Propagation of depth information from local regions in 3-D scenes

The effects of regions with local linear perspective on judgments of the depth separation between two objects in a scene were investigated for scenes consisting of a ground plane, a quadrilateral region, and two poles separated in depth. The poles were either inside or outside the region. Two types of displays were used: motion-parallax dot displays, and a still photograph of a real scene on which computer-generated regions and objects were superimposed. Judged depth separations were greater for regions with greater linear perspective, both for objects inside and outside the region. In most cases, the effect of the region's shape was reduced for objects outside the region. Some systematic differences were found between the two types of displays. For example, adding a region with any shape increased judged depth in motion-parallax displays, but only high-perspective regions increased judged depth in real-scene displays. We conclude that depth information present in local regions affects perceived depth within the region, and that these effects propagate, to a lesser degree, outside the region.     

Is Complex Visual Information Implicated During Language Comprehension? The Case of Cast Shadows

Previous research showed that sensorimotor information affects the perception of properties associated with implied perceptual context during language comprehension. Three experiments addressed a novel question of whether perceptual context may contribute to a simulation of information about such out-of-sight objects as cast shadows. In Experiment 1, participants read a sentence that implied a particular shadow cast on a target (blinds vs. an open window) and then verified the picture of the object onto which a shadow was cast. Responses were faster when the shadow of blinds cast on the object matched that implied by the sentence. However, the data did not show the same matching effect for pictures with cast shadows from an open window. In Experiments 2 and 3, we found that verification times for pictures with no cast shadows were faster when preceded by an “open window” sentence, thus suggesting that reading the sentence does not elicit a visual simulation of any specific shadow. Experiment 3 showed that the objects superimposed with a cast shadow of the blinds and blinds themselves were verified faster after reading a “blinds” sentence. However, the results of an order analysis showed the temporal stability of the “blinds shadows” effect, but the disappearance of the “blinds” effect in the second half of the data. We conclude that the results are compatible, to a lesser or greater extent, with multiple accounts, and discuss our findings in the context of a mental imagery view, a mental simulation view, and an amodal representation view.     

The perception of 3-D shape from shadows cast onto curved surfaces

In a natural environment, cast shadows abound. Objects cast shadows both upon themselves and upon background surfaces. Previous research on the perception of 3-D shape from cast shadows has only examined the informativeness of shadows cast upon flat background surfaces. In outdoor environments, however, background surfaces often possess significant curvature (large rocks, trees, hills, etc.), and this background curvature distorts the shape of cast shadows. The purpose of this study was to determine the extent to which observers can “discount” the distorting effects of curved background surfaces. In our experiments, observers viewed deforming or static shadows of naturally shaped objects, which were cast upon flat and curved background surfaces. The results showed that the discrimination of 3-D object shape from cast shadows was generally invariant over the distortions produced by hemispherical background surfaces. The observers often had difficulty, however, in identifying the shadows cast onto saddle-shaped background surfaces. The variations in curvature which occur in different directions on saddle-shaped background surfaces cause shadow distortions that lead to difficulties in object recognition and discrimination.     

Perception of the motion trajectory of objects from moving cast shadows in infant Japanese macaques (Macaca fuscata)

The shadows cast by moving objects enable human adults and infants to infer the motion trajectories of objects. Nonhuman animals must also be able to discriminate between objects and their shadows and infer the spatial layout of objects from cast shadows. However, the evolutionary and comparative developmental origins of sensitivity to cast shadows have not been investigated. In this study, we used a familiarity/novelty preferential looking procedure to assess the ability of infant macaques, aged 7–24 weeks, to discriminate between a ‘depth’ display containing a ball and cast shadow moving diagonally and an ‘up’ display containing a ball with a diagonal trajectory and a shadow with a horizontal trajectory. The infant macaques could discriminate the trajectories of the balls based on the moving shadows. These findings suggest that the ability to perceive the motion trajectory of an object from the moving shadow is common to both humans and macaques.     

Differential effects of cast shadows on perception and action

In two experiments we investigated the effects of cast shadows on different real-life tasks. In experiment 1, participants were required to make a speeded verbal identification of the target object (perceptual task), whereas in experiment 2 participants were required to reach for and grasp the target object (motor task). In both experiments real three-dimensional (3-D) objects were presented, one at a time, either with their own natural cast shadow (congruent condition) or with the cast shadow of a different object (incongruent condition). Shadows were cast either to the left or to the right of the object. We asked whether the features of the shadow (ie whether it is congruent or incongruent with the object, and whether it is cast to the left or to the right of the object) could influence perception and action differently. Results showed that cast shadows did not influence identification of real 3-D objects (experiment 1), but they affected movement kinematics, producing distractor-like interference, particularly on movement trajectory (experiment 2). These findings suggest a task-dependent influence of cast shadows on human performance. In the case of object-oriented actions, cast shadows may represent further affordances of the object, and as such compete for the control of the action.     

Infants' perception of depth from cast shadows

Five- and 7-month-old infants viewed displays in which cast shadows provided information that two objects were at different distances. The 7-month-olds reached preferentially for the apparently nearer object under monocular-viewing conditions but exhibited no reaching preference under binocular-viewing conditions. These results indicate that 7-month-old infants perceive depth on the basis of cast shadows. The 5-month-olds did not reach preferentially for the apparently nearer object and, therefore, exhibited no evidence of sensitivity to cast shadows as depth information. In a second experiment, 5-month-olds reached preferentially for the nearer of two objects that were similar to those used in the first experiment but were positioned at different distances from the infant. This result indicated that 5-month-olds have the motor skills and motivation necessary to exhibit a reaching preference under the conditions of this study. The results are consistent with the hypothesis that depth perception based on cast shadows first appears between 5 and 7 months of age.     

