The Nervous System Independently Controls Motion and Force

Vikram Chib
Department of Biomedical Engineering, Northwestern Universit

Matthew Krutky
Department of Biomedical Engineering, Northwestern University

Kevin Lynch
Department of Mechanical Engineering, Northwestern University

Ferdinando Mussa-Ivaldi
Department of Physiology Engineering, Northwestern University

     Full text: Not available
     Last modified: January 17, 2007

Abstract
When manipulating objects, we must control our hand motion as well as the interaction forces that arise from contact with the environment. At the level of musculoskeletal biomechanics, motions and forces are coupled by intrinsic limb impedance. However, it has yet to be established whether at the neural level the control of motion and force are coupled or independent. Here we provide evidence for the existence of independent neural controllers for arm motion and interaction forces. This evidence is offered by transcranial magnetic stimulation (TMS) of posterior parietal cortex (PPC) resulting in the differential disruption of the control of motion but not of force.

We designed three experimental conditions where force and position control tasks appeared in combination or separately. Accordingly, our experiment consisted of three blocks: Combined, Force, and Motion. During the Combined block subjects applied force to the handle of the manipulandum in a leftward direction as it was moved to center out positions along a smooth trajectory. This block required the simultaneous control of motion and force. During the Force block subjects maintained an applied force as the manipulandum moved along a very slow constant velocity profile in which inertial effects were negligible. In this condition, the quality of force control was assessed by quantifying the ability of the subjects to maintain constant force vectors at different arm configurations. In the Motion block subjects were asked to track a predetermined movement trajectory. In order to compare performance in this and in the other tasks, subjects were required to track a predictable motion of the manipulandum while maintaining contact with it. A perfect position controller would produce perfect tracking, thus resulting in zero interaction force between the subject and the manipulandum. Deviation from zero force was attributed to errors in the ability of the subject to track the desired trajectory. Single pulse TMS was applied to left PPC during all blocks after learning.

We found (a) that TMS stimulation results in disruption of performance during the Combined and Motion blocks, but not the Force Block. Furthermore, (b) at the end of learning, a simple summation of forces from the Motion block and the Force block describe 80-97% of the variability of forces applied in the Combined block. These results are consistent with the presence, after learning, of independent force and motion controllers. They also suggest that PPC is critical to the neural control of hand motion but not of interaction force.