Repetita iuvant: repetition facilitates online planning of sequential movements

Abstract

Beyond being essential for long-term motor-skill development, movement repetition has immediate benefits on performance, increasing the speed and accuracy of a second execution. While such repetition effects have been reported for single reaching movements, whether they apply to movement sequences, or whether they affect planning or execution processes, has yet to be determined. We addressed these questions in two behavioral experiments using a discrete sequence production (DSP) task in which human volunteers had to perform short sequences of finger movements. In Experiment 1, participants were presented with random sequences and we manipulated whether participants had to execute the sequence (Go), or not (No-Go), and whether the sequence was repeated on the next trial. We establish that repeating a sequence of movements led to immediate improvements in speed without associated accuracy costs. The biggest benefit was observed in the middle part of a sequence, suggesting that the repetition effect likely results from facilitated online planning. This claim was further supported by Experiment 2, in which we kept a set of sequences fixed allowing participants to develop sequence-specific learning: once learning reduced the need for online planning, the benefit of repetition disappeared. Finally, we found that repetition-related improvements only occurred for the trials that had been preceded by sequence production, suggesting that action selection and sequence pre-planning may not be sufficient to reap the benefits of repetition. Together, our results highlight the importance of motor practice for enhancing our ability to link individual sequence elements into skilled sequential behavior. Significance Statement Even for overlearned motor skills such as reaching, repeating the same movement improves subsequent performance. How exactly brain processes associated with motor planning and execution might benefit from repetition, however, remains unclear. Here we report the novel finding of repetition effects for sequential movements. Our results indicate that this benefit of repetition is tied to faster and more accurate online planning of upcoming sequence elements. We also highlight how recent movement experience appears to be required to observe the repetition effect, suggesting that actual practice might be more beneficial to the human sensorimotor system than mental rehearsal for producing short-term performance improvements.