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

Abstract

The Kohnstamm phenomenon is a prolonged involuntary aftercontraction following a sustained voluntary isometric muscle contraction. The control principles of the Kohnstamm have been investigated using mechanical perturbations, but previous studies could not dissociate sensorimotor responses to perturbation from effects of gravity. We induced a horizontal, gravity-independent Kohnstamm movement around the shoulder joint, and applied resistive or assistive torques of 0.5 Nm after 20° angular displacement. A No perturbation control condition was included. Further, participants made velocity-matched voluntary movements, with or without similar perturbations, yielding a 2 × 3 factorial design. Resistive perturbations produced an increase in agonist electromyography (EMG), in both Kohnstamm and voluntary movements, while assistive perturbations produced a decrease. While overall Kohnstamm EMGs were greater than voluntary EMGs, the EMG responses to perturbation, when expressed as a percentage of unperturbed EMG activity, were significantly smaller during Kohnstamm movements than during voluntary movements. The results suggest that the Kohnstamm aftercontraction involves a central drive, coupled with low-gain servo control by a negative feedback loop between afferent input and a central motor command. The combination of strong efferent drive with low reflex gain may characterize involuntary control of postural muscles. Our results question traditional accounts involving purely reflexive mechanisms of postural maintenance. They also question existing high-gain, peripheral accounts of the Kohnstamm phenomenon, as well as accounts involving a central adaptation interacting with muscle receptors via a positive force feedback loop.

Control Modification of Grasp Force Covaries Agency and Performance on Rigid and Compliant Surfaces

“…Any programmable interface for rehabilitation training or assistive device tuning can potentially adapt parameters for greater agency. Parameters include feedback gains ( Nataraj et al, 2010 , 2012b ; De Havas et al, 2018 ) or customized settings within training environments ( Velazquez et al, 2008 ). Systematic and computational approaches to adapt user training through agency would readily apply to any advanced rehabilitation platform (VR, robotics) or powered assistive devices such as exoskeletons ( Farris et al, 2013 ) and neuroprostheses ( Nataraj et al, 2012a , b ; Marasco et al, 2018 ).…”

Neural dynamics of illusory tactile pulling sensations

“…They must be interpreted cautiously and do not represent the totality of neural processes underlying the pulling sensation. Nevertheless, we speculate that activity in the postcentral sulcus might constitute a proprioceptive signal that the hand is stationary, a key subjective feature of the pulling sensation differentiating it from other illusory external force sensations ( De Havas et al., 2017 , 2018 ). It could be that the IPS contributes to the pulling sensation by extracting the orientation of the illusory force vector, while nearby populations of cells in the SPL are involved in mapping the extracted force vector in hand or externally centered coordinates.…”