Motor equivalence in actions by redundant motor systems

Mattos, Daniela
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University of Delaware
The phenomenon of motor equivalence (ME) is one of the most remarkable features of purposeful behaviors in biological systems that allow reorganizing the degrees-of-freedom (DOFs) in the face of perturbations and still accomplishing the functional task. When a small perturbation is introduced to the system, salient variables of task performance are never perfectly unchanged, which poses additional challenges to the estimation of motor equivalence. The Uncontrolled Manifold (UCM) hypothesis proposes that the CNS uses a manifold (UCM) within the space of the elemental variables corresponding to desired values of a particular performance variable. Then, most of the variance in motor elements is expected to be confined to the UCM. Based on this hypothesis, the changes in elemental variables caused by an external perturbation are expected to lie mostly along the UCM, i.e. to represent ME with respect to the performance variable, while a smaller set of deviations in the elemental variables would cause errors in the performance variable (non-motor equivalent, nME). The overall goal of this dissertation was to quantify the ME phenomenon and, more specifically, to determine how different types of feedback contribute to the reorganization of motor elements to maintain stability of task-specific variables. Perturbations were provided during the course of either reaching arm movements or finger pressing tasks. Then, ME was quantified with respect to salient, lower dimensional performance variables at several levels of description including joint angles, muscle groups, finger forces, and finger modes. The outcomes of this study suggest that ME is present throughout the course of actions, even when there is no perturbation. After the perturbation onset, there was an immediate increase in the amount of ME that became larger within time windows of 50 ms post-perturbation, accompanied by a smaller increase in the nME component. These effects remained high after a transient external perturbation. ME effects were also induced by a quick action without any physical perturbation. We observed a large increase in the nME component upon removal of visual feedback, while the changes in the ME component were inconsistent. These results suggest that ME modulation receives contributions from several sources: muscle mechanics, reflexes and voluntary responses. Visual feedback seems to play a critical role in organizing the task-specific stability in multi-finger pressing tasks. These results support the idea that the CNS makes use of the abundant DOFs, allowing for flexibility of motor patterns in the face of perturbations or quick changes in actions.