Impaired effort perception, not reward sensitivity, impacts the effort-reward trade-off in Parkinson’s disease

Deciding whether and how to act depends on a trade-off between the effort required to execute a given action and the potential reward for completing it. Impairments in this effort-reward trade-off have been proposed to underlie reduced movement vigor, or bradykinesia, in Parkinson’s disease, but the underlying mechanism remains unclear. Indeed, several mechanisms could alter the effort-reward trade-off, each with unique implications for understanding bradykinesia. Here, we examined three potential sources of impairment in the effort-reward trade-off in people with Parkinson’s disease (both on and off dopamine medication) by separately interrogating reward sensitivity, perceptions of effort, and the mapping between effort and reward compared to age- and sex-matched neurotypical controls. We found that people with PD exhibited behavior consistent with increased perception of effort during isometric force production, compared to controls. In contrast, using a well-established psychometric test, we found no difference in reward sensitivity between groups. People with Parkinson’s disease also showed no difference from control participants in the slope of the effort-reward trade-off function, suggesting a similar effort-reward mapping between groups. Within the Parkinson’s disease group, none of the factors tested were modulated by dopaminergic medication status. Taken together, our results suggest that changes in perceived effort, rather than reduced reward sensitivity, may be an important contributor to reduced vigor in people with Parkinson’s disease, and this impairment may not be mitigated by dopaminergic medication.