James E. Cutting

Recognizing friends by their walk: Gait perception without familiarity cues

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.

Chapter 3 – Perceiving Layout and Knowing Distances: The Integration, Relative Potency, and Contextual Use of Different Information about Depth*

Publisher Summary This chapter discusses three questions: Why are there so many sources of information about layout? How is it that one perceives layout with near-metric accuracy when none of these sources yields metric information about it? Can one not do better, theoretically, in understanding the perception of layout than simply make a list? The answer to the first question begins with Todds answer. Perceiving layout is extremely important to human beings, so important that it must be redundantly specified so that the redundancy can guard against the failure of any given source or the failure of any of the assumptions on which a given source is based. However, information redundancy is only part of the answer. Different sources of information about layout metrically reinforce and contrast with each other, providing a powerful network of constraints. The answer to the second proceeds from this idea. Through the analysis of depth-threshold functions for nine different sources of information about layout, one can begin to understand how those sources of information sharing the same-shaped functions across distances can help ramify judgments of layout by serving to correct measurement errors in each. Third, on the basis of the analyses and the pattern of functions, it suggests that list making has misled about space and layout. Psychologists and other vision scientists have generally considered layout, space, and distance as a uniform commodity in which observers carry out their day-to-day activities.

Recognizing the sex of a walker from a dynamic point-light display

The sex of human walkers can be recognized without familiarity cues from displays of pointlight sources mounted on major joints. Static versions of these abstract displays do not permit accurate recognition of sex. Variation in the degree of armswing or in walking speed generally interferes with recognition, except that faster speeds are associated somewhat with improved recognition of females. Lights on upper-body joints permit more accurate guesses than do Lights on lower-body joints, but identification is possible even from minimal displays, with lights placed only on the ankles. No feedback was given to observers. Confidence judgments of sex relate to the accuracy of responses in a manner that suggests that viewers know what they are doing.

Temporal and spatial factors in gait perception that influence gender recognition

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.

Infant sensitivity to figural coherence in biomechanical motions

Two experiments assessed infant sensitivity to figural coherence in point-light displays moving as if attached to the major joints of a walking person. Experiment 1 tested whether 3- and 5-month-old infants could discriminate between upright and inverted versions of the walker in both moving and static displays. Using an infant-control habituation paradigm, it was found that both ages discriminated the moving but not the static displays. Experiment 2 was designed to clarify whether or not structural invariants were extracted from these displays. The results revealed that (1) moving point-light displays with equivalent motions but different topographic relations were discriminated while (2) static versions were not, and (3) arrays that varied in the amount of motion present in different portions of the display were also not discriminated. These results are interpreted as indicating that young infants are sensitive to figural coherence in displays of biomechanical motion.

Categories and boundaries in speech and music

Perceptual categories and boundaries arise when Ss respond to continuous variation on a physical dimension in a discontinuous fashion. It is more difficult to discriminate between members of the same category than to discriminate between members of different categories, even though the amount of physical difference between both pairs is the same. Speech stimuli have been the sole class of auditory signals to yield such perception; for example, each different consonant phoneme serves as a category label. Experiment I demonstrates that categories and boundaries occur for both speechand nonspeech stimuli differing in rise time. Experiment II shows that rise time cues categorical differences in both complex and simple nonspeech waveforms. Taken together, these results suggest that certain aspects of speech perception are intimately related to processes and mechanisms exploited in other domains. The many categories in speech may be based on categories that occur elsewhere in auditory perception.

Wayfinding on Foot From Information in Retinal, Not Optical, Flow

People find their way through cluttered environments with ease and without injury. How do they do it? Two approaches to wayfinding 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.

Generation of Synthetic Male and Female Walkers through Manipulation of a Biomechanical Invariant

Synthetic versions of human walkers were generated by computer as point-light displays. Previously it had been determined that the natural gaits of males and females differ according to the extent of movement at the shoulder and the hip. These movements were measured and then used to synthesize the stimuli used in the present study. These stimuli are shown here to be identified by untrained viewers as male when the shoulder movement is greater than the hip movement, and female when the configuration is reversed. Because of the coherence of the display lights representing the shoulder and hip are not necessary for gender recognition, although they do increase performance level. Hypernormality and heavy-footedness in gait are also discussed. Finally, all results are linked to an underlying biomechanical invariant, the center of moment.

Three gradients and the perception of flat and curved surfaces

Researchers of visual perception have long been interested in the perceived slant of a surface and in the gradients that purportedly specify it. Slant is the angle between the line of sight and the tangent to the planar surface at any point, also called the surface normal. Gradients are the sources of information that grade, or change, with visual angle as one looks from ones feet upward to the horizon. The present article explores three gradients--perspective, compression, and density--and the phenomenal impression of flat and curved surfaces. The perspective gradient is measured at right angles to the axis of tilt at any point in the optic array; that is, when looking down a hallway at the tiles of a floor receding in the distance, perspective is measured by the x-axis width of each tile projected on the image plane orthogonal to the line of sight. The compression gradient is the ratio of y/x axis measures on the projected plane. The density gradient is measured by the number of tiles per unit solid visual angle. For flat surfaces and many others, perspective and compression gradients decrease with distance, and the density gradient increases. We discuss the manner in which these gradients change for various types of surfaces. Each gradient is founded on a different assumption about textures on the surfaces around us. In Experiment 1, viewers assessed the three-dimensional character of projections of flat and curved surfaces receding in the distance. They made pairwise judgments of preference and of dissimilarity among eight stimuli in each of four sets. The presence of each gradient was manipulated orthogonally such that each stimulus had zero, one, two, or three gradients appropriate for either a flat surface or a curved surface. Judgments were made were made for surfaces with both regularly shaped and irregularly shaped textures scattered on them. All viewer assessment were then scaled in one dimension. Multiple correlation and regression on the scale values revealed that greater than 98% of the variance in scale values was accounted for by the gradients. For the flat surfaces a mean of 65% of the variance was accounted for by the perspective gradient, 28% by the density gradient, and 6% by the compression gradient. For curved surfaces, on the other hand, a mean of 96% of the variance was accounted for by the compression gradient, and less than 2% by either the perspective gradient or the density gradient.(ABSTRACT TRUNCATED AT 400 WORDS)

Minimodularity and the Perception of Layout

In 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 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 minimodularity are discussed.