Reward processing in health and Parkinson's disease: neural organization and reorganization

It has been suggested that motivational processes mediated by dopaminergic neural systems may be relatively spared in Parkinson's disease (PD) and activation of these pathways may be of therapeutic relevance. To investigate the behavioural and neural correlates of motivation in unmedicated PD patients, we used H(2)(15)O positron emission tomography to measure brain activation patterns related to the processing of monetary rewards of different magnitudes during a spatial search task in PD patients withdrawn from medication, and age-matched healthy controls. Both groups showed increased search efficiency with increasing reward, but demonstrated different patterns of neuronal activation. In healthy controls activity in prefrontal and rhinal cortices, and thalamic activity correlated with reward magnitude. In contrast, activity in the cerebellar vermis in PD patients increased with increasing reward magnitude, suggesting it was sensitive to motivational state. We interpret these relative increases in cerebellar activation as evidence for the presence of compensatory neural mechanisms in unmedicated PD patients.