Expert strategy switching in the control of a bimanual manipulandum with an unstable task

The goal of this study is to better understand how the central nervous system switches between alternative stabilization strategies when presented with an unstable task. A haptic, bimanual manipulandum has been used to emulate an unstable task, which requires subjects to stabilize a virtual mass under the action of a saddle force field with two nonlinear springs, whose stiffness increases with the amount of stretch. Subjects learn to position the mass at various target points by adjusting the rest length, and thus the stiffness of the two springs. From a previous study we know that subjects can stabilize the mass by either 1) applying large forces to stretch the springs and increase the mechanical stiffness of the system beyond a critical level or by 2) applying small force impulses that intermittently adjust the position of the mass. In this study we report the performance of a subject who was trained extensively to use one strategy or the other in order to characterize the mechanism of target switching, from the high-stiffness to the low-stiffness regime and back.