Refinement of efficient encodings of movement in the dorsolateral striatum throughout learning

The dorsal striatum plays a critical role in action selection, movement, and sensorimotor learning. While action-specific striatal ensembles have been described, the mechanisms underlying their formation and evolution during motor learning remain poorly understood. Here, we employed longitudinal two-photon Ca²⁺ imaging of dorsal striatal neurons in head-fixed mice as they learned to self-initiate locomotion. We found that both direct- and indirect-pathway spiny projection neurons (dSPNs and iSPNs, respectively) exhibited robust activation during early locomotor bouts, with activity gradually diminishing across sessions. For dSPNs, action onset and offset ensembles progressively emerged from an initially broad population of nonspecific neurons. In contrast, iSPN ensembles originated from neurons responsive to opposing actions before refining into onset- or offset-specific populations. These findings demonstrate that striatal ensemble activity becomes more selective over time, with a reduction in nonspecific neuronal activation and an increase in the efficiency of striatal encoding for learned motor actions.