Hypothetical joint-related coordinate systems in which populations of motor cortical neurons code direction of voluntary arm movements

Results of the recent electrophysiological experiments by Caminiti et al. suggest that populations of neurons of the motor areas code direction of voluntary arm movement in an intrinsic coordinate system. In this letter I propose a set of joint-related coordinate systems that rotate with the posture of the arm in which populations of motor cortical neurons code direction of the arm movement. In these frames of reference, the movement directions represented vary with the posture of the arm but the preferred directions of the motor cortical neurons do not rotate. It is suggested that the difference in the neuronal activity for different workspaces, which was observed by Caminiti et al., is due to the dependence of the movement direction, represented in one of the joint-related coordinate systems, on the posture of the arm.