The influence of visual perturbations on the neural control of limb stiffness.

Department of Psychology, The University of Western Ontario, 1151 Richmond St., London, Ontario, Canada N6A 5C2.
Journal of Neurophysiology (Impact Factor: 3.04). 08/2008; 101(1):246-57. DOI: 10.1152/jn.90371.2008
Source: PubMed

ABSTRACT To adapt to novel unstable environments, the motor system modulates limb stiffness to produce selective increases in arm stability. The motor system receives information about the environment via somatosensory and proprioceptive signals related to the perturbing forces and visual signals indicating deviations from an expected hand trajectory. Here we investigated whether subjects modulate limb stiffness during adaptation to a purely visual perturbation. In a first experiment, measurements of limb stiffness were taken during adaptation to an elastic force field (EF). Observed changes in stiffness were consistent with previous reports: subjects increased limb stiffness and did so only in the direction of the environmental instability. In a second experiment, stiffness changes were measured during adaptation to a visual perturbing environment that magnified hand-path deviations in the lateral direction. In contrast to the first experiment, subjects trained in this visual task showed no accompanying change in stiffness, despite reliable improvements in movement accuracy. These findings suggest that this sort of visual information alone may not be sufficient to engage neural systems for stiffness control, which may depend on sensory signals more directly related to perturbing forces, such as those arising from proprioception and somatosensation.

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    ABSTRACT: Abstract (max. 150):137 Main text (Intro, Results, Discussion: max. 4000): 1978 Number of References (max. 50): 47 Number of Figures (max. 8): 4
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    ABSTRACT: We tested an innovative method to estimate joint stiffness and damping during multi-joint unfettered arm movements. The technique employs impulsive perturbations and a time-frequency analysis to estimate the arm's mechanical properties along a reaching trajectory. Each single impulsive perturbation provides a continuous estimation on a single reach basis, making our method ideal to investigate motor adaptation in the presence of force fields and to study the control of movement in impaired individuals with limited kinematic repeatability. In contrast with previous dynamic stiffness studies, we found that stiffness varies during movement achieving levels higher than during static postural control. High stiffness was associated with elevated reflexive activity. We observed a decrease in stiffness and a marked reduction in long latency reflexes around the reaching movement velocity peak. This pattern could partly explain the difference between the high stiffness reported in postural studies and the low stiffness measured in dynamic estimation studies where perturbations are typically applied near the peak velocity point.
    Journal of Neurophysiology 08/2013; · 3.04 Impact Factor
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    ABSTRACT: Previous studies have shown that short-term exposure to mirror-reversed visual feedback suppresses rapid online control (ROC) of arm movements in response to a sudden target displacement. Here we tested if the reduced ROC under reversed vision can be observed for natural reaches without target perturbations, i.e. without corrective movements that are driven by visual input perturbation. Second, we ask if such ROC reduction generalizes to movement phases without visual feedback of the hand. Subjects were instructed to perform simple reach movements towards a stationary target position either under normal or physically reversed vision of the hand during the late movement phase. We quantified time-resolved ROC via a coefficient of determination of the reach trajectories over the full course of the movement. As for other measures in previous studies, we found that our perturbation-independent ROC was reduced within a few trials after exposure to reversed visual feedback. The reduced ROC was restricted to late movement phases, and was not observed in early movement phases. We further asked if subjects would be able to re-gain ROC with prolonged exposure to the reversed visual input. ROC gradually and incompletely increased over the course of 400 exposure trials, affecting both early and late movement phases. Our results show that under reversed vision ROC is reduced even for perturbation-independent natural reaches aiming at stationary targets.
    Vision Research 09/2014; · 2.38 Impact Factor


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