title: Changes in motor synergies for tracking movement and responses to perturbations depend on task-irrelevant dimension constraints.
abstract: We investigated the changes in the motor synergies of target-tracking movements of hands and the responses to perturbation when the dimensionalities of target positions were changed. We used uncontrolled manifold (UCM) analyses to quantify the motor synergies. The target was changed from one to two dimensions, and the direction orthogonal to the movement direction was switched from task-irrelevant directions to task-relevant directions. The movement direction was task-relevant in both task conditions. Hence, we evaluated the effects of constraints on the redundant dimensions on movement tracking. Moreover, we could compare the two types of responses to the same directional perturbations in one- and two-dimensional target tasks. In the one-dimensional target task, the perturbation along the movement direction and the orthogonal direction were task-relevant and -irrelevant perturbations, respectively. In the two-dimensional target task, the both perturbations were task-relevant perturbations. The results of the experiments showed that the variabilities of the hand positions in the two-dimensional target-tracking task decreased, but the variances of the joint angles did not significantly change. For the task-irrelevant perturbations, the variances of the joint angles within the UCM that did not affect hand position (UCM component) increased. For the task-relevant perturbations, the UCM component tended to increase when the available UCM was large. These results suggest that humans discriminate whether the perturbations were task-relevant or -irrelevant and then adjust the responses of the joints by utilizing the available UCM.
authors.affiliation: Cognitive Mechanisms Laboratories, Advanced Telecommunications Research Institute International, 2-2-2 Hikaridai, Keihanna Science City, Soraku, Kyoto 619-0288, Japan; Japan Society for the Promotion of Science, Japan. Electronic address: togo@atr.jp.
authors.forename: Shunta
authors.initials: S
authors.lastname: Togo
authors.affiliation: Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Kagawa
authors.affiliation: Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
authors.forename: Yoji
authors.initials: Y
authors.lastname: Uno
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology*; D004576:Electromyography; D006801:Humans; D033182:Intention; D007596:Joints / Q000502:physiology*; D007699:Kinesthesis / Q000502:physiology*; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation; D009949:Orientation / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology; D055815:Young Adult
keywords: Joint coordination; Redundancy; Synergy; Target tracking; Uncontrolled manifold analysis