This thesis aimed at gaining more insight into the neural underpinnings of the cognitive heterogeneity among early-stage PD patients. For this purpose, we scanned 25 patients with PD who were not on dopamine replacement therapy, and 43 healthy controls while performing a set-shifting, working memory, and response inhibition task in an MRI scanner. We, furthermore, induced a "temporary lesion" in healthy participants to mimic the decreased activation of the left dorsal PFC in PD patients. Last, we compared the T1-weighted MRI scans from a large cohort of PD patients (N=93) with those of a group of ad hoc recruited healthy participants (N=46) to investigate between-group differences in brain structure, and assess the relation between structure related measures and performance on neuropsychological tasks within the group of PD patients.

We will first briefly discuss the main findings per research question based on the results reported in the corresponding chapter(s), and subsequently try to interpret our overall results in relation to the existing literature on PD and cognition. We will then try to deduce a working model about cognitive heterogeneity and compensation in PD, and discuss the consequences of our results for the classical model of the basal ganglia. Last, we will consider a number of important methodological issues that apply to this thesis, and end with recommendations for future investigations.