This research study will evaluate functions of memory, thinking, behavior, and daily life activities 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 in Alzheimer's dementia (dementia is a disease where persons become forgetful and confused), a reduction in the amount of acetylcholine, a "neurotransmitter" substance (a chemical messenger that nerve cells need to communicate with each other), may be responsible for some of the memory and behavioral changes. At the present time, the investigators have no clear information on the levels of acetylcholine in the brain of patients with Parkinson's disease who also have memory or behavioral changes.