Primary motor cortex and feedback control: long-loop reflexes reflect joint mechanics and not joint motion

A fundamental problem with multijoint movements is that motion and torque are coupled between different joints. Therefore, in order to accurately compensate for mechanical perturbations, the brain must convert motion of the limb, sensed by proprioceptors, into motor commands that consider these interjoint interactions. In order to understand how the brain performs this transformation, we examined the activity of neurons in primary motor cortex during transient perturbations of the limb. In general, the activity of neurons during the perturbations was more similar to the cell´s response during a motor task as compared to the cell´s response to passive limb motion. This suggests that neural activity appears to reflect not simply the sensory motion during a perturbation. Rather, the neural activity appears to reflect the requisite joint torques to compensate for the perturbations. These results illustrate that an important role of the transcortical long loop reflex is to consider limb mechanics in sensorimotor feedback control.