Figure 1.

Experimental setup. A, One frog hindlimb (the wiping limb) was attached to the endpoint of a robotic arm. The robot both measured the wiping hindlimb endpoint kinematics and applied force perturbations to the wiping limb. The endpoint kinematics were measured relative to the mediolateral (ML)-rostrocaudal (RC) coordinate system. To evoke the hindlimb wiping reflex, a cutaneous electrical stimulus was applied to the other hindlimb (the target limb). A barrier was placed near the animal's midline to prevent the target limb from affecting the measured wiping limb kinematics. Each trial began with the wiping limb in an extended, lateral configuration, as indicated. B, On stimulation of the target limb, both limbs moved toward the midline with the objective of wiping off the noxious stimulus. The trajectory of the recorded, wiping limb is shown for the same trial shown in Figure 2 B. The arrow indicates the direction of movement.