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

Parsa, Behnoosh; O’Shea, Daniel J.; Zatsiorsky, Vladimir M.; Latash, Mark L.

doi:10.1152/jn.00180.2016

Cited by 38 publications

(59 citation statements)

References 62 publications

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“…Scholz and Schöner 2014; also see later in the Discussion), the counter-directional changes in the two components suggest that visual feedback was crucial to ensure stability of the task-related variables. This general conclusion is supported by results from earlier studies by Ranganathan and Newell (2008, 2009) as well as by a recent study exploring changes in the structure of inter-trial variance during unintentional drifts in performance (Parsa et al 2016). …”

Section: Discussionsupporting

confidence: 81%

“…The consistent drifts in F TOT and M TOT in the absence of visual feedback (cf. Slifkin et al 2000; Vaillancourt and Russell 2002; Parsa et al 2016) are leading to nME deviations. The drifts within the UCM, directed toward preferred finger force combinations (modeled as those with smaller costs, Parsa et al 2017), lead to ME deviations.…”

Section: Discussionmentioning

confidence: 99%

“…This was done in another study using only a few conditions and multiple trials per condition (Parsa et al 2016). So, in this study, we have only ME data reflecting stability of the variables that continued to receive visual feedback vs. those that stopped receiving this feedback.…”

Section: Discussionmentioning

confidence: 99%

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Optimality and stability of intentional and unintentional actions: II. Motor equivalence and structure of variance

2016

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We address the nature of unintentional changes in performance in two papers. This second paper tested hypotheses related to stability of task-specific performance variables estimated using the framework of the uncontrolled manifold (UCM) hypothesis. Our first hypothesis was that selective stability of performance variables would be observed even when the magnitudes of those variables drifted unintentionally because of the lack of visual feedback. Our second hypothesis was that stability of a variable would depend on the number of explicit task constraints. Subjects performed four-finger isometric pressing tasks that required the accurate production of a combination of total moment and total force with natural or modified finger involvement under full visual feedback, which was removed later for some or all of the salient variables. We used inter-trial analysis of variance and drifts in the space of finger forces within the UCM and within the orthogonal to the UCM space. The two variance components were used to estimate a synergy index stabilizing the force/moment combination, while the two drift components were used to estimate motor equivalent and non-motor equivalent force changes, respectively. Without visual feedback, both force and moment drifted toward lower absolute magnitudes. The non-motor equivalent component of motion in the finger force space was larger that the motor equivalent component for variables that stopped receiving visual feedback. In contrast, variables that continued to receive visual feedback showed larger motor equivalent component, compared to non-motor equivalent component, over the same time interval. These data falsified the first hypothesis; indeed, selective stabilization of a variable over the duration of a trial allows expecting comparably large motor equivalent components both with and without visual feedback. Adding a new constraint (presented as a target magnitude of middle finger force) resulted in a drop in the synergy index in support of the second hypothesis. We interpret the force drift as a natural relaxation process toward states with lower potential energy in the physical (physiological) system involved in the task. The results show that presenting sensory feedback on a performance variable makes synergies stabilizing that variable dependent on that particular sensory feedback.

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Section: Discussionsupporting

confidence: 81%

Section: Discussionmentioning

confidence: 99%

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Optimality and stability of intentional and unintentional actions: II. Motor equivalence and structure of variance

2016

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“…In isometric conditions, the actual coordinate cannot change; as a result, a slow drift of RC takes place resulting in the force drop. This interpretation has received support in a few recent studies (Jo et al 2016; Parsa et al 2016). …”

Section: Discussionmentioning

confidence: 68%

The synergic control of multi-finger force production: stability of explicit and implicit task components

2016

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Manipulating objects with the hands requires the accurate production of resultant forces including shear forces; effective control of these shear forces also requires the production of internal forces normal to the surface of the object(s) being manipulated. In the present study, we investigated multi-finger synergies stabilizing shear and normal components of force, as well as drifts in both components of force, during isometric pressing tasks requiring a specific magnitude of shear force production. We hypothesized that shear and normal forces would evolve similarly in time, and also show similar stability properties as assessed by the decomposition of inter-trial variance within the uncontrolled manifold hypothesis. Healthy subjects were required to accurately produce total shear and total normal forces with four fingers of the hand during a steady-state force task (with and without visual feedback) and a self-paced force pulse task. The two force components showed similar time profiles during both shear force pulse production and unintentional drift induced by turning the visual feedback off. Only the explicitly instructed components of force, however, were stabilized with multi-finger synergies. No force-stabilizing synergies and no anticipatory synergy adjustments were seen for the normal force in shear force production trials. These unexpected qualitative differences in the control of the two force components – which are produced by some of the same muscles and show high degree of temporal coupling – are interpreted within the theory of control with referent coordinates for salient variables. These observations suggest the existence of two classes of neural variables: one that translates into shifts of referent coordinates and defines changes in magnitude of salient variables, and the other controlling gains in back-coupling loops that define stability of the salient variables. Only the former are shared between the explicit and implicit task components.

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“…This hypothetical mechanism has been referred to as RC-back-coupling (Reschechtko et al 2014; Ambike et al 2015; Zhou et al 2015). Along similar lines, moment of force (M) production may be viewed as a consequence of a change in referent orientation (RO) of the object with respect to its actual orientation (AO): M = k R (RO – AO) where k R is rotational apparent stiffness (Latash et al 2010; Parsa et al 2016). Unintentional changes in M are viewed as consequences of a drift of RO toward AO, which is another example of the hypothetical RC-back-coupling mechanism.…”

Section: Introductionmentioning

confidence: 99%

Optimality and stability of intentional and unintentional actions: I. Origins of drifts in performance

et al. 2016

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We address the nature of unintentional changes in performance in two papers. This first paper tested a hypothesis that unintentional changes in performance variables during continuous tasks without visual feedback are due to two processes. First, there is a drift of the referent coordinate for the salient performance variable toward the actual coordinate of the effector. Second, there is a drift toward minimum of a cost function. We tested this hypothesis in four-finger isometric pressing tasks that required the accurate production of a combination of total moment and total force with natural and modified finger involvement. Subjects performed accurate force/moment production tasks under visual feedback, and then visual feedback was removed for some or all of the salient variables. Analytical inverse optimization was used to compute a cost function. Without visual feedback, both force and moment drifted slowly toward lower absolute magnitudes. Over 15 s, the force drop could reach 20% of its initial magnitude while moment drop could reach 30% of its initial magnitude. Individual finger forces could show drifts toward both higher and lower forces. The cost function estimated using the analytical inverse optimization reduced its value as a consequence of the drift. We interpret the results within the framework of hierarchical control with referent spatial coordinates for salient variables at each level of the hierarchy combined with synergic control of salient variables. The force drift is discussed as a natural relaxation process toward states with lower potential energy in the physical (physiological) system involved in the task.

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

Cited by 38 publications

References 62 publications

Optimality and stability of intentional and unintentional actions: II. Motor equivalence and structure of variance

Optimality and stability of intentional and unintentional actions: II. Motor equivalence and structure of variance

The synergic control of multi-finger force production: stability of explicit and implicit task components

Optimality and stability of intentional and unintentional actions: I. Origins of drifts in performance

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