Direct cortical control of muscle activation in voluntary arm movements: a model.

What neural activity in motor cortex represents and how it controls ongoing movement remain unclear. Suggestions that cortex generates low-level muscle control are discredited by correlations with higher-level parameters of hand movement, but no coherent alternative exists. I argue that the view of low-level control is in principle correct, and that seeming contradictions result from overlooking known properties of the motor periphery. Assuming direct motor cortical activation of muscle groups and taking into account the state dependence of muscle-force production and multijoint mechanics, I show that cortical population output must correlate with hand kinematics in quantitative agreement with experimental observations. The model reinterprets the 'neural population vector' to afford unified control of posture, movement and force production.