The role of internal models in motion planning and control: evidence from grip force adjustments during movements of hand-held loads

Abstract

We investigated the issue of whether or not the CNS makes use of an internal model of the motor apparatus in planning and controlling arm movements. In particular, we tested the ability of subjects to predict different hand-held loads by examining grip force adjustments used to stabilize the load in the hand during arm movements. Subjects grasped a manipulandum using a precision grip with the tips of the thumb and index finger on either side. The grip force (normal to the contact surfaces) and the load force (tangential to the surfaces) were measured, along with the trajectory of the hand. The manipulandum was attached to two servo-controlled linear motors used to create inertial and viscous loads as well as a composite load, including inertial, viscous, and elastic components. The form of the hand trajectory was independent of load for some subjects but varied systematically across load conditions in others. Nevertheless, under all load conditions and in all subjects, grip force was modulated in parallel with, and thus anticipated, fluctuations in load force despite the marked variation in the form of the load function. This indicates that the CNS is able to predict the load force and the kinematics of hand movement on which the load depends. We suggest this prediction is based on an internal model of the motor apparatus and external load and is used to determine the grip forces required to stabilize the load.

Fig. 1.

A, Top view of the experimental setup. Subjects grasp a manipulandum attached to two force-served linear motors mounted at right angles to give motion in the horizontal plane. B, Side view of the manipulandum instrumented with force sensors to measure grip force normal to the contact surface and load forces tangential to the surface.

Fig. 2.

Single kinematic and force records from one subject under the three load conditions. Shaded regionsindicate the horizontal load force (HF) resisting the movement and the primary kinematic variable on which this component of the load depended. Under all three load conditions, grip force (GF) is adjusted in parallel with fluctuations in load force (LF), the resultant load tangential to the grasp surface. All calibration bars start at zero. Dashed vertical lines indicate movement onset.

Fig. 3.

Overlaid kinematic and kinetic records taken from the last 20 trials under the inertial load condition (after adaptation to the load). Five push and five pull trials are shown for two subjects. Calibration values for bars are given in the bottom left panel. All bars start at zero.

Fig. 4.

Overlaid kinematic and kinetic records taken from the last 20 trials under the viscous load condition (after adaptation to the load). Five push and five pull trials are shown for two subjects. Calibration values for bars are given in the bottom left panel. All bars start at zero.

Fig. 5.

Overlaid kinematic and kinetic records taken from the last 20 trials under the composite load condition (after adaptation to the load). Five push and five pull trials are shown for two subjects. Calibrations for bars are given in the bottom left panel. All bars start at zero.

Fig. 6.

Five hand velocity profiles from the last 10 pull trials in each of the three load conditions for four subjects. Velocity profiles were normalized with respect to area and peak velocity and aligned with respect to peak velocity. Whereas one-half of the subjects produced profiles that were invariant across loads (illustrated byS8 and S6), the others did not (illustrated by S10 and S3).