The brain is made up of cells called neurons. The neurons communicate with one another and secrete chemicals called neurotransmitters. The neurotransmitters bind to specific sites on other neurons called receptors. Acetylcholine (ACh) is a neurotransmitter that binds to ACh receptors. In both aging and AD, the number of neurons that secrete ACh decreases and the function of some ACh receptors changes. This study will use positron emission tomography (PET) scans of the brain to study the effects of age and AD on muscarinic type 2 [M2], a type of ACh receptor.