Revisiting the importance of common body motion in human action perception.

Human actions are complex dynamic stimuli comprised of two principle motion components: 1) common body motion, which represents the translation of the body when a person moves through space, and 2) relative limb movements, resulting from articulation of limbs after factoring out common body motion. Historically, most research in biological motion has focused primarily on relative limb movements while discounting the role of common body motion in human action perception. The current study examined the relative contribution of posture change resulting from relative limb movements and translation of body position resulting from common body motion in discriminating human walking versus running actions. We found that faster translation speeds of common body motion evoked significantly more responses consistent with running when discriminating ambiguous actions morphed between walking and running. Furthermore, this influence was systematically modulated by the uncertainty associated with intrinsic cues as determined by the degree of limited-lifetime spatial sampling. The contribution of common body motion increased monotonically as the reliability of inferring posture changes on the basis of intrinsic cues decreased. These results highlight the importance of translational body movements and their interaction with posture change as a result of relative limb movements in discriminating human actions when visual input information is sparse and noisy.