All subjects will undergo written informed consent and a screening evaluation including baseline clinical laboratory testing, a baseline physical and neurological evaluation and baseline cognitive evaluations using the MMSE, the ANAM computerized cognitive battery, and other tests of executive function. All subjects will be evaluated with United Parkinson Disease Rating Scales (UPDRS) following an overnight withdrawal of anti-parkinson medication. Subjects will be asked to undergo an injection of [123I] 5-IA followed by SPECT imaging as described below. A second [123I] 5-IA and SPECT imaging study will be obtained for reliability testing between 2 weeks and 2 months following the initial [123I] 5IA imaging session.