The purpose of this study is to evaluate the ability to identify individuals with dopaminergic degeneration in group of patients with a clinical diagnosis of either dementia with Lewy bodies (DLB) or idiopathic Parkinson's disease and to differentiate them from Alzheimer's disease (AD) and control subjects.

Inclusion Criteria DLB:

• Male or female > 50 years of age
• Meet the diagnostic criteria for probable DLB as established by the DLB Consortium (McKeith et al., 2005)

Inclusion Criteria AD:

• Male or female > 50 years of age
• Meet the NINCDS criteria for probable AD and have a Mini Mental State Examination (MMSE) score at screening between 10 and 24 inclusive

Inclusion Criteria PD:

• Male or female > 50 years of age

• Have probable PD according to the following criteria (Gelb et al., 1999):

  • Presence of 2 of the following 3 features: rest tremor, rigidity, bradykinesia;
  • Documented history of a sustained (>6 months) improvement to Levodopa (L-DOPA) or a dopamine agonist
  • Absence of atypical clinical features or other possible signs or symptoms suggesting another cause of parkinsonism such as a history of frequent falls as a prominent early feature, localized brain lesion(s) or neuroleptic use
  • Asymmetric onset

• A diagnosis of PD made within the 4 years prior to enrollment

Normal subjects:

• Are males or females > 50 years of age
• Have a MMSE score > 29, and are cognitively normal on the psychometric test battery at screening
• Have no signs or symptoms of clinically meaningful parkinsonism

Exclusion Criteria:

• Have a history or current diagnosis of other neurologic disease
• Have evidence of clinically significant cerebrovascular disease
• Have evidence from MRI or other biomarker studies that suggests the presence of a CNS pathology other than that associated with the study diseases