VISUAL-PERCEPTION OF SUPPORT-SURFACE DEFORMABILITY FROM HUMAN-BODY KINEMATICS

Riccio and Stoffregen (1988) argued that dynamical properties of a sur face of support influence the control of behavior. This implies that t he kinematic form of events, including the movements of a behaving ani mal, will be influenced by the dynamics (e.g., rigidity, tilt, frictio n) 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 surfa ce properties by the behaving animal or by observers who view the anim als behavior. We evaluated the latter possibility by presenting observ ers with point-light displays (Johansson, 1973) of an actor on rigid a nd deformable surfaces. In Experiment 1, observers identified support- surface deformability as the dimension of variability, selecting it fr om a variety of dynamical dimensions. In the remaining experiments obs ervers differentiated the surfaces across a wide range of behaviors in cluding walking, running, and push-ups. The surfaces were not differen tiated for other behaviors, such as hopping and sit-ups. The data cann ot be accounted for in terms of sensitivity to deformation (a nondynam ic property) rather than to deformability (a dynamic property). We con clude that observers are sensitive to the deformability of support sur faces, and that this sensitivity can be based solely on kinematic stim ulation. This is consistent with Riccio and Stoffregen's (1988) theory of the perception and control of bodily orientation.