Study of Individuals With Parkinson's Symptoms But in Whom There is Diagnostic Uncertainty

* Subjects will be referred to the Institute for Neurodegenerative Disorders (IND) by practicing general neurologists with genuine uncertainty regarding the subject's diagnosis. * Subjects with suspected PD or PS will be evaluated clinically and with DAT imaging, using b-CIT and SPECT. The DAT imaging procedure will take place over two days: * On the first day participants are injected with \[123I\]ß CIT, an investigational radioactive material that localizes in the brain. Study participants will also have a thorough neurologic examination and standard neuropsychological testing, including testing of memory, concentration, abstraction and visual spatial functions. * Twenty-four hours later study participants return to the Institute for Neurodegenerative Disorders where an investigational scanning procedure will be used to obtain SPECT (single photon emission computed tomography) images of the brain. * Subjects will be asked to return within 3 months following the imaging study to have a repeat neurological examination by the two study neurologists at IND. * Subjects will be asked to return at about 6 months and possibly again at one year following the imaging study for a final clinical evaluation by one of the study neurologists at IND.