Parkinson's disease (PD) is one of the most common movement disorders, affecting 1 percent to 5 percent of the population older than 60 years (Farrer, 2006). It is a progressive neurodegenerative disorder with primary motor symptoms including resting tremor (shaking), rigidity (stiffness), bradykinesia (slowness), and impaired postural reflexes (dysbalance) (Lang & Lozano, 1998). The hallmark of PD pathophysiology is a selective loss of dopaminergic neurons in the substantia nigra pars compacta in the midbrain (Greenfield & Bosanquet, 1953). The gradual degeneration of nigro-striatal projections leads to a progressive loss of the neurotransmitter dopamine in the striatum, which is the major input structure to the basal ganglia. When the loss of dopamine exceeds 50 percent to 80 percent, parkinsonian symptoms become clinically evident (Ehringer & Hornykiewicz, 1960; Hornykiewicz, 1963). Although PD is often categorized as a movement disorder with well-defined motor symptomatology, non-motor symptoms have become more widely recognized. The non-motor symptoms, particularly cognitive and behavioral dysfunction, are not only common, but occur across all stages of the disease and may dramatically affect the quality of patients' life as well as that of their caregivers (Herlofson & Larsen, 2002). The cognitive symptoms are sometimes present before the onset of the motor symptoms or emerge with disease progression, dementia being among the most common in the advanced stages of the disease (Chaudhuri, Healy, & Schapira, 2006). The cognitive symptoms occur in up to 88 percent of PD patients and affect neuropsychiatric, sleep, autonomic, and sensory domains (Shulman, Taback, Bean, & Weiner, 2001). A comprehensive look at PD pathogenesis and disease management, therefore, should include cognitive as well as motor symptomology (Chaudhuri, Yates, & Martinez-Martin, 2005). However, because cognitive functioning is not always apparent, and because it encompasses a wide variety of behavioral and mental functions, it is more difficult to diagnose and manage than motor symptoms.