Stability of Multi-finger Action in Different State Spaces

Open Access
Reschechtko, Sasha
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 04, 2014
Committee Members:
  • Mark Latash, Thesis Advisor
  • redundancy
  • abundance
  • synergy
  • referent configuration
  • uncontrolled manifold hypothesis
  • equifinality
  • enslaving
This thesis investigates the task-specific stability of action by the human hand during isometric accurate force production tasks. These tasks were performed using different numbers of instructed fingers, a subset of which were subjected to transient spatial perturbations. Subjects utilized visual feedback to produce a constant target force with index, index and middle, index, middle, and ring, and all four fingers; after reaching this target force, the visual feedback was removed and the subjects were instructed “do not interfere” while their index fingers (and, in some conditions, ring fingers) were perturbed by the “inverse piano” apparatus. Results were analyzed in the space of finger forces as well as hypothetical finger commands (modes), which reflect the inter-dependence of force production by each digit (enslaving). The main results were: (1) Inter-trial variance during steady states, in the space of modes and forces, was higher in directions which did not affect total force production (within the uncontrolled manifold or UCM); (2) Perturbations resulted in large deviations of finger forces and modes within the UCM (i.e. large motor equivalent, or ME, displacements were observed); (3) Deviations in forces and modes resulting from the perturbation showed larger variances within the UCM. Strikingly, no significant effect of the number of fingers involved was seen for any condition, including the single-finger condition, despite the apparent differences in system redundancy in each condition. Together, these results suggest that all tasks carried out with the whole hand are effectively four-finger tasks with only quantitative differences in the involvement of instructed and non-instructed fingers. Additionally, high volatility of enslaving was observed as a result of transient perturbations, which led to significantly higher enslaving in certain conditions. These results are considered in the framework of the UCM hypothesis and control with hierarchically organized referent body configurations which ensure task-specific stability of the system by means of synergic interactions.