Unintentional movements produced by back-coupling between the actual and referent body configurations: violations of equifinality in multi-joint positional tasks

Abstract

We tested several predictions of a recent theory that combines the ideas of control with referent configurations, hierarchical control, and the uncontrolled manifold (UCM) hypothesis. In particular, we tested a hypothesis that unintentional changes in hand coordinate can happen following a long-lasting transient perturbation. The subjects grasped a handle with the right hand, occupied an initial position against a bias force produced by the Hapticmaster robot, and then tried not to react to changes in the robot-produced force. Changes in the force were smooth and transient; they always ended with the same force as the bias force. The force-change amplitude and the time the force was kept at the new level (dwell time) varied across conditions. After the transient force change was over, the handle rested in a position that differed significantly from the initial position. The amplitude of this unintentional movement increased with the amplitude of transient force change and with the dwell time. In the new position, the across-trials joint configuration variance was mostly confined to a sub-space compatible with the average handle coordinate and orientation (the UCMs for these variables). We view these results as the first experimental support for the hypothesis on back-coupling between the referent and actual body configurations during multi-joint actions. The results suggest that, even under the instruction “not to react to transient force changes”, the subjects may be unable to prevent unintentional drift of the referent configuration. The structure of joint configuration variance after such movements was similar to that in earlier reports on joint configuration variance after intentional movements. We conclude that the intentional and unintentional movements are products of a single neural system that can lead to intentional and unintentional shifts of the referent body configuration.

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.…”

“…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).…”

“…Corcos et al 1992). A series of recent observations have suggested that, if body motion toward RC is impossible, RC may drift toward the current actual configuration-a phenomenon addressed as RC-back-coupling (Wilhelm et al 2013;Zhou et al 2014;Ambike et al 2014). The notion of RC-back-coupling has been introduced to account for unintentional drifts of performance in kinetic and kinematic variables observed when subjects, who began an action with the assistance of visual feedback, were instructed to continue to produce the action after the visual feedback was removed.…”

“…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.…”

Stability of multifinger action in different state spaces

“…Another sort of back-coupling has recently been suggested based on observations of unintentional movements in experiments when actual body configuration was kept away from the RC for long time intervals (Ambike et al 2014;Zhou et al 2014b). This process, addressed as RC-back-coupling, is thought to result in a relatively slow drift of RC toward the actual body configuration.…”