Perspective Cues Research Articles

Further Explorations of the Facing Bias in Biological Motion Perception: Perspective Cues, Observer Sex, and Response Times

The human visual system has evolved to be highly sensitive to visual information about other persons and their movements as is illustrated by the effortless perception of point-light figures or ‘biological motion’. When presented orthographically, a point-light walker is interpreted in two anatomically plausible ways: As ‘facing the viewer’ or as ‘facing away’ from the viewer. However, human observers show a ‘facing bias’: They perceive such a point-light walker as facing towards them in about 70-80% of the cases. In studies exploring the role of social and biological relevance as a possible account for the facing bias, we found a ‘figure gender effect’: Male point-light figures elicit a stronger facing bias than female point-light figures. Moreover, we also found an ‘observer gender effect’: The ‘figure gender effect’ was stronger for male than for female observers. In the present study we presented to 11 males and 11 females point-light walkers of which, very subtly, the perspective information was manipulated by modifying the earlier reported ‘perspective technique’. Proportions of ‘facing the viewer’ responses and reaction times were recorded. Results show that human observers, even in the absence of local shape or size cues, easily pick up on perspective cues, confirming recent demonstrations of high visual sensitivity to cues on whether another person is potentially approaching. We also found a consistent difference in how male and female observers respond to stimulus variations (figure gender or perspective cues) that cause variations in the perceived in-depth orientation of a point-light walker. Thus, the ‘figure gender effect’ is possibly caused by changes in the relative locations and motions of the dots that the perceptual system tends to interpret as perspective cues. Third, reaction time measures confirmed the existence of the facing bias and recent research showing faster detection of approaching than receding biological motion.

Recovering 3-D Shape: Roles of Absolute and Relative Disparity, Retinal Size, and Viewing Distance as Studied with Reverse-Perspective Stimuli

When viewing reverspective stimuli, data-driven signals such as disparity, motion parallax, etc, help to recover veridical three-dimensional (3-D) shape. They compete against schema-driven influences such as experience with perspective, foreshortening, and other pictorial cues that favor the perception of an illusory depth inversion. We used three scaled-size versions of a reverspective to study the roles of retinal size, binocular disparity, and viewing distance--that influences both vergence and accommodation--in recovering the true 3-D shape. Experiment 1 used three conditions, in each of which a parameter was kept fixed across the three stimulus sizes: (a) fixed retinal size, (b) fixed viewing distance, (c) fixed disparity. The predominance of the veridical percept was recorded. Generally, the illusion strength was the same when the viewing distance was fixed, despite significantly different disparities and retinal sizes; conversely, illusion strength changed significantly in fixed-disparity and fixed-retinal-size conditions. Experiment 2 confirmed the results of experiment 1b (roughly equal performances for fixed viewing distance, independent of size) for two additional distances. Viewing distance and "scaled disparity" (disparity divided by retinal size) are good predictors of the data trends. We propose that disparity scaling is supported by both mathematical and 3-D shape considerations.

The Influence of Ground Contact and Visible Horizon on Perception of Distance and Size under Severely Degraded Vision

For low vision navigation, misperceiving the locations of hazards can have serious consequences. Potential sources of such misperceptions are hazards that are not visually associated with the ground plane, thus, depriving the viewer of important perspective cues for egocentric distance. In Experiment 1, we assessed absolute distance and size judgments to targets on stands under degraded vision conditions. Normally sighted observers wore blur goggles that severely reduced acuity and contrast, and viewed targets placed on either detectable or undetectable stands. Participants in the detectable stand condition demonstrated accurate distance judgments, whereas participants in the undetectable stand condition overestimated target distances. Similarly, the perceived size of targets in the undetectable stand condition was judged to be significantly larger than in the detectable stand condition, suggesting a perceptual coupling of size and distance in conditions of degraded vision. In Experiment 2, we investigated size and implied distance perception of targets elevated above a visible horizon for individuals in an induced state of degraded vision. When participants' size judgments are inserted into the size-distance invariance hypothesis (SDIH) formula, distance to above-horizon objects increased compared to those below the horizon. Together, our results emphasize the importance of salient visible ground-contact information for accurate distance perception. The absence of this ground-contact information could be the source of perceptual errors leading to potential hazards for low vision individuals with severely degraded acuity and contrast sensitivity.

