Distinguishing Between Alzheimer's Disease, Lewy Body Dementia, and Parkinson's Disease

Inclusion Criteria: * Subjects must meet the standard clinical criteria of the syndromes of interest * All subjects must have enough cognitive abilities to complete study procedures, which will be operationally defined as having a Mini Mental State Examination (MMSE) score greater or equal to 10. * Subjects must be on stable cognitive and psychoactive medication regimen for the preceding four weeks of enrollment. * Subjects must have a responsible study partner that either lives with them or is in regular contact with them at least 4 out of 7 days per week. * Subjects must have visual and auditory acuity adequate for testing. Exclusion Criteria: * Any other condition (other than the primary diagnosis), which in the opinion of the investigators might contribute to the syndrome of dementia or complicate its assessment. * active medical disorder that could preclude participation in this protocol * Women who are pregnant or are breast feeding * severe renal impairment as defined by glomerular filtration rate (GFR) less than 30 (may have increased radiation exposure with the DaTscan). * Subjects in whom English is not the 1st language * Subjects with educational level less than 12 years * Subjects who have ever participated in an experimental study with an amyloid targeting agent unless it can be documented that the subject received only placebo during the course of the trial.