Bimanual control of an unstable task: Stiffness vs. intermittent control strategy

The goal of this study is to better understand how the central nervous system chooses a stable control strategy when presented with an unstable task. A haptic, bimanual manipulandum has been used to implement an unstable task, which requires subjects to stabilize a mass-load under the action of a saddle force field with two non-linear springs, whose stiffness increases when stretched. Subjects learn to position and stabilize the mass at various target points by adjusting the length, and thus the stiffness of the two springs. Subjects can stabilize the mass either by 1) applying large forces to stretch the springs and increase the mechanical stiffness of the system (stiffness strategy) or by 2) applying force impulses that intermittently adjust the position of the mass (intermittent strategy). The former control policy is simple but fatiguing, while the latter one is less fatiguing but computationally more challenging. Preliminary results indicate that human subjects can adopt one strategy or the other and the choice is correlated with the bimanual orientation of the arms. Specifically, when the hands are aligned along the unstable manifold of the force field, subjects tend to use the stiffness strategy, whereas when the hands are aligned along the stable manifold, subjects tend to use the intermittent strategy.