Force and Position Control in Humans - The Role of Augmented Feedback

Summary

Controlling an identical movement with position or force feedback results in different neural activation and motor behavior. This protocol describes how to investigate behavioral changes by looking at neuromuscular fatigue and how to evaluate motor cortical (inhibitory) activity using subthreshold TMS with respect to the interpretation of augmented feedback.

Abstract

During motor behaviour, humans interact with the environment by for example manipulating objects and this is only possible because sensory feedback is constantly integrated into the central nervous system and these sensory inputs need to be weighted in order meet the task specific goals. Additional feedback presented as augmented feedback was shown to have an impact on motor control and motor learning. A number of studies investigated whether force or position feedback has an influence on motor control and neural activation. However, as in the previous studies the presentation of the force and position feedback was always identical, a recent study assessed whether not only the content but also the interpretation of the feedback has an influence on the time to fatigue of a sustained submaximal contraction and the (inhibitory) activity of the primary motor cortex using subthreshold transcranial magnetic stimulation. This paper describes one possible way to investigate the influence of the interpretation of feedback on motor behaviour by investigating the time to fatigue of submaximal sustained contractions together with the neuromuscular adaptations that can be investigated using surface EMG. Furthermore, the current protocol also describes how motor cortical (inhibitory) activity can be investigated using subthreshold TMS, a method known to act solely on the cortical level. The results show that when participants interpret the feedback as position feedback, they display a significantly shorter time to fatigue of a submaximal sustained contraction. Furthermore, subjects also displayed an increased inhibitory activity of the primary cortex when they believed to receive position feedback compared when they believed to receive force feedback. Accordingly, the results show that interpretation of feedback results in differences on a behavioural level (time to fatigue) that is also reflected in interpretation-specific differences in the amount of inhibitory M1 activity.

Introduction

Sensory feedback is crucial to perform movements. Daily activities are hardly possible in the absence of proprioception ¹. Furthermore, motor learning is influenced by proprioceptive integration ² or cutaneous perception ³. Healthy humans with intact sensation are able to weight the sensory inputs arising from various sensory sources in order to meet situation-specific needs ⁴. This sensory weighing enables humans to perform difficult tasks with high precision even when some aspects of the sensory information are unreliable or even absent (e.g., walking in the dark or with eyes closed).

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Protocol

The protocol described here followed the guidelines of the ethics committee of the University of Freiburg and was in accordance with the declaration of Helsinki (1964).

1. Ethical Approval - Subject Instruction

1. Before the actual experiment, instruct all subjects about the purpose of the study and potential risk factors. When applying transcranial magnetic stimulation (TMS), there are some medical risks including any history of epileptic seizures, metal implants in eyes and/or head, diseases of the cardiovascular system and pregnancy. Exclude any subject affirming to one of these risk factors from the study.
2. Include onl....

Results

Interpretation of feedback

In procedure described here, subjects were instructed in a way that they believed in half of their trials to have received position feedback and in the other half of the trials to have received force feedback. In fact, they were tricked in half of their trials as they the pF-group always received position feedback and the fF-group always received force feedback.

Discussion

The present study investigated if the interpretation of augmented feedback influences the time to fatigue of a sustained submaximal contraction and the neural processing of the primary motor cortex. The results show that as soon as the participants interpreted the feedback as position feedback (compared to force feedback), the time to fatigue was significantly shorter and the inhibitory activity of the motor cortex (measured as the amount of EMG suppression caused by subTMS) is greater. As the task did not change between.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
torquemeter	LCB 130, ME-Mebsysteme, Neuendorf, Germany		Part of robotic device built for force and position recordings
potentiometer	type 120574, Megatron, Putzbrunn, Germany		Part of robotic device built for force and position recordings
EMG electrodes	Blue sensor P, Ambu, Bad Nauheim, Germany
TMS coil	Magstim
TMS machine	Magstim Company Ltd., Whitland, UK
Recording software	Labview-Based		custom written software