substance called dopamine in parts of the brain. Medicines such as levodopa, which help restore dopamine levels, can greatly improve function; however, little is known about how these drugs work. In normal aging, dopamine decreases slightly in certain parts of the brain, but the importance of this decline is poorly understood. This study may provide new information about Parkinson's disease and normal aging that might lead to better treatment strategies.