Hierarchical Control of Prehension

Author:

Gorniak, Stacey Lynn

Graduate Program:

Kinesiology

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

May 27, 2009

Committee Members:

Mark Latash, Dissertation Advisor/Co-Advisor
Mark Latash, Committee Chair/Co-Chair
Vladimir M Zatsiorsky, Committee Member
Stephen Jacob Piazza, Committee Member
Arkady Tempelman, Committee Member

Keywords:

prehension
bimanual
synergies
finger
hand
motor control

Abstract:

The current dissertation addresses strategies used by humans in the control of hand-held objects using one- and two-hand grasps. The following objectives were set in this dissertation: to test the hypothesis on hierarchical organization of bimanual synergies for fragile and non-fragile objects, to investigate the mechanical output of the fingers involved in both bimanual prehension and prehension of fragile and non-fragile objects, and to investigate bimanual prehension in the framework of the referent configuration hypothesis. The series of experiments investigating both kinetic and kinematic synergies resulted in the following conclusions: (1) The CNS can form synergies at both levels of the hierarchy involved in prehensile tasks. In natural tasks, salient performance variables are stabilized at both hierarchical levels, while variables that do not have to obey strict constraints are typically stabilized at the upper level only. (2) In comparison to unimanual prehension, force and moments of force produced by opposing effectors differs quantitatively in bimanual prehension. Specifically, bimanual prehension is marked by lower values of normal forces and safety margin measures as well as a decrease in the synergy index at the higher hierarchical level. (3) Synergies are present at both levels of an assumed control hierarchy in kinematic tasks involving object transport with both hands. In the presence of external perturbations to the upper limbs, tactile information from the hands is used to maintain stable grasp of an object, while the lack of such sensory information leads to loss of stability of the distance between the hands and dissociation between the limb trajectories. (4) The mechanical output and variability of the mechanical output of digits involved in prehension of fragile objects are qualitatively similar to patterns found during prehension of non-fragile objects in both unimanual and bimanual tasks. (5) The application of the reference configuration hypothesis to prehension provides insight into the central nervous system controls movement.