The natural variability of a dexterous motor skill is stably encoded in the cortex of freely behaving mice
Skilled, goal-directed movements exhibit trial-to-trial variability even in experts, particularly in response to dynamic environmental conditions or when perfect repetition is not required for success. Identifying where, to what extent, and how stably this variability is encoded in the nervous system is essential for understanding how learned movements are robustly maintained over time yet flexibly executed on each trial. We record calcium fluorescence activity in forelimb motor cortex (M1), a key node in the multi-areal network responsible for movement control, in freely-moving mice of both sexes as they performed a self-paced, precision reach-to-grasp task. High trial counts and rich single-trial variability enable rigorous statistical analysis of moment-to-moment movement encoding across matched behavioral sets over five days. Approximately 80% of recorded neurons significantly encoded paw, digit, and head movements during reaching, as quantified using linear models. Across days, encoding similarity shows a small but measurable decline that increases with the interval between recording sessions. This drift is heterogeneously distributed across the population, with many neurons retaining high encoding similarity even in sessions four days apart, as assessed using shuffle controls and comparison to encoding for trial-averaged movements. Thus, over the timescale examined, M1 is capable of maintaining stable encoding of movement details at the level of single cells, even for complex, sensory-guided tasks like reach-to-grasp. Together, these results raise the question of whether downstream circuits support consistent behavior by preferentially relying on neurons with greater stability or instead through population-level readout that is robust to a modest level of representational change.