BACKGROUND In Parkinson's disease (PD), cognitive dysfunction is frequently reported - approximately 50% of PD patients experience cognitive impairment (Litvan et al., 2011). Of these impairments, executive dysfunction is most frequently reported early in the disease trajectory (Bosboom, Stoffers, & Wolters, 2004; Muslimovic, Post, Speelman, & Schmand, 2005), while impairments in other cognitive domains (i.e. attention, episodic memory, visuospatial functions) are also highly prevalent (Bosboom et al., 2004). The majority of PD patients ultimately develops PD dementia (PD-D; Aarsland, Andersen, Larsen, Lolk, & Kragh-Sorensen, 2003; Hely, Reid, Adena, Halliday, & Morris, 2008). Moreover, about 10% of the PD patients develops PD-D every year (Aarsland & Kurz, 2010). Cognitive dysfunctions in PD have a significant negative influence on the quality of life (Klepac, Trkulja, Relja, & Babic, 2008), while treatment of these dysfunctions is in its infancy.