Excited to share my latest work with @jonathanamichaels.bsky.social @diedrichsenjorn.bsky.social & @andpru.bsky.social! We asked: How does the motor cortex account for arm posture when generating movement? Paper 👉 www.biorxiv.org/content/10.1... 1/10

The problem: Moving your arm to grab coffee requires different muscle commands depending on where your arm starts. We know the brain must incorporate posture when planning movement—but how neural dynamics achieve this remains unclear. 2/10

Why this problem exists: Most insights come from center-out tasks, where all movements start from one spot. Here, reach direction and final posture are always correlated—making it impossible to separate movement dynamics from posture encoding. 3/10

Key finding: High-density recordings from M1 & PMd revealed a compositional neural geometry with 3 components. 1️⃣ A posture subspace: fixed points for each target, visited whenever the arm rested at that location before or after a reach. 5/10

2️⃣ Rotational dynamics: transitions linking posture-specific fixed points.These rotations were systematic—similar rotations produced similar reach directions—and their projection continuously updated posture. 6/10

3️⃣ A condition-independent shift dimension: a trajectory reflecting trial progression, unfolding similarly across all movements, regardless of direction or posture. 7/10

Modeling: Using modular RNNs, we asked what’s required for this compositional structure to emerge. It turns out this solution is common whenever the effector is complex enough to demand posture-dependent control policies. 8/10

Conclusion: Motor cortex dynamics are compositional. They simultaneously: - generate movement - maintain an internal representation of posture - track task progress 9/10