Wilhelm L, Zatsiorsky VM, Latash ML. Equifinality and its violations in a redundant system: multifinger accurate force production. J Neurophysiol 110: 1965-1973. First published July 31, 2013 doi:10.1152/jn.00461.2013.-We explored a hypothesis that transient perturbations applied to a redundant system result in equifinality in the space of task-related performance variables but not in the space of elemental variables. The subjects pressed with four fingers and produced an accurate constant total force level. The "inverse piano" device was used to lift and lower one of the fingers smoothly. The subjects were instructed "not to intervene voluntarily" with possible force changes. Analysis was performed in spaces of finger forces and finger modes (hypothetical neural commands to fingers) as elemental variables. Lifting a finger led to an increase in its force and a decrease in the forces of the other three fingers; the total force increased. Lowering the finger back led to a drop in the force of the perturbed finger. At the final state, the sum of the variances of finger forces/modes computed across repetitive trials was significantly higher than the variance of the total force/mode. Most variance of the individual finger force/mode changes between the preperturbation and postperturbation states was compatible with constant total force. We conclude that a transient perturbation applied to a redundant system leads to relatively small variance in the task-related performance variable (equifinality), whereas in the space of elemental variables much more variance occurs that does not lead to total force changes. We interpret the results within a general theoretical scheme that incorporates the ideas of hierarchically organized control, control with referent configurations, synergic control, and the uncontrolled manifold hypothesis. redundancy; abundance; equifinality; finger; force; referent configuration THE EQUILIBRIUM-POINT HYPOTHESIS has been, for the past halfcentury, one of the most controversial and influential hypotheses in the field of motor control (Feldman 1986;Feldman and Levin 1995). Recently, it has been developed in the form of a referent configuration (RC) hypothesis (Feldman 2009). Whereas the original equilibrium-point hypothesis described the neural control of a single muscle and muscle pairs acting at a joint (Feldman 1986), the RC hypothesis generalized the main ideas to the control of multimuscle, whole body actions involving redundant sets of effectors. According to the RC hypothesis, voluntary movements are produced by shifts of neural variables leading to changes in referent values for task-specific, salient variables given the external force field. Analysis of movement equifinality in the presence of transient force perturbations has been an important source of experimental evidence quoted as both supporting the equilibrium-point hypothesis (Bizzi et al. 1976;Kelso and Holt 1980;Latash and Gottlieb 1990;Schmidt and McGown 1980) and creating problems for it (DiZio and Lackner 1995;Hinder and Mi...

During single-finger force production, the non-instructed fingers unintentionally produce force (finger enslaving). In this study, enslaving effects were compared between the dominant and non-dominant hands. The test consisted of a series of maximum voluntary contractions with different finger combinations. Enslaving matrices were calculated by means of training an artificial neural network. The dominant hand was found to be stronger, but there was found to be no difference between the overall enslaving effects in the dominant and non-dominant hands. There was no correlation between the magnitude of finger enslaving and the performance in such tests as the Edinburgh Handedness Inventory, the Grooved Pegboard test, and the Jebsen-Taylor Hand Function test. Each one of those three tests showed a significant difference between the dominant and non-dominant hand performances. Eleven subjects were retested after two months, and it was found that enslaving effects did not fluctuate significantly between the two testing sessions. While the dominant and non-dominant hands are involved differently in everyday tasks, e.g. in writing or eating, this practice does not cause significant differences in enslaving between the hands.

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