Stabilization of the total force in multi-finger pressing tasks studied with the ‘inverse piano’ technique

et al. 2016

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.

Section: Two Types Of Unintentional Force Changementioning

confidence: 99%

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

Parsa

O’Shea

et al. 2016

“…Data were recorded using an inverse piano device (for a detailed overview of the inverse piano, see Martin et al 2011a). The device consisted of four unidirectional piezoelectric force transducers (208C02; PCB Piezotronics, Depew, NY) mounted on individual linear actuators (PS01-23x80; LinMot, Spreitenbach, Switzerland).…”

Section: Equipmentmentioning

confidence: 99%

“…Note that lifting a finger is expected to be transmitted mechanically to other fingers of the hand and lead to a drop in their forces. Although such effects could contribute to force changes observed during the finger lifting and lowering processes, their contribution has been estimated as small (Martin et al 2011a) leading to a conclusion on a major role of neural components in those counterdirectional force changes. We also emphasize that the main results of our study were obtained in the comparison of finger forces (and modes) before and after the lifting-lowering sequence when all of the fingers were at the same position.…”

Section: The Ucm Concept and Stability Of Actionmentioning

confidence: 99%

“…So, equifinality can be studied by using both transient force perturbation during a movement task and transient positional perturbations during a force production task. In this study, we used a device called the "inverse piano" (Martin et al 2011a) that allows introducing positional perturbations (smooth changes in the finger configuration) during accurate force production tasks. We predicted that a transient change in position of a finger would lead to relatively small variance of the total force produced by four fingers of a hand (F TOT ) compared with variance in the space of elemental variables: commands to fingers.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Equifinality and its violations in a redundant system: multifinger accurate force production

Wilhelm

Latash

2013

Wilhelm L, Zatsiorsky VM, Latash ML. Equifinality and its violations in a redundant system: multifinger accurate force production. J Neurophysiol 110: 1965-1973. First published July 31, 2013 doi:10.1152/jn.00461.2013.-We explored a hypothesis that transient perturbations applied to a redundant system result in equifinality in the space of task-related performance variables but not in the space of elemental variables. The subjects pressed with four fingers and produced an accurate constant total force level. The "inverse piano" device was used to lift and lower one of the fingers smoothly. The subjects were instructed "not to intervene voluntarily" with possible force changes. Analysis was performed in spaces of finger forces and finger modes (hypothetical neural commands to fingers) as elemental variables. Lifting a finger led to an increase in its force and a decrease in the forces of the other three fingers; the total force increased. Lowering the finger back led to a drop in the force of the perturbed finger. At the final state, the sum of the variances of finger forces/modes computed across repetitive trials was significantly higher than the variance of the total force/mode. Most variance of the individual finger force/mode changes between the preperturbation and postperturbation states was compatible with constant total force. We conclude that a transient perturbation applied to a redundant system leads to relatively small variance in the task-related performance variable (equifinality), whereas in the space of elemental variables much more variance occurs that does not lead to total force changes. We interpret the results within a general theoretical scheme that incorporates the ideas of hierarchically organized control, control with referent configurations, synergic control, and the uncontrolled manifold hypothesis. redundancy; abundance; equifinality; finger; force; referent configuration THE EQUILIBRIUM-POINT HYPOTHESIS has been, for the past halfcentury, one of the most controversial and influential hypotheses in the field of motor control (Feldman 1986;Feldman and Levin 1995). Recently, it has been developed in the form of a referent configuration (RC) hypothesis (Feldman 2009). Whereas the original equilibrium-point hypothesis described the neural control of a single muscle and muscle pairs acting at a joint (Feldman 1986), the RC hypothesis generalized the main ideas to the control of multimuscle, whole body actions involving redundant sets of effectors. According to the RC hypothesis, voluntary movements are produced by shifts of neural variables leading to changes in referent values for task-specific, salient variables given the external force field. Analysis of movement equifinality in the presence of transient force perturbations has been an important source of experimental evidence quoted as both supporting the equilibrium-point hypothesis (Bizzi et al. 1976;Kelso and Holt 1980;Latash and Gottlieb 1990;Schmidt and McGown 1980) and creating problems for it (DiZio and Lackner 1995;Hinder and Mi...

“…In the first of these studies, subjects were asked to produce a constant amount of force by pressing on force sensors with the four fingers of the right hand. A transient perturbation (liftingand-lowering a finger with the "inverse piano" device; Martin et al 2011) led to changes in all finger forces organized in such a way that intertrial variance in the space of finger forces was relatively high while variance of the total force was low. This was associated with V UCM Ͼ V ORT reflecting relatively low stability of the four-finger system in directions that span the UCM compared with the direction orthogonal to the UCM.…”

mentioning

confidence: 99%

Stability of multifinger action in different state spaces

Reschechtko

Latash

2014

Reschechtko S, Zatsiorsky VM, Latash ML. Stability of multifinger action in different state spaces. J Neurophysiol 112: 3209-3218, 2014. First published September 24, 2014 doi:10.1152/jn.00395.2014.-We investigated stability of action by a multifinger system with three methods: analysis of intertrial variance, application of transient perturbations, and analysis of the system's motion in different state spaces. The "inverse piano" device was used to apply transient (lifting-andlowering) perturbations to individual fingers during single-and twofinger accurate force production tasks. In each trial, the perturbation was applied either to a finger explicitly involved in the task or one that was not. We hypothesized that, in one-finger tasks, task-specific stability would be observed in the redundant space of finger forces but not in the nonredundant space of finger modes (commands to explicitly involved fingers). In two-finger tasks, we expected that perturbations applied to a nontask finger would not contribute to task-specific stability in mode space. In contrast to our expectations, analyses in both force and mode spaces showed lower stability in directions that did not change total force output compared with directions that did cause changes in total force. In addition, the transient perturbations led to a significant increase in the enslaving index. We consider these results within a theoretical scheme of control with referent body configurations organized hierarchically, using multiple few-to-many mappings organized in a synergic way. The observed volatility of enslaving, greater equifinality of total force compared with elemental variables, and large magnitude of motor equivalent motion in both force and mode spaces provide support for the concept of task-specific stability of performance and the existence of multiple neural loops, which ensure this stability. redundancy; abundance; synergy; referent configuration; uncontrolled manifold hypothesis; equifinality; enslaving ONE OF THE FUNDAMENTAL PROBLEMS of motor control is that of redundancy (Bernstein 1967): at any level of description of the system involved in movement production, the number of elements (and of their output variables, elemental variables) is higher than the number of constraints associated with typical motor tasks. Recently, the problem of motor redundancy has been revisited in the context of the principle of motor abundance (Gelfand and Latash 1998;Latash 2012). This approach posits a way for the central nervous system (CNS) to take advantage of all of the elemental variables at its disposal to ensure stability of movement, rather than requiring the CNS to achieve a single, optimal solution by eliminating redundant degrees of freedom. Thus the principle of abundance avoids requiring computational, and physiologically unfeasible, control schemes, which involve explicit instruction to all body structures involved in a given action.A theoretical scheme has been developed, which unites the principle of abundance with the idea of hierarchical...