Hippocampal skill memory expansion drives online performance dynamics during skill learning

Motor skill learning depends on refining action sequences through practice, yet how learning unfolds moment-to-moment within trials remains unclear. Here, healthy participants learned generative sequential keypress skills at different speeds. Within trials, we identified reproducible high initial skill (HIS) segments, brief epochs of elevated performance followed by decline, that emerged only during learning of repeating sequences and were absent during practice of pseudo-randomized, non-repeating sequences. HIS keypress content scaled with execution speed and increased with practice. This expansion of HIS segment content continued after overall performance plateaued. Neural analyses showed increased theta-gamma phase-amplitude coupling (θ/γ PAC) and beta-band bursts during HIS segments. Hippocampal θ/γ PAC predicted HIS content, suggesting a mechanism for binding successive actions into progressively larger skill-memory representations during early learning. Together, these findings identify expansion of skill memory, rather than fatigue, as a key driver of early skill learning.