Intact reward learning but elevated delay discounting in Parkinson's disease patients with impulsive-compulsive spectrum behaviors.

It has been postulated that impulsive-compulsive spectrum behaviors (ICBs) in Parkinson's disease (PD) reflect overvaluation of rewards, resulting from excessive dopaminergic transmission in the ventral striatum. However, as the ventral striatum is also strongly implicated in delay discounting, an alternative explanation would be that, similar to stimulant-dependent individuals, PD patients with ICBs impulsively discount future rewards. To test these hypotheses, we investigated whether 36 medicated PD patients with and without ICBs differed from controls on measures of stimulus-reinforcement learning and delay discounting. There was a clear double dissociation between reward learning and impulsivity in PD patients with and without ICBs. Although PD patients without ICBs were impaired at learning stimulus-reward associations for high-probability stimuli, PD patients with ICBs were able to learn such associations equally as well as controls. By contrast, PD patients with ICBs showed highly elevated delay discounting, whereas PD patients without ICBs did not differ from controls on this measure. These results contradict the hypothesis that ICBs in PD result from overvaluation of rewards. Instead, our data are more consistent with a model in which excessive dopaminergic transmission induces a strong preference for immediate over future rewards, driving maladaptive behavior in PD patients with ICBs.