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Motor cortex generates output necessary for the execution of a wide range of motor behaviours. Although neural representations of movement have been described throughout motor cortex, how population activity in output layers relates to the execution of distinct motor actions is less well explored. To address this, we imaged layer 5B population activity in mice performing a two-action forelimb task. We found most neurons convey a generalised movement signal, with action-type-specific signalling restricted to relatively small, spatially intermingled subpopulations of neurons. Deep layer population dynamics largely reflect dense, action-invariant signals that correlate with movement timing, while embedded sparse action-type representations reflect distinct forelimb actions. We suggest that sparse coding of action-type enhances the number of possible output configurations necessary for behavioural flexibility and the execution of a wide repertoire of behavioural actions.

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