Reward-based improvements in motor sequence learning are differentially affected by dopamine

Reward is a powerful tool to enhance human motor behaviour with research showing that it promotes motor sequence learning through increases in both motor vigour and movement fusion. Specifically, during a sequential reaching movement, monetary incentive leads to increased speed of each movement (vigour effect), whilst reward-based performance feedback increases speed of transition between movements (fusion effect). Therefore, motor sequence learning can be driven by distinct reward types with dissociable underlying processes. The neurotransmitter dopamine has been implicated to modulate motor vigour and regulate movement fusion. However, in humans, it is unclear if the same dopaminergic mechanism underlies both processes. To address this, we used a complex sequential reaching task in which rewards were based on movement times (MT). Crucially, MTs could be reduced via : 1) enhanced speed of individual movements (vigour effect) and/or 2) enhanced speed of transition between movements (fusion effect). 92 participants were randomly assigned to a reward and no reward group and were given either 2.5mg of the dopamine antagonist haloperidol or a placebo. Our results demonstrate that haloperidol impaired the reward-based effects on motor vigour whilst not affecting movement fusion. Thus, we illustrate that whilst both strategies are reward sensitive, they rely on dissociable mechanisms.