Bridging chunks during complex movement sequence execution

Executing complex movement sequences from memory is crucial for skillful motor actions such as musical performance and sports. While chunking helps memorization during practice, transitions between chunks may become vulnerable during execution. Here, we developed a novel paradigm combining chunked motor sequence memorization with online sensory perturbation to show that perturbations at chunk junctions induced larger errors than those within chunks and increased pupil diameter, reflecting elevated attentional load. Subsequent training focusing on bridging chunk junctions improved performance stability, as evidenced by reduced force variability during perturbation. EEG data further revealed elevated frontal theta power in response to perturbation, indicating greater cognitive effort. Critically, this disruption was reduced after training, suggesting more efficient memory retrieval. Together, these findings underscore the vulnerability of motor sequence retrieval at chunk junctions and highlight the potential of targeted training to stabilize execution and reduce cognitive load.