Visual Perception of Support-Surface Deformability From Human Body Kinematics

Riccio and Stoffregen (1988) argued that dynamical properties of a surface of support influence the control of behavior. This implies that the kinematic form of events, including the movements of a behaving animal, will be influenced by the dynamics (e.g., rigidity, tilt, friction) of the surfaces on which they occur. For any behavior in which the execution is constrained (influenced) by a surface, the kinematics of the body should provide information about the dynamics of the surface. Such kinematics could be used for perception of action-relevant surface properties by the behaving animal or by observers who view the animal's behavior. We evaluated the latter possibility by presenting observers with point-light displays (Johansson, 1973) of an actor on rigid and deformable surfaces. In Experi- ment 1, observers identified support-surface deformability as the dimension of variability, selecting it from a variety of dynamical dimensions. In the remaining experiments observers differentiated the surfaces across a wide range of behaviors including walking, running, and push-ups. The surfaces were not differentiated for other behaviors, such as hopping and sit-ups. The data cannot be accounted for in terms of sensitivity to deformation (a nondynamic property) rather than to deformability (a dynamic property). We conclude that observers are sensitive to the deformability of support surfaces, and that this sensitivity can be based solely on kinematic stimulation. This is consistent with Riccio and Stoffregen's (1988) theory of the perception and control of bodily orientation.