Viewers can recognize themselves and others in an abstract display of their movements_ Light sources mounted on joints prominent during the act of walking are sufficient cues for identification. No other information, no feedback, and little practice with such a display are needed_ This procedure, developed by Johansson, holds promise for inquiry into the dimensions and features of event perception: It is both naturalistic and experimentally manageable_ People often believe that they can recognize friends by their walk. Unfortunately, this belief and the previous research on the topic (e.g., Wolff, 1943) are confounded by familiarity cues, size and shape cues, or other nongait sources of information such as probabilities of seeing a person at a given place or time. We demonstrate that viewers can recognize themselves and others in a dynamic display of their movements when these factors are controlled.We were stimulated by the work of Johansson

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People find their way through cluttered environments with ease and without injury. How do they do it? Two approaches to wayfmding are considered: Differential motion parallax (DMP) is a retinal motion invariant of near and far objects moving against fixation; the information in optical flow (IOF) is a radial pattern of vectors, relying on decomposition of retinal flow. Evidence is presented that DMP guides wayfinding during natural gait, accounting for errors as well as correct responses. Evidence against IOF is also presented, and a space-time aliasing artifact that can contaminate IOF displays is explored. Finally, DMP and IOF are separated, showing they can yield different results in different environments. Thus, it is concluded that (a) DMP and IOF are different, (b) DMP and not IOF is used for wayfinding, (c) moving observers do not usually decompose retinal flow, and (d) optical flow may be a mathematical fiction with no psychological reality.One of the most compelling of all visual phenomena occurs when one hurtles through the environment. The resulting radial streams of motion by surrounding objects, sometimes called optical flow, have captured the imagination of writers, artists, and cinematographers, as well as psychologists, neuroscientists, and computer scientists. This global motion was probably first noticed by the general populace in the mid19th century, but then only in industrializing nations and with the widespread use of railroads.The reasons for the relatively recent focus on optical flow are probably threefold: On a train one could, for the first time, travel (a) at velocities greater than about 4 eye heights/ s for a sustained period of time; 1 (b) on a relatively smooth roadbed that eliminated the bouncing caused by one's own footfall or that of a horse, or by the jostling of a coach; and (c) with free time to look about, unfettered by the demands of guiding one's course through the environment. Naturally, railway travel offered much more than noticeable optical flow. Not all of it was good. Indeed, around 1860 The Lancet published a series of articles on putative ill effects of rail travel This research was supported by National Science Foundation Grant BNS-8818971 to James E. Cutting.Experiment 1 was reported at the 30th Annual Meeting of the Psychonomic Society, Atlanta, November 1989; parts of Experiments 2 and 3 at the Society's 31st Annual Meeting, New Orleans, November 1990; and parts of Experiments 7 and 8 at its 32nd Annual Meeting, San Francisco, November 1991. Experiments 1, 2, 4, 5, and 6 were also reported at a perception-action workshop at Storrs, Connecticut, in July 1990.We thank Laurence G. Kaplan for initial and sustaining help in programming the Personal Iris Workstation; David Sabean for discussions of 19th-century perception; Gretchen Van de Walle, Jack Loomis, and several anonymous reviewers for their comments; and Julian Hochberg and Nan Karwan for various discussions of this project over the past 6 yearsCorrespondence concerning this article should be addressed to J...

Several temporal and spatial factors affect gender recognition of a walker when portrayed, without familiarity cues, as a dynamic point-light display. We demonstrate that, among temporal parameters, the duration of the dynamic stimulus. must be longer than 1.6 sec, but that 2.7 sec is fully adequate. Given the speed of our walkers, the recognition threshold appears to be roughly two step cycles. In addition, presentation rate of the stimulus must be near to normal, perhaps because nonnormal rates alter apparent gravity and obscure the normal relationship between output and conservation of energy. We demonstrate that, among spatial factors, the discreteness of the joint information must be maintained for accurate recognition. We go on to argue that it is the information about the shoulder and the hip of a walker that is of primary importance. Finally, inversion of the stimulus display produces the unexpected effect of reversing the apparent sex of most walkers. That is, when presented upside down, male walkers appear female and female walkers appear male.If one removes familiarity cues from a dynamic display, nothing is left for the viewer to perceive except formless relations projected over time. Yet these formless relations can yield rich and vivid percepts. This fact proves embarrassing for any view of form perception that relies exclusively on superficial features such as lines and angles. The present studies explore how formless relations that afford gender recognition in human walkers survive temporal and spatial distortion. Our goal is that through this type of study we can learn more about the nature of those relations and how they are perceived.For nearly a century researchers have attached points of light that glow in the dark to the limbs of walkers for the purposes of studying gait (see Bernstein, 1967). In fact, through this technique and others, human gait may be the most studied of all biological movement. Nevertheless, nearly all of this work has focused on how the movement is produced. Until the work of Johansson (1973Johansson ( , 1975Johansson ( , 1976, few have studied how that movement is perceived.Using a point-light technique developed by Johansson, we have begun to study various aspects of gait recognition. Previously, we had found that individuals can recognize themselves and others without familiarity cues from such a dynamic display (Cutting & Kozlowski, 1977). Next, we (Kozlowski & Cutting, 1977) determined that viewers could idenThis paper is based on the Master's thesis of the first author. This research was supported by a small research grant from Wesleyan University to each of the authors. We thank Robert J. White for his continued help, and Carol Fowler, Dennis Proffitt, and David Stier for instructive comments. Reprint requests should be sent to J. Cutting or L. Kozlowski, Department of Psychology, Wesleyan University, Middletown, Connecticut 06457.tify the gender of a walker from that display, but not from static tokens taken out of the dynamic sequences. Variations in walking s...

SUMMARYIn natural vision, information overspecifies the relative distances between objects and their layout in three dimensions. Directed perception applies (Cutting, 1986), rather than direct or indirect perception, because any single source of information (or cue) might be adequate to reveal relative depth (or local depth order), but many are present and useful to observers. Such overspecification presents the theoretical problem of how perceivers use this multiplicity of information to arrive at a unitary appreciation of distance between objects in the environment.This article examines three models of directed perception: selection, in which only one source of information is used; addition, in which all sources are used in simple combination; and multiplication, in which interactions among sources can occur. To establish perceptual overspecification, we created stimuli with four possible sources of monocular spatial information, using all combinations of the presence or absence of relative size, height in the projection plane, occlusion, and motion parallax. Visual stimuli were computer generated and consisted of three untextured parallel planes arranged in depth. Three tasks were used: one of magnitude estimation of exocentric distance within a stimulus, one of dissimilarity judgment in how a pair of stimuli revealed depth, and one of choice judgment within a pair as to which one revealed depth best.Grouped and individual results of the one direct and two indirect scaling tasks suggest that perceivers use these sources of information in an additive fashion. That is, one source (or cue) is generally substitutable for another, and the more sources that are present, the more depth is revealed. This pattern of results suggests independent use of information by four separate, functional subsystems within the visual system, here called minimodules. Evidence for and advantages of mmimodularity are discussed.How do humans perceive distances between objects? Since ping between them many-to-many; on the latter the relation