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

Authors

  • Kaat Alaerts,

    1. Research Center of Movement Control and Neuroplasticity, Department of Biomedical Kinesiology, Group Biomedical Sciences, Katholieke Universiteit Leuven, Tervuursevest 101 – B-3001 Heverlee, Belgium
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    • *

      K.A. and P.S. contributed equally in designing, performing and analysing the experiments as well as writing the manuscript.

  • Patrice Senot,

    1. D.S.B.T.A. Section of Human Physiology, Faculty of Medicine, University of Ferrara, Via Fossato di Mortara 17/19 – 44100 Ferrara, Italy
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    • *

      K.A. and P.S. contributed equally in designing, performing and analysing the experiments as well as writing the manuscript.

  • Stephan P. Swinnen,

    1. Research Center of Movement Control and Neuroplasticity, Department of Biomedical Kinesiology, Group Biomedical Sciences, Katholieke Universiteit Leuven, Tervuursevest 101 – B-3001 Heverlee, Belgium
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  • Laila Craighero,

    1. D.S.B.T.A. Section of Human Physiology, Faculty of Medicine, University of Ferrara, Via Fossato di Mortara 17/19 – 44100 Ferrara, Italy
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    • N.W., L.C. and L.F. contributed equally in developing the research question, designing and supervising the experiments and writing the manuscript.

  • Nicole Wenderoth,

    1. Research Center of Movement Control and Neuroplasticity, Department of Biomedical Kinesiology, Group Biomedical Sciences, Katholieke Universiteit Leuven, Tervuursevest 101 – B-3001 Heverlee, Belgium
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    • N.W., L.C. and L.F. contributed equally in developing the research question, designing and supervising the experiments and writing the manuscript.

  • Luciano Fadiga

    1. D.S.B.T.A. Section of Human Physiology, Faculty of Medicine, University of Ferrara, Via Fossato di Mortara 17/19 – 44100 Ferrara, Italy
    2. The Italian Institute of Technology, Department RBCS, Via Morego 30 – Genova, Italy
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    • N.W., L.C. and L.F. contributed equally in developing the research question, designing and supervising the experiments and writing the manuscript.


Dr N. Wenderoth and Dr L. Fadiga, as above.
E-mail: nici.wenderoth@faber.kuleuven.be and fdl@unife.it

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.

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