The effect of externally generated loading on predictive grip force modulation

Alice G Witney,Daniel M Wolpert

doi:10.1016/j.neulet.2006.10.026

Abstract

A characteristic of skilled movement is the ability of the CNS to predict the consequences of motor commands. When we lift an object there is an anticipatory increase in grip force that prevents a grasped object from slipping. When an object is pulled from our grasp by an external force, a reflexive modulation in grip force prevents slippage. Here we examine how external perturbations to a grasped object influence anticipatory grip force during object manipulation using a bimanual task, with each hand holding a computer-controlled object. Subjects were instructed to maintain the position of the object held in the right hand. Loading was applied to this restrained object: either self-generated by the action of their left hand or externally generated by a motor. The magnitude of the grip force response to self-generated loading increased after the object was loaded, and the latency of this response remained predictive of load force. This implies that external and self-generated loading increase the anticipatory grip force response. Unlinked trials, where the subject's moved their left hand but no loading was generated on the right-hand object were used to assess the presence of purely predictive control of grip force. External loading soon after self-generated loading maintained an existing predictive response once the linkage between the subject's action and object loading had been removed. However, external loading had no influence as the existing prediction decays. Therefore, the predictive grip force response during object manipulation can be significantly modified by object loading from an external source.

Highlights

When the position of our body [7] or a hand held object [8] is compromised through perturbations, our motor system acts to maintain stability
If we self-generate a perturbation, for instance when one hand pulls on an object held in the other, efference copy of our motor command enables us to anticipate the consequences of our action [3,4,5]
If a perturbation to an object is caused by loading from an external source, for instance when someone pulls on the object that we are holding, our motor system will instead reflexively adjust grip force to compensate for the alteration in load force [2]

Summary

Introduction

When the position of our body [7] or a hand held object [8] is compromised through perturbations, our motor system acts to maintain stability. Different loadings occurred on the right-hand object, either self or externally generated, and subjects were instructed to maintain this object’s position. In the ‘Decay of Prediction’ experiment, whether previous external loading influences the predictive or the reflexive component of the grip force response.

Objectives

Results

Conclusion