Perceptual Effects When Scaling Screen Size of Stereo 3D Presentations

Stereo three-dimensional (3D) content that is shot for the cinema may go on to be displayed on a number of different types of display devices, from domestic televisions to mobile devices. Stereo 3D content is scaled appropriately for the display device, but this scaling process alters the stereo 3D properties of the scene. For instance, when scaling cinema screen content to be shown on a domestic television screen, disparities are reduced, and the perceived depth of objects in the scene are affected in a nonlinear way. This paper explores the case of displaying stereo 3D cinema content on screens of a different size, both larger and smaller. It then explores in more detail the issues of showing cinema content on a domestic television, which include nonlinear scaling of depth. Experiments presented here show that the effects of this depth distortion are mitigated by two-dimensional perspective cues within the 3D scene, thus limiting the perceived distortion. The effect can be further mitigated by introducing a positive planar shift (of around 1%), which also improves comfort by keeping the depth range in the center of the comfort zone, reducing the vergence-accommodation conflict.

Depth in Box Spaces

Human observers adjust the frontal view of a wireframe box on a computer screen so as to look equally deep and wide, so that in the intended setting the box looks like a cube. Perspective cues are limited to the size-distance effect, since all angles are fixed. Both the size on the screen, and the viewing distance from the observer to the screen were varied. All observers prefer a template view of a cube over a veridical rendering, independent of picture size and viewing distance. If the rendering shows greater or lesser foreshortening than the template, the box appears like a long corridor or a shallow slab, that is, like a 'deformed' cube. Thus observers ignore 'veridicality'. This does not fit an 'inverse optics' model. We discuss a model of 'vision as optical user interface'.

The Role of Stereopsis, Motion Parallax, Perspective and Angle Polarity in Perceiving 3-D Shape

We studied how stimulus attributes (angle polarity and perspective) and data-driven signals (motion parallax and binocular disparity) affect recovery of 3-D shape. We used physical stimuli, which consisted of two congruent trapezoids forming a dihedral angle. To study the effects of the stimulus attributes, we used 2 × 2 combinations of convex/concave angles and proper/reverse perspective cues. To study the effects of binocular disparity and motion parallax, we used 2 × 2 combinations of monocular/binocular viewing with moving/stationary observers. The task was to report the depth of the right vertical edge relative to a fixation point positioned at a different depth. In Experiment 1 observers also had the option of reporting that the right vertical edge and fixation point were at the same depth. However, in Experiment 2, observers were only given two response options: is the right vertical edge in front of/behind the fixation point? We found that across all stimulus configurations, perspective is a stronger cue than angle polarity in recovering 3-D shape; we also confirm the bias to perceive convex compared to concave angles. In terms of data-driven signals, binocular disparity recovered 3-D shape better than motion parallax. Interestingly, motion parallax improved performance for monocular viewing but not for binocular viewing.

Automatic generation of 3D outdoor and indoor building scenes from a single image

In this paper, a novel approach for creating 3D models of building scenes is presented. The proposed method is fully automated and fast, and accurately reconstructs both outdoor images of a building and indoor scenes, with perspective cues in real-time, using only one image. It combines the extracted line segments to identify the vanishing points of the image, the orientation, the different planes that are depicted in the image and concludes whether the image depicts indoor or outdoor scenes. In addition, the proposed method efficiently eliminates the perspective distortion and produces an accurate 3D model of the scene without any intervention from the user. The main innovation of the method is that it uses only one image for the 3D reconstruction, while other state-of-the-art methods rely on the processing of multiple images. A website and a database of 100 images were created to prove the efficiency of the proposed method in terms of time needed for the 3D reconstruction, its automation and 3D model accuracy and can be used by anyone so as to easily produce user-generated 3D content: http://3d-test.iti.gr:8080/3d-test/3D_recon/

Emotional triangles: A test of emotion-based attentional capture by simple geometric shapes

Previous work has proposed that simple geometric shapes, carrying the features present within negative or threatening faces are especially effective at capturing or guiding attention. Here we test this account and provide converging evidence for a threat-based attentional advantage. Experiment 1 found that downward-pointing triangles continue to be detected more efficiently than upward-pointing triangles when: (i) both overall RT and search slope measures are obtained; and (ii) when the set size is varied and the stimuli are presented in random configurations. Experiment 2 tested and ruled out an alternative account of the selection advantage, based on differences between triangle shape consistencies with scene perspective cues. Overall, the data provide converging evidence that simple geometric shapes, which might be particularly important in providing emotional signals in faces, can also attract attention preferentially even when presented outside of a face context.

