Hierarchical control of static prehension: II. Multi-digit synergies

Abstract

The purpose of this study was to explore the ability of the central nervous system (CNS) to organize synergies at two levels of a hypothetical control hierarchy involved in two-hand multi-finger prehension tasks with one or more persons participating in the task together. At the higher level of the hierarchy, the total force and moment of force produced on an object are distributed between the thumb and the virtual finger (an imagined finger with mechanical output equal to the involved fingers of the hand), while at the lower level the virtual finger action is distributed among the four fingers. We tested a hypothesis that the CNS is able to organize synergies at only one level of the hierarchy. The subjects held vertically one of the two handles, a narrow one and a wide one. They used the four fingers of the right hand opposed by either the right hand thumb, the left hand thumb, the left hand index finger, the thumb of an experimenter, the index finger of an experimenter, or an inanimate object. Forces and moments of force produced by each digit were recorded. Indices of synergies stabilizing the mechanical output variables at each of the two levels were computed. Contrary to the expectations, force and moment of force stabilizing synergies were found at one or both levels of the hierarchy across all tasks. Unimanual tasks exhibited higher synergy indices compared to all tasks, while intrapersonal synergy indices were higher than those of interpersonal synergies. The results suggest that both feed-forward and feedback mechanisms may be used to create force and moment of force stabilizing synergies. We invoke the notion of chain effects and generalize it for relations among variance components related to stabilization of different mechanical variables. The reference configuration hypothesis offers a fruitful framework for analysis of prehension synergies.

Prehension synergies and hand function in early-stage Parkinson’s disease

“…The synergy index, Δ V, was computed in the same way as in the force pulse production task. Note that Δ V > 0 indicates a synergy stabilizing a certain performance variable at the selected level (Shim et al 2005; Gorniak et al 2009). This index was log-transformed (Δ V Z ) using a Fischer transformation applied to the boundaries of each level.…”

Motor control theories and their applications

“…Initially this involved examining the ratio of the two components. More recent investigations have used the relative difference between the components (Gorniak et al 2007, 2009b), in part because the ratio has a floor effect and is not normally distributed. The idea is that a larger value of this measure indicates a stronger synergy.…”

Prehension Synergies During Fatigue of a Single Digit: Adaptations in Control With Referent Configurations

“…It has been shown in the past that there is an inherent trade-off between synergies at two hierarchical levels (for details cf. Gorniak, Zatsiorsky, & Latash, 2007; Gorniak et al, 2009; Latash, Gorniak, & Zatsiorsky, 2008). In particular, an increase in V UCM at the upper level of the hierarchy leads to higher ΔV, but also to an increase in variance of the VF force.…”

“…At the TH-VF level, it can be expected that if one of the elements (TH or VF) is weakened due to fatigue and produces lower force, then the other element has to provide mechanical compensation to ensure that the grasped object remains in static equilibrium (Gorniak, Zatsiorsky, & Latash, 2009; Shim, Latash, & Zatsiorsky, 2005a). In contrast, fatigue of only one finger may have little effect on the variability of the total output of the VF (Singh et al, 2010b).…”