Impaired planning in Parkinson's disease is reflected by reduced brain activation and connectivity

OBJECTIVE Parkinson's disease (PD) often entails impairments of executive functions, such as planning. Although widely held that these impairments arise from dopaminergic denervation of the striatum, not all executive functions are affected early on, and the underlying neural dynamics are not fully understood. In a combined longitudinal and cross-sectional study, we investigated how planning deficits progress over time in the early stages of PD compared to matched healthy controls. We used functional magnetic resonance imaging (fMRI) to identify accompanying neural dynamics. METHODS Seventeen PD patients and 20 healthy controls performed a parametric Tower of London task at two time points separated by ∼3 years (baseline and follow-up). We assessed task performance longitudinally in both groups; at follow-up, a subset of participants (14 patients, 19 controls) performed a parallel version of the task during fMRI. We performed meta-analyses to localize regions-of-interest (ROIs), that is, the bilateral dorsolateral prefrontal cortex (DLPFC), inferior parietal cortex, and caudate nucleus, and performed group-by-task analyses and within-group regression analyses of planning-related neural activation. We studied task-related functional connectivity of seeds in the DLPFC and caudate nucleus. RESULTS PD patients, compared with controls, showed impaired task performance at both time-points, while both groups showed similar performance reductions from baseline to follow-up. Compared to controls, patients showed lower planning-related brain activation together with decreased functional connectivity. CONCLUSION These findings support the notion that planning is affected early in the PD disease course, and that this impairment in planning is accompanied by decreases in both task-related brain activity and connectivity.