The Effect of Looming and Receding Sounds on the Perceived In-Depth Orientation of Depth-Ambiguous Biological Motion Figures

BackgroundThe focus in the research on biological motion perception traditionally has been restricted to the visual modality. Recent neurophysiological and behavioural evidence, however, supports the idea that actions are not represented merely visually but rather audiovisually. The goal of the present study was to test whether the perceived in-depth orientation of depth-ambiguous point-light walkers (plws) is affected by the presentation of looming or receding sounds synchronized with the footsteps.Methodology/Principal FindingsIn Experiment 1 orthographic frontal/back projections of plws were presented either without sound or with sounds of which the intensity level was rising (looming), falling (receding) or stationary. Despite instructions to ignore the sounds and to only report the visually perceived in-depth orientation, plws accompanied with looming sounds were more often judged to be facing the viewer whereas plws paired with receding sounds were more often judged to be facing away from the viewer. To test whether the effects observed in Experiment 1 act at a perceptual level rather than at the decisional level, in Experiment 2 observers perceptually compared orthographic plws without sound or paired with either looming or receding sounds to plws without sound but with perspective cues making them objectively either facing towards or facing away from the viewer. Judging whether either an orthographic plw or a plw with looming (receding) perspective cues is visually most looming becomes harder (easier) when the orthographic plw is paired with looming sounds.Conclusions/SignificanceThe present results suggest that looming and receding sounds alter the judgements of the in-depth orientation of depth-ambiguous point-light walkers. While looming sounds are demonstrated to act at a perceptual level and make plws look more looming, it remains a challenge for future research to clarify at what level in the processing hierarchy receding sounds affect how observers judge the in-depth perception of plws.

Stereoscopic perception of real depths at large distances

There has been no direct examination of stereoscopic depth perception at very large observation distances and depths. We measured perceptions of depth magnitude at distances where it is frequently reported without evidence that stereopsis is non-functional. We adapted methods pioneered at distances up to 9 m by R. S. Allison, B. J. Gillam, and E. Vecellio (2009) for use in a 381-m-long railway tunnel. Pairs of Light Emitting Diode (LED) targets were presented either in complete darkness or with the environment lit as far as the nearest LED (the observation distance). We found that binocular, but not monocular, estimates of the depth between pairs of LEDs increased with their physical depths up to the maximum depth separation tested (248 m). Binocular estimates of depth were much larger with a lit foreground than in darkness and increased as the observation distance increased from 20 to 40 m, indicating that binocular disparity can be scaled for much larger distances than previously realized. Since these observation distances were well beyond the range of vertical disparity and oculomotor cues, this scaling must rely on perspective cues. We also ran control experiments at smaller distances, which showed that estimates of depth and distance correlate poorly and that our metric estimation method gives similar results to a comparison method under the same conditions.

Susceptibility to the Shepard illusion in participants with autism: reduced top‐down influences within perception?

Previous research [Ropar & Mitchell, 2002] has shown that autistic individuals are somewhat immune to biases induced by top-down processes, particularly the influence of previous knowledge on perception. In order to test this hypothesis within perception, 18 participants with autism who had measured intelligence in the normal range were compared against 18 matched controls in their susceptibility to the Shepard illusion. The illusion consists in misperceiving the shape of a parallelogram in the presence of depth cues. It is attributed [Mitchell, Ropar, Ackroyd, & Rajendran, 2005] to the effect of top-down constraints within perception. The task involved adjusting a stimulus to the dimensions of a template on a computer screen. Both groups were susceptible to the illusion and the illusion effect was stronger when three-dimensional perspective cues were prominent. Notably, participants with autism were less susceptible to the illusion than typically developing individuals. The findings raise the possibility that in some instances top-down influences are attenuated in individuals with autism.

