Evidence for subjective values guiding posture and movement coordination in a free-endpoint whole-body reaching task.

P M Hilt,C Papaxanthis,P J Stapley,T Pozzo,B Berret

doi:10.1038/srep23868

Abstract

When moving, humans must overcome intrinsic (body centered) and extrinsic (target-related) redundancy, requiring decisions when selecting one motor solution among several potential ones. During classical reaching studies the position of a salient target determines where the participant should reach, constraining the associated motor decisions. We aimed at investigating implicit variables guiding action selection when faced with the complexity of human-environment interaction. Subjects had to perform whole body reaching movements towards a uniform surface. We observed little variation in the self-chosen motor strategy across repeated trials while movements were variable across subjects being on a continuum from a pure ‘knee flexion’ associated with a downward center of mass (CoM) displacement to an ‘ankle dorsi-flexion’ associated with an upward CoM displacement. Two optimality criteria replicated these two strategies: a mix between mechanical energy expenditure and joint smoothness and a minimization of the amount of torques. Our results illustrate the presence of idiosyncratic values guiding posture and movement coordination that can be combined in a flexible manner as a function of context and subject. A first value accounts for the reach efficiency of the movement at the price of selecting possibly unstable postures. The other predicts stable dynamic equilibrium but requires larger energy expenditure and jerk.

Highlights

Sensorimotor values guiding decision making behaviors for voluntary movement
If we assume that the behavior adopted reflects these idiosyncratic values, the task could verify whether motor decisions rely upon the exclusion of competing options or on the combination of a few motor preferences
The protocol used here (Fig. 1) created considerable inter-individual differences in strategy that led to varying amounts of CoM and finger trajectories as a result of the different joint configurations adopted to achieve the task of reaching to an unspecified target in the normal base of support (NBoS) condition

Summary

Introduction

Sensorimotor values guiding decision making behaviors for voluntary movement. We suggest that previous protocols have only partially addressed the question of free decision-making processes in motor control. A movement that does not explicitly provide an endpoint to reach towards introduces spatial ambiguity and exposes the subject to a number of subjective and free choices[9,10] To address these limitations, we designed a protocol where: i) the motor output was not constrained and ii) the reward associated with the different motor choices was not externally but internally driven. Current theoretical approaches propose that the coordination of reaching and equilibrium modules follow a hierarchical organization where the perturbation due to upper limb movement is compensated for in advance by a postural component (for a review see[11]). If the two components act together to facilitate the execution of the movement[12] a combination of “competing” mechanisms, trading-off the equilibrium and reaching components of the task, may be predicted

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Conclusion