From Motion Plans to Motor Execution

The second lecture will deal with motor execution. I shall describe current ideas of how the brain carries out desired motion plans. One fundamental problem in executing desired motions is associated with transforming hand trajectories into joint rotations. This problem is rather complicated given the excess number of degrees of freedom available at the joint versus the hand levels and the many possible combinations of joint rotations that can be used to carry out any given task. I will discuss several of the currently existing ideas on how the brain may resolve these problems, based on human and primate studies and research dealing with the control of the octopus' flexible arms. Another problem associated with motor execution is the inverse dynamics problem, namely establishing what forces and torques are needed to execute desired joint rotations. I will discuss current models addressing this issue. One such model proposes that the brain explicitly calculates the required forces and torques based on the use of internal models. An alternative model, however, suggests that the brain may control both posture and movement by taking advantage of the visco-elastic properties of muscles and by choosing appropriate limb stiffness and viscosity. In this context I will describe recent studies focusing on force control during grasping and manipulation of objects.