Brief Summary: This study seeks to establish the sensitivity and specificity of what appears to be a unique brainstem biomarker of Parkinson's Disease (PD) - an electrically induced olygosynaptic nasotrigeminal reflex response - in differentiating early stage PD from normal controls and from patients with various other neurodegenerative diseases. This study will additionally compare the biomarker to olfactory testing.

Detailed Description: Parkinson's disease (PD), a devastating age-related disease that is clinically defined by its effects on the motor system, afflicts more than six million people worldwide, imposing enormous burdens on patients, relatives, caretakers, and society in general. Diagnostic errors are common, particularly as symptoms first arise. The most common misdiagnoses are Alzheimer's disease (AD), essential tremor, and vascular pseudo-Parkinson's Disease. An accurate diagnosis is typically made at a later stage of the disease when marked and irreversible damage has occurred within the motor control system of the brain. Sensitive and specific biomarkers of the early stages of PD are urgently needed. Identification of such markers is critical for the development and assessment of medications and other interventions designed to eliminate or reduce the gradual and irreversible decline of neurons involved in the disorder. This study seeks to establish the sensitivity and specificity of what appears to be a unique brainstem biomarker of PD - an electrically induced trigeminal nerve blink reflex response - in differentiation of early stage PD from normal controls and such neurodegenerative diseases as early stage AD, progressive supranuclear palsy (PSP), and diffuse Lewy Body disease (DLBD). This study will additionally compare the biomarker to olfactory test results.