Random change in cortical load representation suggests distinct control of posture and movement.

Accurately maintaining a fixed limb posture and quickly moving between postures underlies both everyday skills, including holding and lifting a cup of coffee, and expert skills, such as an Olympic wrestler's holding and throwing an opponent. A fundamental question in limb motor control is whether the brain manages these contrasting goals of posture and movement through a single, robust control process or whether each engages a specialized control process. We addressed this question by examining how individual neurons in the primary motor cortex of macaque monkeys represent mechanical loads during posture and movement tasks. Notably, approximately half of the neurons that expressed load-related activity did so exclusively during either posture only or movement only. Further, those neurons with load-related activity during both tasks randomly switched their magnitude of response between tasks. These random changes in load representation suggest specialized control processes, one for posture and one for movement.