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.

Motor cortical excitability during voluntary inhibition of involuntary tic movements

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Mental Control in Musical Imagery: A Dual Component Model

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Low Gain Servo Control During the Kohnstamm Phenomenon Reveals Dissociation Between Low-Level Control Mechanisms for Involuntary vs. Voluntary Arm Movements

“…Classically, participants hold the arm straight and vertical by their side and push outwards against a solid surface for 30 s, by a strong voluntary contraction of the lateral deltoid muscle. Upon relaxation the arm rises involuntarily (Kohnstamm, 1915 ; De Havas et al, 2016 ). This aftercontraction is electromyographically (EMG) similar to a slow voluntary movement (Mathis et al, 1996 ) and can be elicited in many skeletal muscles (Forbes et al, 1926 ).…”