Force requirements of observed object lifting are encoded by the observer’s motor system: a TMS study

Abstract

Several transcranial magnetic stimulation (TMS) studies have reported facilitation of the primary motor cortex (M1) during the mere observation of actions. This facilitation was shown to be highly congruent, in terms of somatotopy, with the observed action, even at the level of single muscles. With the present study, we investigated whether this muscle-specific facilitation of the observer's motor system reflects the degree of muscular force that is exerted in an observed action. Two separate TMS experiments are reported in which corticospinal excitability was measured in the hand area of M1 while subjects observed the lifting of objects of different weights. The type of action 'grasping-and-lifting-the-object' was always identical, but the grip force varied according to the object's weight. In accordance to previous findings, excitability of M1 was shown to modulate in a muscle-specific way, such that only the cortical representation areas in M1 that control the specific muscles used in the observed lifting action became increasingly facilitated. Moreover, muscle-specific M1 facilitation was shown to modulate to the force requirements of the observed actions, such that M1 excitability was considerably higher when observing heavy object lifting compared with light object lifting. Overall, these results indicate that different levels of observed grip force are mirrored onto the observer's motor system in a highly muscle-specific manner. The measured force-dependent modulations of corticospinal excitability in M1 are hypothesized to be functionally relevant for scaling the observed grip force in the observer's own motor system. In turn, this mechanism may contribute, at least partly, to the observer's ability to infer the weight of the lifted object.

Dissociated Representations of Deceptive Intentions and Kinematic Adaptations in the Observer's Motor System

“…Thus, we observed that, once the actual weight of the object became known in the deceived condition, the FDI muscle was involved in adjusting the lifting of the object according to its actual weight. In particular, in the central and last phases of truthful lifting, the FDI muscle was more activated for the heavy than for the light object (third bin: P = 0.023; fourth bin: P < 0.001; fifth bin: P < 0.001), in keeping with the sensitivity of the hand muscles to object weight (Alaerts et al 2010a(Alaerts et al , 2010bSenot et al 2011;Tidoni et al 2013). Importantly, this weight-sensitivity was reflected into a reduced activation of the FDI while the actors were lifting an object that was expected to be lighter than its actual weight as compared to when they received truthful information.…”

“…More importantly, the FDI CSE is more facilitated during the observation of fooling than genuine hand actions exerted upon objects of the same weight (Tidoni et al 2013). As well, not only is the ECR muscle activity involved in lifting action execution, but the ECR CSE is highly sensitive to changes in the kinematic pattern and exerted force during action observation (Alaerts et al 2010a(Alaerts et al , 2010b. However, there is no evidence about the ECR involvement in deceptive behaviors (see electromyography, EMG, recording during action execution).…”

“…These 2 muscles were selected for their motor role during reaching-tograsp and lifting movements and sensitivity to genuine versus deceptive intentions during action observation. Indeed, the FDI muscle activity is directly involved in controlling the strength of the grip during lifting (Alaerts et al 2010a(Alaerts et al , 2010bSenot et al 2011). More importantly, the FDI CSE is more facilitated during the observation of fooling than genuine hand actions exerted upon objects of the same weight (Tidoni et al 2013).…”

Placebo-Induced Changes in Excitatory and Inhibitory Corticospinal Circuits during Motor Performance

“…Trials for which the FDI MEP amplitude was lower than the mean EMG background_rms were discarded (Alaerts et al, 2010). Furthermore, data of all the measured neurophysiological parameters (i.e., FDI amp , ADM amp , FDI csp , FDI EMG background_rms , and ADM EMG background_rms ) were inspected to exclude outliers (i.e., values 2 SD above or below the mean value for each subject in each session).…”

The role of observers' gaze behaviour when watching object manipulation tasks: predicting and evaluating the consequences of action

“…For example, observing a video of an actor lifting an object increases the excitability of the representation of muscles involved in lifting in motor cortex, and this increases scales with the required force (i.e. object weight) [24,39].…”