Open Access
parsa, Behnoosh
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 31, 2016
Committee Members:
  • Mark L. Latash, Thesis Advisor
  • Robert L. Sainburg, Committee Member
  • Stephen J. Piazza, Committee Member
  • redundancy
  • abundance
  • synergy
  • referent configuration
  • uncontrolled manifold hypothesis
  • visual feedback
  • finger
  • unintentional movement
  • optimization
  • cost function
This thesis investigates intentional and unintentional human actions during multi-finger isometric force-moment production tasks. In the first study, a multi-finger isometric force-moment task was used for studying the nature of force and moment unintentional drift, and the force-moment stabilizing synergies. In this experiment, subjects were utilizing visual feedback to maintain a certain amount of force and moment. At some point after subjects had stabilized their performance at the target, either force, or moment, or both feedbacks were turned off. The finger forces were studied under the uncontrolled-manifold and referent configuration hypothesis. This study showed that the absolute value of force and moment drops without visual feedback. However, drifts in individual finger forces could be in different directions; 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 the minimum of potential energy. The interaction between drifts of the hand referent coordinate and referent orientation led to counter-directional drifts in individual finger forces. The results also demonstrate that the sensory information used to create multi-finger synergies is necessary for their presence over the task duration. In the second study, subjects performed a set of force-moment tasks with different levels of force and moment. These data were used to compute a cost function using the analytical inverse optimization method. Then, the subjects were asked to produce accurate force-moment tasks with a non-preferred sharing among the fingers. This was done by asking the subjects to reduce the middle finger force to half of its magnitude during the comfortable task performance. Adding this new constraint to an ongoing task resulted in a drop of variance within the uncontrolled manifold leading to a decline in the synergy index. Then, the visual feedback was removed for one, two, or all three of the main variables (force, moment, and middle finger force). The studies showed that the observed drift in performance variables was due to two processes, a drift of the referent coordinate toward the actual coordinate leading to a decrease in the finger forces, and a movement in the space of finger forces in a direction that did not affect the performance, but it reduced the cost of performing the task. Overall, these results fit naturally the scheme of hierarchical control using changes in referent coordinates for relevant variables.