Cue weighting in stereoacuity with lines and closed forms

It might be supposed that depth perception provides an ideal situation in which to study cue weighting — the end result is usually an unequivocal indication of depth and there are cues based on horizontal disparity as well as cues that we shall simply describe as “perspective “ cues. The latter include the subtense at the eyes of vertically orientated lines of equal length but different distance, the angles that horizontally orientated elements of closed figures make with the vertically orientated elements in the retinal projections of the rectilinear figures we use, and the angle between the projections of horizontal elements of stimuli that do not lie in a fronto-parallel plane. One major difficulty is that these factors cannot readily be measured in isolation (although monocular viewing gives some hope of measuring “perspective” cues in the absence of horizontal disparity). A second problem is not knowing the extent to which the weights that might attach to each class of cue are fixed (in early experience, say, and influenced, perhaps, by the extent of each observer's phorias) or remain plastic and change with the task, the stimuli, and the experimental requirements. (We have some evidence that any “weight” is not completely plastic from the fact that observers' performance is drastically affected by making perspective cues irrelevant; irrelevant cues should be, but appear not to be, ignored in our 2-AF-C tasks with feedback.) A third problem is not knowing the extent to which the weights are optimal. We report an attempt to solve some of these issues.

Vertical and horizontal references determined by linear perspective and optic flow information

Judging the height and laterality of an object requires vertical and horizontal references that coincide respectively, with the judged eye level and midline of the body trunk. What visual cues determine these references? We previously found that the focus of expansion (FOE) of the optic flow determines judged eye level (Wu et al, VSS02). Here, we extended the study to the horizontal reference. In a virtual environment, observers judged the midline while viewing optical flow display with its FOE shifted horizontally (±10, ±20 and ±30deg). As predicted, horizontal midline was biased to the FOE [rate=0.20 (left); 0.29 (right)]. Arguably, besides showing the utility of the optic flow, our study also implies that other stable cues, eg linear perspective cue (Sedgwick, 1986), can help determine the references. To test this, observers judged their eye levels in the dark. In condition 1, two fluorescent lines (2.5m long) were placed on the right wall with their relative orientation varied to cause false linear perspective. Compared to baseline (parallel lines/true linear perspective), judged eye level was affected by false linear perspective. For example, with one line parallel to the floor, judged eye level changed with the orientation of the second line. If the two lines converged below the actual eye level, judged eye level shifted down. That the effect was mainly due to false linear perspective was shown by removing the line parallel to the floor; the orientation of the second line produced only about 1/9th of the effect. In condition 2, we placed a fluorescent line on each side of a wall (1.5m apart). While the pair of lines remained parallel at the same height, their orientation relative to the floor varied (0, 5, 10deg). Again, relative to baseline, judged eye level changed at a rate of 0.18 as the orientation of the lines varied, compared to a single tilted line (rate=0.06). These findings highlight the role of environmental constants in space vision.

Judging absolute distance by relying on linear perspective and texture density cues

Judging absolute distance by relying on linear perspective and texture density cues

Disparity scaling and correction for inclined surfaces

The horizontal disparity pattern associated with slant about a horizontal axis can be represented locally as a horizontal shear disparity. If the eyes are torsionally aligned, slant about a horizontal axis is given to close approximation by: S = tan-1 [ (1/m) tan(H) ] where H is the horizontal shear disparity and m is the eyes' vergence. Thus, to estimate slant veridically, the visual system must “scale” disparity by taking distance into account (m). The size disparity created by rotation about the vertical axis must be “corrected” for changes in azimuth (the eyes' version), but an analysis of binocular viewing geometry reveals that such correction is not required if shear disparity is measured in a particular way. We examined the estimation of slant about the horizontal axis from stereoscopic information alone when the distance and azimuth of the stimulus is varied. The stimuli were random-dot planes viewed in a custom haploscope. Perspective cues to slant were minimized by use of a sparse texture, back-projection, and randomly oriented clipping windows. Observers were asked to adjust the slant of a test plane until it matched the perceived slant of a standard plane. In the first experiment, we presented the test and standard planes at different distances (but the same azimuth). We found that observers took distance into account nearly veridically in this task. Thus, disparity scaling occurs with reasonably high accuracy. In the second experiment, we examined whether the vertical disparity gradient or eye-position signals is the predominant signal in disparity scaling. The former predominated. In the third experiment, we presented the test and standard planes at different azimuths (but the same vergence). We found that observers were able to estimate slant nearly veridically across azimuths as expected from use of horizontal shear disparity.

