Fig. 1: The overview of the experiments.

A Experimental procedure of the Experiments 1 and 2. Participants underwent either the decomposition training or repetition training of the experimental task. The participants who underwent the decomposition training took a 5-min break every 5 blocks, whereas the participants who underwent the repetition training took a 3-minute break every 5 trials. All participants performed the experimental task before and after the training. B (a) Experimental task. The task required the alternate repetition of two movement patterns. Each pattern consists of synchronous strikes of two keys with releasing another two keys; one using the right index and ring fingers for the strikes (movement pattern A) and another using the right middle and little fingers for the strikes (movement pattern B). I, M, R, and L indicate the index, middle, ring, and little fingers, respectively. (b) Skill decomposition training with visual feedback on the timing error of synchronous key motions with the four fingers. The training required practicing each of the two constituent movement patterns separately. For example, when practicing the pattern A, each participant kept depressing the keys with the middle and ring fingers during the preparation phase. Once the cue was provided, these two fingers lifted the keys whereas both the index and ring fingers depressed the adjacent keys synchronously. Participants in the skilled and non-skilled FB groups received the visual feedback (FB) on the timing of the key depression and key release by each of the four fingers within a time window of 0–100 ms (a sublet). When the difference in the timing between the initial and last key motions (timing error in the sublet) was lower than 10 ms or larger than 20 ms, circles corresponding to each finger displayed in the monitor turned black or blue, respectively; otherwise, it turned red. Each participant was instructed to minimize the timing error.