Model-free decision-making underlies motor errors in rapid sequential movements under threat

Our movements, especially sequential ones, are usually goal-directed, i.e., coupled with task-level goals. Consequently, cognitive strategies for decision-making and motor performance are likely to influence each other. However, evidence linking decision-making strategies and motor performance remains elusive. Here, we designed a modified version of the two-step task, named the two-step sequential movement task, where participants had to conduct rapid sequential finger movements to obtain rewards (n = 40). In the shock session, participants received an electrical shock if they made an erroneous or slow movement, while in the no-shock session, they only received zero reward. We found that participants who prioritised model-free decision-making committed more motor errors in the presence of the shock stimulus (shock sessions) than those who prioritised model-based decision-making. Using a mediation analysis, we also revealed a strong link between the balance of the model-based and the model-free learning strategies and sequential movement performances. These results suggested that model-free decision-making produces more motor errors than model-based decision-making in rapid sequential movements under the threat of stressful stimuli.