Abstract
It has been largely proved that while judging a force humans mainly rely on the motor commands produced to interact with that force (i.e., sense of effort). Despite of a large bulk of previous investigations interested in understanding the contributions of the descending and ascending signals in force perception, very few attempts have been made to link a measure of neural output (i.e., EMG) to the psychophysical performance. Indeed, the amount of correlation between EMG activity and perceptual decisions can be interpreted as an estimate of the contribution of central signals involved in the sensation of force. In this study we investigated this correlation by measuring the muscular activity of eight arm muscles while participants performed a quasi-isometric force detection task. Here we showed a method to quantitatively describe muscular activity (“muscle-metric function”) that was directly comparable to the description of the participants' psychophysical decisions about the stimulus force. We observed that under our experimental conditions, muscle-metric absolute thresholds and the shape of the muscle-metric curves were closely related to those provided by the psychophysics. In fact a global measure of the muscles considered was able to predict approximately 60% of the perceptual decisions total variance. Moreover the inter-subjects differences in psychophysical sensitivity showed high correlation with both participants' muscles sensitivity and participants' joint torques. Overall, our findings gave insights into both the role played by the corticospinal motor commands while performing a force detection task and the influence of the gravitational muscular torque on the estimation of vertical forces.
Highlights
To carry out most of our everyday actions, both internal forces—e.g., muscular torques as well as external forces—e.g., gravity—must be taken into account
From one of the few studies quantifying [11,12] the correlation between muscles activity and subjects’ perceived muscular effort that provided an index of dis/ concordance, we show a method to quantitatively describe muscular activity (“muscle-metric function”) that was directly comparable to the description of psychophysical decisions in a force detection task
By modeling the EMG changes with respect to the forces applied on their arm, we were able, for first time, to provide a quantitative description of the muscular activity (“muscle-metric curve”) that was directly comparable to the description of the psychophysical judgments described by the psychometric function
Summary
To carry out most of our everyday actions, both internal forces—e.g., muscular torques as well as external forces—e.g., gravity—must be taken into account. Arm Muscles Activity and Unloading Force Perception the external force will result from the combination of two kinds of signals: one derived from the descending motor commands necessary to counteract the external force and to maintain the posture, and the second one derived from the resulting afferent somaesthetic signals. Several different approaches, such as psychophysics, analysis of kinematics and dynamics, electromyography (EMG), and computational modeling of motor control, have been used to investigate both the mechanisms of how the central nervous system (CNS) interacts with external forces and how it merges descending and ascending signals to perform a perceptual decision. The common observation of these works of a perceptual sensitivity reduction when efferent signals were manipulated gave behavioral support to the idea that force perception is mainly mediated by central signals [5,16,18,19]
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