Finger interaction in a three-dimensional pressing task
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Accurate control of forces produced by the fingers is essential for performing object manipulation. This study examines the indices of finger interaction when accurate time profiles of force are produced in different directions, while using one of the fingers or all four fingers of the hand. We hypothesized that patterns of unintended force production among shear force components may involve features not observed in the earlier studies of vertical force production. In particular, we expected to see unintended forces generated by non-task fingers not in the direction of the instructed force but in the opposite direction as well as substantial force production in directions orthogonal to the instructed direction. We also tested a hypothesis that multi-finger synergies, quantified using the framework of the uncontrolled manifold hypothesis, will help reduce across-trials variance of both total force magnitude and direction. Young, healthy subjects were required to produce accurate ramps of force in five different directions by pressing on force sensors with the fingers of the right (dominant) hand. The index finger induced the smallest unintended forces in non-task fingers. The little finger showed the smallest unintended forces when it was a non-task finger. Task fingers showed substantial force production in directions orthogonal to the intended force direction. During four-finger tasks, individual force vectors typically pointed off the task direction, with these deviations nearly perfectly matched to produce a resultant force in the task direction. Multi-finger synergy indices reflected strong co-variation in the space of finger modes (commands to fingers) that reduced variability of the total force magnitude and direction across trials. The synergy indices increased in magnitude over the first 30% of the trial time and then stayed at a nearly constant level. The synergy index for stabilization of total force magnitude was higher for shear force components when compared to the downward pressing force component. The results suggest complex interactions between enslaving and synergic force adjustments, possibly reflecting the experience with everyday prehensile tasks. For the first time, the data document multi-finger synergies stabilizing both shear force magnitude and force vector direction. These synergies may play a major role in stabilizing the hand action during object manipulation.
KeywordsFinger Force Synergy Pressing Enslaving
The study was in part supported by NIH grants AG-018751, NS-035032, and AR-048563.
- Gorniak SL, Duarte M, Latash ML (2008) Do synergies improve accuracy? A study of speed-accuracy trade-offs during finger force production. Mot Control 12:151–172Google Scholar
- Latash ML, Scholz JF, Danion F, Schöner G (2002a) Finger coordination during discrete and oscillatory force production tasks. Exp Brain Res 146:412–432Google Scholar
- Latash ML, Scholz JP, Schöner G (2007) Toward a new theory of motor synergies. Mot Control 11:275–307Google Scholar
- Latash ML, Friedman J, Kim SW, Feldman AG, Zatsiorsky VM (2010) Prehension synergies and control with referent hand configurations. Exp Brain Res (in press)Google Scholar
- Li ZM, Dun S, Harkness DA, Brininger TL (2004) Motion enslaving among multiple fingers of the human hand. Mot Control 8:1–15Google Scholar
- Rouiller EM (1996) Multiple hand representations in the motor cortical areas. In: Wing AM, Haggard P, Flanagan JR (eds) Hand and brain. The neurophysiology and psychology of hand movements. Academic Press, San Diego, pp 99–124Google Scholar
- Vaillancourt DE, Slifkin AB, Newell KM (2002) Inter-digit individuation and force variability in the precision grip of young, elderly, and Parkinson’s disease participants. Mot Control 6:113–128Google Scholar