Reinforcement drives within- not between-trial motor adaptation.

While reinforcement has repeatedly been shown to impact motor adaptation, previous inconsistent findings hinder our mechanistic understanding. To conciliate contradictory views, we used two distinct motor tasks to investigate whether reinforcement differentially modulates two core motor learning processes: between-trial learning resulting from motor planning and within-trial learning supported by the online correction of ongoing movements. We observed successful adaptation, retention and highly similar temporal profiles of adaptation for both tasks, in line with the notion that motor adaptation of upper limb movements is likely to result from updates in both motor planning and online correction. Reinforcement enhanced performance in the task allowing for within-trial adaptation. Conversely, higher degrees of early motor variability were associated with faster between-trial but not within-trial adaptation. These results highlight a dissociation of factors affecting motor adaptation: Reinforcement seems to primarily impact within-trial learning linked to the online correction of ongoing movements. In contrast, early motor variability might benefit between-trial learning by informing motor planning potentially via an exploration of the motor output space. Our results help explain previous inconsistent findings and add to our theoretical understanding by demonstrating that the effects of reinforcement and motor variability may depend on whether adaptation is driven by within- or between-trial learning.