Unsteady steady-states: Central causes of unintentional forcedrift

摘要

We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely, the fingertip referent coordinate (RFT) and its apparent stiffness (CFT). The system's state is defined by a point in the {RFT; CFT} space. We tested the hypothesis that, after visual feedback removal, this point (1) moves along directions leading to drop in the output fingertip force, and (2) has even greater motion along directions that leaves the force unchanged. Subjects produced a prescribed fingertip force using visual feedback, and attempted to maintain this force for 15 s after the feedback was removed. We used the “inverse piano” apparatus to apply small and smooth positional perturbations to fingers at various times after visual feedback removal. The time courses of RFT and CFT showed that force drop was mostly due to a drift in RFT towards the actual fingertip position. Three analysis techniques, namely, hyperbolic regression, surrogate dataanalysis, and computation of motor-equivalent and non-motor-equivalent motions,suggested strong co-variation in RFT and CFT stabilizingthe force magnitude. Finally, the changes in the two hypothetical controlvariables {RFT; CFT} relative to their average trends alsodisplayed covariation. On the whole the findings suggest that unintentionalforce drop is associated with (a) a slow drift of the referent coordinate thatpulls the system towards a low-energy state, and (b) a faster synergic motion ofRFT and CFT that tends to stabilize the outputfingertip force about the slowly-drifting equilibrium point.