Learning multi-finger synergies: an uncontrolled manifold analysis
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We used the uncontrolled manifold (UCM) approach to study the synergy formation during learning an unusual multi-finger task. The subjects produced accurate force ramps with challenging sets of four fingers (two per hand). We tested hypotheses on stabilization of the contributions of subsets of effectors to the task force (FTASK) and to the moment in the frontal plane (force-stabilization and moment-stabilization, respectively). Force signals were used to compute magnitudes of hypothetical independent signals, modes. The variance of the mode magnitudes across repetitions of the task was partitioned into two components, within the UCM (VUCM), which did not affect the average value of a selected performance variable (force or moment), and orthogonal to the UCM (VORT), which affected the variable. Prior to practice, subjects showed high error indices and failed to show stabilization of each hand’s contribution to FTASK (VORT≥VUCM), while the pronation-supination moment was stabilized by the fingers of each hand (VORT<VUCM). The total forces produced by each of the two hands showed negative covariation across trials, which supported the force-stabilization hypothesis but not moment-stabilization hypothesis. Both force-stabilization and moment-stabilization hypotheses were supported by analysis of mode magnitudes to all eight fingers. Over 2 days of practice, the performance of the subjects improved considerably. This was accompanied by the emergence of within-a-hand force-stabilization for each of the two hands without deterioration of moment-stabilization. Quantitatively better within-a-hand force-stabilization was seen in male subjects as compared to females throughout the course of the experiment. Force-stabilization by all eight fingers improved quantitatively with practice. Practice also resulted in higher finger forces in maximal force production (MVC) trials and higher forces produced by unintended fingers in single-finger MVC trials (higher enslaving). We conclude that the UCM approach allows quantifying changes in the coordination of effectors during practice, and offers insights into the microstructure of this coordination with respect to different performance variables and different subsets of effectors. The approach can be used to test whether new synergies emerge in the process of practice.
KeywordsFinger Force production Synergy Effects of practice
We are grateful to Jae Kun Shim and Halla Olafsdottir for their help with running the experiments and to John Scholz for many helpful discussions. The study was partly supported by NIH grants NS-35032 and AG-018751.
- Bernstein NA (1967) The coordination and regulation of movements. Pergamon Press, LondonGoogle Scholar
- Bernstein NA (1996) On dexterity and its development. In: Latash ML, Turvey MT (eds) Dexterity and its development. Erlbaum, Mahwah, NJ, pp 277–304Google Scholar
- Feldman AG (1986) Once more on the equilibrium-point hypothesis (λ model) for motor control. J Mot Behav 18:17–54Google Scholar
- Feldman AG, Levin MF (1995) The origin and use of positional frames of reference in motor control. Behav Brain Sci 18:723–804Google Scholar
- Gelfand IM, Tsetlin ML (1966) On mathematical modeling of the mechanisms of the central nervous system. In: Gelfand IM, Gurfinkel VS, Fomin SV, Tsetlin ML (eds) Models of the structural-functional organization of certain biological systems. Nauka, Moscow (in Russian, a translation is available in 1971 edn by MIT Press), pp 9–26Google Scholar
- Latash ML (1993) Control of human movement. Human Kinetics, Urbana, ILGoogle Scholar
- Latash ML, Scholz JP, Danion F, Schöner G (2002b) Finger coordination during discrete and oscillatory force production tasks. Exp Brain Res 146:412–432Google Scholar
- Turvey MT (1990) The challenge of a physical account of action: a personal view. In: Whiting HTA, Meijer OG, van Wieringen PCW (eds) The natural-physical approach to movement control. VU University Press, Amsterdam, pp 57–92Google Scholar
- Vereijken B, van Emmerick REA, Whiting HTA, Newell KM (1992) Free(z)ing degrees of freedom in skill acquisition. J Mot Behav 24:133–142Google Scholar