In order to grasp an object, the human nervous system must transform the intended hand displacement into control signals distributed to motor neurons and ultimately to muscles. The aim of this thesis is to describe the nature of the internal representations that the human motor system uses to perform reaching movements. / The aim of the first study was to provide a clear and simple way to test whether dynamical information is coded by the nervous system in an extrinsic, Cartesian, versus intrinsic, muscle- or joint-based, system of coordinates. As a means to determine the frame for reference used by the motor system, we examined how adaptation to externally applied forces transfers across different arm configurations. We trained subjects to make reaching movements while holding a robotic arm that applied forces proportional and perpendicular to the tangential velocity of the hand. While in the first trials hand paths were substantially deviated, subjects rapidly adapted to the new dynamic condition; they learned to compensate for the forces in order to restore the kinematics observed in the absence of load. Learning of the new dynamics transferred across movements performed in different regions of the workspace when the relation between joint displacements and experienced torques remained unchanged, rather than when the mapping between hand displacements and forces was preserved. This provided support to the idea that dynamics are encoded in muscle- or joint-based coordinates. / The results of the first study described a process of generalization that relies on the invariance of the mapping between torques and joint displacements. While this clearly points to an intrinsic coding of dynamics, it does not explain whether or how generalization over the workspace occurs when the pattern of torques changes with the configuration of the arm. In the second study, subjects learned a force field in which the forces acted always in the same direction relative to an external frame of reference, which defines a mapping between joint displacements and torques that varies with the configuration of the arm. Our idea was to test if in the absence of invariance in the pattern of torques, generalization would occur on the basis of the invariance in the direction of the forces represented in an extrinsic system of coordinates. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.85577 |
| Date | January 2004 |
| Creators | Malfait, Nicole |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Psychology.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 002210730, proquestno: AAINR12897, Theses scanned by UMI/ProQuest. |
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