Determining the point of subjective ambiguity of ambiguous biological-motion figures with perspective cues

Orthographic frontal/back projections of biological-motion figures are bistable: The point-light figure in principle can be perceived either as facing toward the viewer or as facing away from the viewer. Some point-light actions--for example, walking--elicit a strong "facing bias": Despite the absence of objective cues to depth, observers tend to interpret the figure as facing toward the viewer in most of the cases. In this article, we present and experimentally validate a technique that affords full experimental control of the perceived in-depth orientation of point-light figures. We demonstrate that by parametrically manipulating the amount of perspective information in the stimulus, it is possible to obtain any desired level of subjective ambiguity. Directions for future research, in which this technique can be fruitfully implemented, are suggested. Program code of a demo is provided that can be modified easily for program code of new experiments. The demo and QuickTime movie files illustrating our perspective manipulation technique may be downloaded from http://brm.psychonomic-journals.org/content/supplemental.

Perspective-Based Illusory Movement in a Flat Billboard—An Explanation

We describe a compelling motion illusion elicited by a huge billboard placed along a street, depicting a building that contains strong perspective cues. When observers move fast along the opposite sidewalk, they perceive the depicted building as rotating in their direction of travel. This is a special case of the 'following', or 'pointing out of the picture', illusion that elicits a strong illusory motion percept. Here we discuss the cause of the illusory motion and suggest that the brain relies on the depicted perspective cues to infer a 3-D shape and a concomitant motion that is incompatible with the physical pictorial surface.

Do bantams (Gallus gallus domesticus) experience amodal completion? An analysis of visual search performance.

Whereas many mammals (some primates and mice) experience amodal completion, previous data split for avian species. However, experimental procedures have been quite different among the species, and thus a direct comparison of various avian species in the same experimental situation is needed. We tested whether bantams (Gallus gallus domesticus) would complete partly occluded figures using a visual search task on the touch monitor, which was successfully used in our previous study with pigeons. First, we trained 3 participants to search for a notched red diamond (a target) among complete diamonds (distracters). Next, white squares accompanied each figure with a small gap of a fixed size. In test, the location of the accompanying white squares sometimes changed. In some trials, the white squares exactly covered, or "occluded," the notched portion of the target. Humans are known to have great difficulty in finding such targets due to "automatic" completion of the notched part. However, bantams met no such difficulty at all. This result and the demonstration by Forkman (1998) of hens' amodal completion of figures placed on a perspective background, suggest that the perspective cue may have an important role in amodal completion in this species.

Orientation Disparity: A Cue for 3D Orientation?

Orientation disparity, the difference in orientation that results when a texture element on a slanted surface is projected to the two eyes, has been proposed as a binocular cue for 3D orientation. Since orientation disparity is confounded with position disparity, neither behavioral nor neurophysiological experiments have successfully isolated its contribution to slant estimates or established whether the visual system uses it. Using a modified disparity energy model, we simulated a population of binocular visual cortical neurons tuned to orientation disparity and measured the amount of Fisher information contained in the activity patterns. We evaluated the potential contribution of orientation disparity to 3D orientation estimation and delimited the stimulus conditions under which it is a reliable cue. Our results suggest that orientation disparity is an efficient source of information about 3D orientation and that it is plausible that the visual system could have mechanisms that are sensitive to it. Although orientation disparity is neither necessary nor sufficient for estimating slant, it appears that it could be useful when combined with estimates from position disparity gradients and monocular perspective cues.

Perceptual Incongruence Influences Bistability and Cortical Activation

We employed a parametric psychophysical design in combination with functional imaging to examine the influence of metric changes in perceptual incongruence on perceptual alternation rates and cortical responses. Subjects viewed a bistable stimulus defined by incongruent depth cues; bistability resulted from incongruence between binocular disparity and monocular perspective cues that specify different slants (slant rivalry). Psychophysical results revealed that perceptual alternation rates were positively correlated with the degree of perceived incongruence. Functional imaging revealed systematic increases in activity that paralleled the psychophysical results within anterior intraparietal sulcus, prior to the onset of perceptual alternations. We suggest that this cortical activity predicts the frequency of subsequent alternations, implying a putative causal role for these areas in initiating bistable perception. In contrast, areas implicated in form and depth processing (LOC and V3A) were sensitive to the degree of slant, but failed to show increases in activity when these cues were in conflict.