Force production with a subset of fingers: two aspects of unintentional finger involvement.

We explored the relations between unintentional finger force production (enslaving) and multi-finger force-stabilizing synergies using the framework of the uncontrolled manifold (UCM) hypothesis. In particular, we formulated three mutually exclusive hypotheses on the relations between the synergy indices for the force produced by the instructed finger, enslaved fingers, and all the fingers. The subjects performed accurate force production task with two fingers of a hand (index and ring) while the visual feedback showed the force produced by all the fingers (unknown to the subjects). Synergy index reflecting inter-trial variance analysis confirmed strongest synergies stabilizing the total force and weakest synergies stabilizing the force by the enslaved fingers. Analysis of 60-s trials with and without continuous visual feedback on total force demonstrated an increase in the enslaving index. Analysis of the diffusion plots (Hurst index) confirmed larger stability along the UCM for the enslaved finger force. Overall, our results show that the traditional view on enslaving as a robust phenomenon ensuring linear relations between the intended and unintended finger forces is likely wrong: Enslaving increases with time as reflected in the smaller indices of force-stabilizing synergies. These findings also suggest that enslaving is defined predominantly by neural factors.