Synergies in the space of control variables within the equilibrium-point hypothesis

Ambike, Satyajit; Mattos, Daniela; Zatsiorsky, Vladimir M.; Latash, Mark L.

doi:10.1016/j.neuroscience.2015.12.012

Cited by 67 publications

(75 citation statements)

References 44 publications

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“…The drift in performance, e.g., in F TOT , suggests a drift at the highest level of the hierarchy, at which F TOT results from a combination of two commands reflected in RC and k (apparent stiffness) defined for a virtual finger (VF, an imagined digit with the same resultant force as all the actual digits combined, Arbib et al 1975). A recent study has shown large intertrial variations in RC and k that kept variance of F TOT very low (Ambike et al 2016b). During the slow F TOT drift, a larger drift is expected within the UCM for F TOT in the {RC; k} space compared with the drift in ORT.…”

Section: Direction Of Force Drift In An Abundant Systemmentioning

confidence: 95%

“…Comparably fast force drifts were also observed in multifinger accurate force production tasks when fingers were perturbed using an "inverse piano" apparatus (Reschechtko et al 2014;Wilhelm et al 2013). To explore drifts in both RC and k, one would have to apply perturbations to the fingers as it was done in a recent study (Ambike et al 2016b), but this may by itself accelerate the drift process. This method remains to be refined if it is to be used in a future study.…”

Section: Origins Of Unintentional Change In Performancementioning

confidence: 99%

“…According to this view, force production by a finger in isometric conditions is associated with setting a referent coordinate (RC) for the fingertip and a magnitude of apparent stiffness (k, which is also a reflection of shifts in spatial RCs for the participating muscles) (Ambike et al 2016b;Feldman 2015;Pilon et al 2007). Given the constant actual coordinate of the effector (AC), force magnitude F ϭ k(RC Ϫ AC).…”

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confidence: 99%

See 2 more Smart Citations

On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

Parsa

O’Shea

Zatsiorsky

et al. 2016

Journal of Neurophysiology

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Parsa B, O'Shea DJ, Zatsiorsky VM, Latash ML. On the nature of unintentional action: a study of force/moment drifts during multifinger tasks. J Neurophysiol 116: 698 -708, 2016. First published May 18, 2016 doi:10.1152/jn.00180.2016.-We explored the origins of unintentional changes in performance during accurate force production in isometric conditions seen after turning visual feedback off. The idea of control with referent spatial coordinates suggests that these phenomena could result from drifts of the referent coordinate for the effector. Subjects performed accurate force/moment production tasks by pressing with the fingers of a hand on force sensors. Turning the visual feedback off resulted in slow drifts of both total force and total moment to lower magnitudes of these variables; these drifts were more pronounced in the right hand of the right-handed subjects. Drifts in individual finger forces could be in different direction; in particular, fingers that produced moments of force against the required total moment showed an increase in their forces. The force/moment drift was associated with a drop in the index of synergy stabilizing performance under visual feedback. The drifts in directions that changed performance (non-motor equivalent) and in directions that did not (motor equivalent) were of about the same magnitude. The results suggest that control with referent coordinates is associated with drifts of those referent coordinates toward the corresponding actual coordinates of the hand, a reflection of the natural tendency of physical systems to move toward a minimum of potential energy. The interaction between drifts of the hand referent coordinate and referent orientation leads to counterdirectional drifts in individual finger forces. The results also demonstrate that the sensory information used to create multifinger synergies is necessary for their presence over the task duration.

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Section: Direction Of Force Drift In An Abundant Systemmentioning

confidence: 95%

Section: Origins Of Unintentional Change In Performancementioning

confidence: 99%

mentioning

confidence: 99%

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On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

Parsa

O’Shea

Zatsiorsky

et al. 2016

Journal of Neurophysiology

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“…However, they may be measured in different units at the level of performance: R is a referent spatial coordinate that the effector would approach in the absence of external constraints, while C is the apparent stiffness of the effector (see Latash 2010; Ambike et al 2016b). We recently explored inter-trial variation in R and C during constant force production by a finger under continuous visual feedback and found large inter-trial variance in both variables (called R FT and C FT ; FT = fingertip) associated with a low variance in total force (Ambike et al 2016b). In other words, R and C variation was primarily confined to the UCM for the required level of output force.…”

Section: Introductionmentioning

confidence: 99%

“…In the present study, we explored the unintentional force drift phenomenon using the same method of {R FT ; C FT } estimation as in the previous study (Ambike et al 2016b). Our main hypothesis was that the slow drift in fingertip force after visual feedback removal would be associated with drifts in both R FT and C FT variables along the corresponding UCM as well as orthogonal to it.…”

Section: Introductionmentioning

confidence: 99%

Unsteady steady-states: central causes of unintentional force drift

Ambike

Mattos²,

Zatsiorsky

et al. 2016

Exp Brain Res

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We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely, the fingertip referent coordinate (RFT) and its apparent stiffness (CFT). The system's state is defined by a point in the {RFT; CFT} space. We tested the hypothesis that, after visual feedback removal, this point (1) moves along directions leading to drop in the output fingertip force, and (2) has even greater motion along directions that leaves the force unchanged. Subjects produced a prescribed fingertip force using visual feedback, and attempted to maintain this force for 15 s after the feedback was removed. We used the “inverse piano” apparatus to apply small and smooth positional perturbations to fingers at various times after visual feedback removal. The time courses of RFT and CFT showed that force drop was mostly due to a drift in RFT towards the actual fingertip position. Three analysis techniques, namely, hyperbolic regression, surrogate data analysis, and computation of motor-equivalent and non-motor-equivalent motions, suggested strong co-variation in RFT and CFT stabilizing the force magnitude. Finally, the changes in the two hypothetical control variables {RFT; CFT} relative to their average trends also displayed covariation. On the whole the findings suggest that unintentional force drop is associated with (a) a slow drift of the referent coordinate that pulls the system towards a low-energy state, and (b) a faster synergic motion of RFT and CFT that tends to stabilize the output fingertip force about the slowly-drifting equilibrium point.

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Fifty Years of Physics of Living Systems

Latash

2016

Advances in Experimental Medicine and Biology

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The equilibrium-point hypothesis and its more recent version, the referent configuration hypothesis, represent the physical approach to the neural control of action. This hypothesis can be naturally combined with the idea of hierarchical control of movements and of synergic organization of the abundant systems involved in all actions. Any action starts with defining trajectories of a few referent coordinates for a handful of salient task-specific variables. Further, referent coordinates at hierarchically lower levels emerge down to thresholds of the tonic stretch reflex for the participating muscles. Stability of performance with respect to salient variables is reflected in the structure of inter-trial variance and phenomena of motor equivalence. Three lines of recent research within this framework are reviewed. First, synergic adjustments of the referent coordinate and apparent stiffness have been demonstrated during finger force production supporting the main idea of control with referent coordinates. Second, the notion of unintentional voluntary movements has been introduced reflecting unintentional drifts in referent coordinates. Two types of unintentional movements have been observed with different characteristic times. Third, this framework has been applied to studies of impaired movements in neurological patients. Overall, the physical approach searching for laws of nature underlying biological movement has been highly stimulating and productive.

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Synergies in the space of control variables within the equilibrium-point hypothesis

Cited by 67 publications

References 44 publications

On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

Unsteady steady-states: central causes of unintentional force drift

Fifty Years of Physics of Living Systems

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