Cue-induced changes in the stability of finger force-production tasks revealed by the uncontrolled-manifold analysis

A motor system configured to maximize the stability of its current state cannot dexterously transition between states. Yet, we routinely resolve the stability-dexterity conflict and rapidly change our current behavior without allowing it to become unstable prior to the desired transition. The phenomenon called anticipatory synergy adjustment (ASA) partly describes how the central nervous system handles this conflict. ASA is a continuous decrease in the stability of the current motor state beginning 150-400 ms prior to a rapid state transition accomplished using redundant sets of motor inputs (more input variables than task-specific output variables). So far, ASAs have been observed only when the timing of the upcoming transition is known. We utilized a multi-finger, isometric force production task to demonstrate that compared to a condition where no state transition is expected, the stability of the current state is lower by about 12% when a participant is cued to make a transition - even when the nature and timing of that transition are unknown. This result (stage-1 ASA) is distinct from its traditional version (stage-2 ASA), and it describes early destabilization that occurs solely in response to the expectation to move. Stage-2 ASA occurs later, only if the timing of the transition is known sufficiently in advance. Stage-1 ASA lasts much longer (~ 1.5 seconds), and may scale in response to the perceived difficulty of the upcoming task. Therefore, this work reveals a much-refined view of the processes that underlie the resolution of the stability-dexterity conflict.

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