Enhanced or impaired cognitive function in Parkinson's disease as a function of dopaminergic medication and task demands

We investigated how dopamine (DA) systems contribute to cognitive performan ce in the domain of learning and attentional flexibility by examining effec ts of withdrawing DA-ergic medication in patients with Parkinson's disease (PD). Medication remediated impairments in switching between two tasks, tho ught to depend on circuitry connecting the dorsolateral prefrontal cortex a nd the posterior parietal cortex to the dorsal caudate nucleus, which is pr ofoundly DA-depleted in PD. By contrast, the same medication impaired proba bilistic reversal learning that implicates orbitofrontal cortex-ventral str iatal circuitry, which is relatively spared of DA loss in PD. Hence, DA-erg ic medication improves or impairs cognitive performance depending on the na ture of the task and the basal level of DA function in underlying cortico-s triatal circuitry.