Continuous estimation of reaching space in superficial layers of the motor cortex

Motor cortex plays a key role in controlling voluntary arm movements towards spatial targets. The cortical representation of spatial information has been extensively studied and was found to range from combinations of muscle synergies to cognitive maps of locations in space. How such abstract representations of target space evolve during a behavior, how they integrate with other behavioral features and what role they play in movement control is less clear. Here we addressed these questions by recording the activity of layer 2/3 (L2/3) neurons in the motor cortex using two-photon calcium imaging in head-restrained mice, while they reached for water droplets presented at different spatial locations around their snout. Our results reveal that a majority (>80%) of L2/3 neurons with task-related activity are target-space selective and their activity is contingent on a single target position in an ego-centric reference frame. This spatial framework is preferentially organized along three cardinal directions (Center, Left and Right). Surprisingly, the coding of target space is not limited to the activity during movement planning or execution, but is also predominant during preceding and subsequent phases of the task, and even persists beyond water consumption. More importantly, target specificity is independent of the movement kinematics and is immediately updated when the target is moved to a new position. Our findings suggest that, rather than descending motor commands, the ensemble of L2/3 neurons in the motor cortex conjointly encode internal (behavioral) and external (spatial) aspects of the task, playing a role in higher-order representations related to estimation processes of the ongoing actions.