We addressed the issue of how display orientation affects the perception of biological motion. In Experiment 1, spontaneous recognition of a point-light walker improved abruptly with image-plane display rotation from inverted to upright orientation. Withina range of orientations from 180°to 90°,it was dramatically impeded. Using ROC analysis, we showed (Experiments 2 and 3) that despite prior familiarization with a point-light figure at all orientations, its detectability within a mask decreased with a change in orientation from upright to a range of 90°-180°. In Experiment 4, a priming effect in biological motion was observed only if a prime corresponded to a range of deviations from upright orientation within which the display was spontaneously recognizable. The findings indicate that display orientation nonmonotonically affects the perception of biological motion. Moreover, top-down influence on the perception of biological motion is limited by display orientation.The ability to correctly identify an event is ofgreat ecological importance for an organism, because it supports functional behavior in a continuously changing environment. Since the classic work ofGestalt psychology, there has been phenomenological evidence for veridical perception of simple events depicted only by rigid motions of several points. As will be shown later, the visual system is highly sensitive to information about the invariant structure in complex everyday events, such as biological motion patterns that consist ofa set of moving dots on the main joints of an invisible walker.Despite the potential perceptual ambiguity, humans readily extract the invariant structure from biological motion. In his initial study, Gunnar Johansson (1973Johansson ( , 1976 showed that adults need only 0.1-0.2 sec to identify displays with filmed biological motion. Mather and West (1993) extended these findings to perception ofanimated point-light figures of quadrupeds. Preschoolers 3-4 years of age were able to recognize point-light huParts of the study were presented at the 12th-14th Annual Meetings of the International Society for Psychophysics and at the I9th-2 I st European Conferences on Visual Perception. M.P. was supported by the Deutsche Forschungsgemeinschaft (436RUS 17/14/97)and by the MaxPlanck-Gesellschaft. Parts of the paper were done at the Institut fur Arbeitsphysiologie, Dortmund, the Institut fiir Psychologie, ChristianAlbrechts-Universitat zu Kiel, and the MPI for Biological Cybernetics, Tiibingen. We thank Niels Birbaumer, Isabelle and Heinrich Biilthoff, C. Richard Cavonius, Walter H. Ehrenstein, Dieter Heyer, Laurence Maloney, Rainer Mausfeld, and Sverker Runeson for stimulating discussions and Bennett Bertenthal for providing us with the walker-generating program. We especially thank John C. Baird for valuable advice on an earlier version of the manuscript and Arseny Sokolov for moral support. We also thank two anonymous reviewers for helpful comments.
