The underlying goal of this study is to assess [123I] 5-IA and SPECT imaging as a tool to detect nicotinic receptor activity in the brain of PD patients. All study procedures will be conducted at the Institute for Neurodegenerative Disorders (IND) and Molecular NeuroImaging (MNI) in New Haven, CT. Approximately 10 patients with a diagnosis of PD without cognitive changes will be recruited to participate in this study. Patients will be eligible to participate if they have a diagnosis of PD of more than 2 years duration and have no significant cognitive changes.