Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease (de Lau and Breteler,2006). PD is characterized by the progressive loss of dopamine neurons in the substantia nigra pars compacta (SNpc), which leads to striatal dopamine denervation, particularly affecting the dorsolateral part of the putamen (Kish et al., 1988; Fearnley and Lees, 1991). As this part of the putamen is mostly involved in motor performance, motor dysfunction is the chief manifestation of PD. However, it has become increasingly recognized that other brain systems and functions are impaired in PD. Thus, behavior disorders and cognitive dysfunction are often present in PD, and constitute a major source of disability (Dubois and Pillon, 1997; Schrag et al., 2000; Aarsland et al., 2001; Weintraub et al., 2004). The actual prevalence rates of cognitive dysfunction and dementia in PD are difficult to determine, as they will depend upon whether extensive neuropsychological analysis was used or not, as well as several confounding factors (e.g. age, presence of depression). In general, it is accepted that at least some 30% of PD subjects will end up developing dementia (Dubois and Pillon, 1997). The risk of dementia can be up to sixfold higher in PD compared to subjects without PD (Aarsland et al., 2001).

The underlying mechanisms by which these non-motor manifestations of PD actually come into play are still poorly understood.