Stability of multifinger action in different state spaces

Abstract

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...

The nature of constant and cyclic force production: unintentional force-drift characteristics

“…There was a much quicker change in the force peak-to-peak amplitude during the cyclic F-task after the visual feedback disappeared as compared to the drift in the mean force and in the force level in constant F-tasks. The characteristic time of the force amplitude increase was, in fact, similar to the times reported for experiments with perturbations (Wilhelm et al 2013;Zhou et al 2014;Reschechtko et al 2014). Moreover, the change in the force amplitude was in the opposite direction (an increase) as compared to the prediction of Hypothesis 3.…”

“…4c,d,5b) suggests another process within the ORT manifold. Note that its timing is similar to the time characteristics reported in earlier studies with external perturbations of constant force-production kinematic and kinetic actions (Wilhelm et al 2013;Zhou et al 2014;Reschechtko et al 2014). These observations suggest that both externally driven and intentional processes can trigger a relatively quick motion of RC along ORT.…”

“…The former hypothesis suggests that these unintentional force changes were mediated by the same mechanism as voluntary force production Zhou et al 2015). All the force changes above were well approximated with exponential functions with a time constant of about 10-15 s. Similar unintentional movements, but with much shorter time constants (about 1 s), have been documented in experiments with relatively short-lasting, transient perturbations (Wilhelm et al 2013;Zhou et al 2014;Reschechtko et al 2014). When the perturbation was over, major changes in both kinematic and kinetic variables were observed despite the instruction to the subject not to react to the perturbations ("do not interfere voluntarily, " Feldman 1966;Latash 1994).…”

“…Two types of RC-back-coupling have been suggested to account for relatively fast (with characteristic times of about 1 s; Zhou et al 2014;Reschechtko et al 2014) and relatively slow (with characteristic times of 10-15 s; Ambike et al 2015) unintentional drifts in performance. The former is observed in tasks with quick force changes, for example those induced by transient perturbations, while the latter is seen in steady-state tasks.…”

Force-stabilizing synergies in motor tasks involving two actors

“…A force pulse must occur in the ORT space, which is characterized by higher stability and faster processes with time constants on the order of 1 s, as shown in studies with unintentional force and position drift following external perturbations applied to the system (Wilhelm et al 2013; Zhou et al 2014; Reschechtko et al 2014). The present results suggest that perturbation of a system in the ORT direction, either externally imposed or intentional, can lead to a faster sharing drift, i.e.…”