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Mitchell Tillman and Satyajit Ambike

The authors examined how the stability of the current total isometric force (F T) produced by four fingers is influenced by previous and expected voluntary changes in F T. The authors employed the synergy index obtained from the across-trial uncontrolled manifold analysis to quantify the stability of F T. The authors compared two tasks with similar histories of F T changes; one in which participants expected changes in F T in the future, and one in which they expected no changes in F T. The stability of F T was lower in the former task, indicating the existence of a novel type of anticipatory synergy adjustment. Disparate histories of F T changes yield inconsistent changes in stability, driven by individual differences in the covariation in the finger forces that leave F T invariant. Future research should focus on exploring these individual differences to better understand how previous and expected behavior changes influence the stability of the current motor behavior.

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Satyajit Ambike, Daniela Mattos, Vladimir Zatsiorsky and Mark Latash

Cyclic isometric finger-force patterns established using visual feedback show systematic drifts when the feedback is removed. Force changes at multiple time scales and in opposite directions have been reported. For further characterization of these drifts, healthy subjects produced isometric, cyclic finger force with and without visual feedback at various initial amplitudes and frequencies. We hypothesized that on feedback removal, the amplitude will be attracted toward a preferred value that is frequency dependent. We found that the amplitude always increased after feedback removal. The magnitude of the amplitude increase changed with initial frequency, but it was invariant over the explored range of initial amplitudes. Thus, the existence of a preferred amplitude of force oscillations was not supported. We interpret these results within the referent configuration and the referent configuration back-coupling hypotheses. These data will inform a mathematical model of finger-force drifts. However, currently, they raise more questions than they answer, and a coherent account of finger-force drifts remains a challenge.