Two ways to learn in visuomotor adaptation.

Previous research has demonstrated significant interindividual variability in the recruitment of fast-explicit and slow-implicit processes during motor adaptation. In addition, we previously identified qualitative individual differences in adaptation linked to the formation and updating of new memory processes. Here, we investigated quantitative and qualitative differences in visuomotor adaptation with a design incorporating repeated learning and forgetting blocks, allowing for precise estimation of individual learning and forgetting rates in fast-slow adaptation models. Participants engaged in a two-day online visuomotor adaptation task. They first adapted to a 30° perturbation to eight targets in three learning blocks. Approximately 24 h later, they performed a no-feedback retention and a relearning block. We clustered the participants into strong and weak learners based on adaptation levels at the end of day 1 and fitted a fast-slow system to the adaptation data. Strong learners exhibited a strong negative correlation between the estimated slow and fast processes, which predicted 24-h retention and savings, respectively, supporting the engagement of a fast-slow system. The individual differences in the recruitment of the two processes were attributed to a wide range of learning rates. Conversely, weak learners exhibited retention but no savings, supporting the engagement of a single slow system. Finally, both during baseline and adaptation, reaction times were shorter for weak learners. Our findings thus reveal two distinct ways to learn in visuomotor adaptation and highlight the necessity of considering both quantitative and qualitative individual differences in studies of motor learning.NEW & NOTEWORTHY Humans show large variability in their capacity to learn motor skills. Here, we demonstrate using an online experiment that the between-subject variability in motor adaptation has two sources: a qualitative difference, whereas some learners recruit a single slow process and others recruit both a fast and a slow process, and, among the latter group of learners, large quantitative differences in the competitive recruitment of the two processes.