Stability of movement by a multi-element system: drifts and random walk within and orthogonal to the uncontrolled manifold during multi-finger force production.

We tested one of the central assumptions of the uncontrolled manifold (UCM) hypothesis, that stability within and orthogonal to the UCM differs in a task-specific way. This was done by exploring the effects of visual feedback on the fast random walk (RW) and slow drifts during multi-finger force production. Healthy participants used the index and middle fingers of both hands to produce an accurate total force magnitude with different initial sharing of the force between the hands. After 5 s, visual feedback was manipulated-kept for both force and sharing, for only one of those variables, or turned off. The subjects tried to keep their performance unchanged for 55 s. Trajectories both along and orthogonal to the UCM for total force showed fast RW and slow drifts. The diffusion plots confirmed persistent RW within the first 0.2 s and anti-persistent RW after 0.5 s. Persistent RW was similar across visual feedback conditions and larger orthogonal to the UCM. Its Hurst index correlated between the UCM and orthogonal to the UCM direction across participants. Anti-persistent RW depended strongly on visual feedback. Drift magnitude and characteristic time depended strongly on visual feedback, being similar along and orthogonal to the UCM. We conclude that RW destabilizes the state of the system, thus encouraging exploration of nearby states over short time intervals, and contributes to its stability over larger time intervals. Visual feedback plays a more important role in structuring the stability of performance compared to the explicit task formulation. RW and drift exploration promise new insights into the organization of stability in abundant systems and a potential biomarker for clinical studies.