title: Practice reduces task relevant variance modulation and forms nominal trajectory.
abstract: Humans are capable of achieving complex tasks with redundant degrees of freedom. Much attention has been paid to task relevant variance modulation as an indication of online feedback control strategies to cope with motor variability. Meanwhile, it has been discussed that the brain learns internal models of environments to realize feedforward control with nominal trajectories. Here we examined trajectory variance in both spatial and temporal domains to elucidate the relative contribution of these control schemas. We asked subjects to learn reaching movements with multiple via-points, and found that hand trajectories converged to stereotyped trajectories with the reduction of task relevant variance modulation as learning proceeded. Furthermore, variance reduction was not always associated with task constraints but was highly correlated with the velocity profile. A model assuming noise both on the nominal trajectory and motor command was able to reproduce the observed variance modulation, supporting an expression of nominal trajectories in the brain. The learning-related decrease in task-relevant modulation revealed a reduction in the influence of optimal feedback around the task constraints. After practice, the major part of computation seems to be taken over by the feedforward controller around the nominal trajectory with feedback added only when it becomes necessary.
authors.affiliation: ATR Brain Information Communication Research Laboratory Group, Keihanna Science City, Kyoto, Japan.
authors.forename: Rieko
authors.initials: R
authors.lastname: Osu
authors.affiliation: Department of Intelligent Systems Design Engineering, Toyama Prefectural University, Imizu, Toyama, Japan.
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Morishige
authors.affiliation: Institute for Cognitive Systems, Technical University of Munich, Munich, Germany.
authors.forename: Jun
authors.initials: J
authors.lastname: Nakanishi
authors.affiliation: Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Miyamoto
authors.affiliation: ATR Brain Information Communication Research Laboratory Group, Keihanna Science City, Kyoto, Japan.
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Kawato
mesh_terms: D000328:Adult; D003198:Computer Simulation; D005260:Female; D006801:Humans; D007858:Learning; D008297:Male; D008962:Models, Theoretical; D012044:Regression Analysis; D013647:Task Performance and Analysis; D013997:Time Factors; D055815:Young Adult