FIG. 4.

Human trajectories when negotiating the assisting perturbation: (a) Early trial: the subject did not exploit any contraction region. (b) Late trial: the subject encountered the perturbation entirely inside a contraction region. The black arrowhead indicates the starting point of the trajectory. $P^{-}$ denotes the instant just before the perturbation, while $P^{+}$ denotes the instant after the perturbation. The exact ellipse indicates pendulum oscillations for zero cup velocity at the target box.