Hand Knob Area of Motor Cortex in People with Tetraplegia Represents the Whole Body in a Modular Way

Decades after the motor homunculus was first proposed, it is still unknown how different body parts are intermixed and interrelated in human motor cortex at single-neuron resolution. Using microelectrode arrays, we studied how face, head, arm and leg movements on both sides of the body are represented in hand knob area of precentral gyrus in people with tetraplegia. Contrary to the traditional somatotopy, we found strong representation of all movements. Probing further, we found that ipsilateral and contralateral movements, and homologous arm and leg movements (e.g. wrist and ankle), had a correlated representation. Additionally, there were neural dimensions where the limb was represented independently of the movement. Together, these patterns formed a “modular” code that might facilitate skill transfer across limbs. We also investigated dual-effector movement, finding that more strongly represented effectors suppressed the activity of weaker effectors. Finally, we leveraged these results to improve discrete brain-computer interfaces by spreading targets across all limbs.