This research study will evaluate functions of memory, thinking, eye movements and walking and how these relate to the measurement of certain chemicals (acetylcholine and dopamine) in the brain using an imaging procedure called positron emission tomography (PET). You may know that the brain chemical dopamine, a "neurotransmitter" substance (a chemical messenger that nerve cells need to communicate with each other), is important for the brain to control movements and that the brain chemical acetylcholine may have functions related to mental concentration and attention. At the present time, the investigators have no clear information how these two chemicals in the brain of patients with Parkinson's disease are related to the risk of falling.