Hand coordination along the unstable axis.
Evolution of the mediolateral component (x-axis) of the movement of the two hands and the virtual mass during the first two trials in the adaptation phase for two representative subjects: S6 (left panel) who was classified as a PSS user and S10 (right panel) who was classified as a SSS user. Note that at start of the adaptation phase both the hands and the virtual mass were located in the origin of the force field. The vertical line indicates the end of the first trial at a peripheral target, i.e. the time required to stabilize the mass for an uninterrupted period of 4 s. The interval following the vertical line plots the transition and the stabilization at the central target. The arrow in the right panel highlights the point at which the SSS subject separated the two hands along the unstable direction.