Null effects of levodopa on reward- and error-based motor adaptation, savings, and anterograde interference

Abstract

Dopamine signaling is thought to mediate reward-based learning. We tested for a role of dopamine in motor adaptation by administering the dopamine precursor levodopa to healthy participants in two experiments involving reaching movements. Levodopa has been shown to impair reward-based learning in cognitive tasks. Thus, we hypothesized that levodopa would selectively impair aspects of motor adaptation that depend on reinforcement of rewarding actions.In the first experiment, participants performed two separate tasks in which adaptation was driven either by visual feedback of the hand position or binary reward feedback. We used EEG to measure event-related potentials evoked by task feedback. We hypothesized that levodopa would specifically diminish adaptation and the neural responses to feedback in the reward learning task. However, levodopa did not affect motor adaptation in either task nor did it diminish event-related potentials elicited by reward outcomes.In the second experiment, participants learned to compensate for mechanical force field perturbations applied to the hand during reaching. Previous exposure to a particular force field can result in savings during subsequent adaptation to the same force field and interference during adaptation to an opposite force field. We hypothesized that levodopa would diminish savings and anterograde interference, as previous work suggests that these phenomena result from a reinforcement learning process. However, we found no reliable effects of levodopa.These results suggest that reward-based motor adaptation, savings, and interference may not depend on the same dopaminergic mechanisms which have been shown to be disrupted by levodopa during various cognitive tasks.New and NoteworthyMotor adaptation relies on multiple processes including reinforcement of successful actions. Cognitive reinforcement learning is impaired by levodopa-induced disruption of dopamine function. We administered levodopa to healthy adults who participated in multiple motor adaptation tasks. We found no effects of levodopa on any component of motor adaptation. This suggests that motor adaptation may not depend on the same dopaminergic mechanisms as cognitive forms or reinforcement learning which have been shown to be impaired by levodopa.

Individual Differences in Sensorimotor Adaptation Are Conserved Over Time and Across Force-Field Tasks

“…When the ANOVA revealed a significant difference across testing sessions, post-hoc comparisons between individual testing sessions were performed using F -tests ( Meteyard and Davies, 2020 ). Savings was quantified by comparing kinematic and clamp-based measures in Early Adaptation across testing sessions ( Leow et al, 2012 ; Coltman et al, 2019 ; Nguyen et al, 2019 ; Palidis et al, 2020 ). Measures of adaptation were compared across tasks ( Experiment 2 ) using paired t -tests (two-tailed).…”