title: Voluntary motor commands reveal awareness and control of involuntary movement.
abstract: The capacity to inhibit actions is central to voluntary motor control. However, the control mechanisms and subjective experience involved in voluntarily stopping an involuntary movement remain poorly understood. Here we examined, in humans, the voluntary inhibition of the Kohnstamm phenomenon, in which sustained voluntary contraction of shoulder abductors is followed by involuntary arm raising. Participants were instructed to stop the involuntary movement, hold the arm in a constant position, and 'release' the inhibition after ∼2s. Participants achieved this by modulating agonist muscle activity, rather than by antagonist contraction. Specifically, agonist muscle activity plateaued during this voluntary inhibition, and resumed its previous increase thereafter. There was no discernible antagonist activation. Thus, some central signal appeared to temporarily counter the involuntary motor drive, without directly affecting the Kohnstamm generator itself. We hypothesise a form of "negative motor command" to account for this novel finding. We next tested the specificity of the negative motor command, by inducing bilateral Kohnstamm movements, and instructing voluntary inhibition for one arm only. The results suggested negative motor commands responsible for inhibition are initially broad, affecting both arms, and then become focused. Finally, a psychophysical investigation found that the perceived force of the aftercontraction was significantly overestimated, relative to voluntary contractions with similar EMG levels. This finding is consistent with the hypothesis that the Kohnstamm generator does not provide an efference copy signal. Our results shed new light on this interesting class of involuntary movement, and provide new information about voluntary inhibition of action.
authors.affiliation: Institute of Cognitive Neuroscience, University College London, London WC1N 3AR, United Kingdom. Electronic address: j.havas.12@ucl.ac.uk.
authors.forename: Jack
authors.initials: J
authors.lastname: De Havas
authors.affiliation: Institute of Cognitive Neuroscience, University College London, London WC1N 3AR, United Kingdom; Institute of Neuroinformatics, University of Zurich and ETH Zurich, Zurich CH-8057, Switzerland; Neuroscience Center Zurich, University of Zurich and ETH Zurich, Zurich CH-8057, Switzerland.
authors.forename: Arko
authors.initials: A
authors.lastname: Ghosh
authors.affiliation: NTT Communication Science Laboratories, Nippon Telegraph and Telephone Corporation, Wakamiya 3-1, Morinosato, Atsugi, Kanagawa-pref. 243-0198, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Gomi
authors.affiliation: Institute of Cognitive Neuroscience, University College London, London WC1N 3AR, United Kingdom.
authors.forename: Patrick
authors.initials: P
authors.lastname: Haggard
mesh_terms: D000328:Adult; D001364:Awareness; D001696:Biomechanical Phenomena; D057645:Deltoid Muscle / Q000502:physiology; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement; D009119:Muscle Contraction; D011597:Psychomotor Performance; D014836:Volition; D055815:Young Adult
keywords: Action awareness; Bilateral movement; Inhibition; Involuntary movement; Motor control; Negative motor command