My shadow,myself: Cast-body shadows are embodied

Objects that serve as extensions of the body can produce a sensation of embodiment, feeling as if they are a part of us. We investigated the characteristics that drive an object’s embodiment, examining whether cast-body shadows, a purely visual stimulus, are embodied. Tools are represented as an extension of the body when they enable observers to interact with distant targets, perceptually distorting space. We examined whether perceptual distortion would also result from exposure to cast-body shadows in two separate distance estimation perceptual matching tasks. If observers represent cast-body shadows as extensions of their bodies, then when these shadows extend toward a target, it should appear closer than when no shadow is present (Experiment 1). This effect should not occur when a non-cast-body shadow is cast toward a target (Experiment 2). We found perceptual distortions in both cast-body shadow and tool-use conditions, but not in our non-cast-body shadow condition. These results suggest that, although cast-body shadows do not enable interaction with objects or provide direct tactile feedback, observers nonetheless represent their shadows as if they were a part of them.     

Infants’ perception of depth from cast shadows

Five- and 7-month-old infants viewed displays in which cast shadows provided information that two objects were at different distances. The 7-month-olds reached preferentially for the apparently nearer object under monocular-viewing conditions but exhibited no reaching preference under binocularviewing conditions. These results indicate that 7-month-old infants perceive depth on the basis of cast shadows. The 5-month-olds did not reach preferentially for the apparently nearer object and, therefore, exhibited no evidence of sensitivity to cast shadows as depth information. In a second experiment, 5-month-olds reached preferentially for the nearer of two objects that were similar to those used in the first experiment but were positioned at different distances from the infant. This result indicated that 5-month-olds have the motor skills and motivation necessary to exhibit a reaching preference under the conditions of this study. The results are consistent with the hypothesis that depth perception based on cast shadows first appears between 5 and 7 months of age.     

Cast shadow can modulate the judged final position of a moving target

Observers tend to localize the final position of a suddenly vanished moving target farther along in the direction of the target motion (representational momentum). We report here that such localization errors are mediated by perceived motion rather than by retinal motion. By manipulating the cast shadow of a moving target, we induced illusory motion to a target stimulus while keeping the retinal motion constant. Participants indicated the vanishing point of the target by directing a mouse cursor. The resulting magnitude of localization errors was modulated on the basis of the induced direction of the target. Such systematic localization biases were not obtained in a control condition in which the motion paths of the ball and shadow were switched. Our results suggest that cues to object motion trajectory, such as cast shadows, are used for the localization task, supporting a view that a predictive mechanism is responsible for the production of localization errors.     

Spatial judgments in the horizontal and vertical planes from different vantage points

Todorovi? (2008 Perception 37 106-125) reported that there are systematic errors in the perception of 3-D space when viewing 2-D linear perspective drawings depending on the observer's vantage point. Because these findings were restricted to the horizontal plane, the current study was designed to determine the nature of these errors in the vertical plane. Participants viewed an image containing multiple colonnades aligned on parallel converging lines receding to a vanishing point. They were asked to judge where, in the physical room, the next column should be placed. The results support Todorovi? in that systematic deviations in the spatial judgments depended on vantage point for both the horizontal and vertical planes. However, there are also marked differences between the two planes. While judgments in both planes failed to compensate adequately for the vantage-point shift, the vertical plane induced greater distortions of the stimulus image itself within each vantage point.     

Why cast shadows are expendable: insensitivity of human observers and the inherent ambiguity of cast shadows in pictorial art

The kinds of visual cues artists choose to use or not use in their work can offer insight into perceptual processes. On the basis of the observed paucity of the use of cast shadow in pictorial art, we hypothesized that cast shadows might be relatively expendable as pictorial cues. In this study, we investigated two potential reasons for this expendability: first, viewers might be insensitive to much of the information that cast shadows provide; and, second, ambiguities about what is shadow and what is pigment can often be resolved only through motion-something that static media are ill-equipped to deal with. In experiment 1, we used a visual-search paradigm in which viewers had to determine if there were odd cast shadows in sets of 4, 8, 16, and 32 objects. In experiment 2, viewers had to discriminate between shadow/pigment ambiguities in both still and moving images. Our results demonstrate that viewers are neither particularly sensitive to static cast-shadow incongruities, nor are they able to disambiguate cast shadow from pigment without continuous motion information. Taken together, these results may help explain why cast shadows are relatively rare in static pictorial work.     

The copycat solution to the shadow correspondence problem

Cast shadows (henceforth simply 'shadows') depicted in artworks can fulfil some of their perceptual roles, such as helping the retrieval of 3-D structure and of relative positions of objects, without having to be geometrically or physically accurate. The visual system displays a wide tolerance for shadows that are mostly inaccurate, at times making use of the scarce accurate but relevant information that can still be retrieved from them. However, the extent of the tolerable inaccuracy is at present still unexplored. I propose that inaccuracy can be not only totally acceptable but in some cases is even likely to be preferable to accuracy if the main perceptual role of shadows is to help locate in a scene the objects that cast them. I examined a small but effective historical corpus of pictorial endeavours, from which it appears that in some cases painters have used a copycat strategy for drawing the terminator of a shadow, ie they have produced a replica of the visible profile of the caster, which in the norm yields an impossible shadow. The copycat strategy is perceptually effective for solving the correspondence problem of associating a shadow with its caster; copycatting can be more effective than other solutions, including the depiction of the geometrically correct shadow, and is complementary to coarser solutions to the correspondence problem. These phenomena provide insight into the computations effectively used by the brain to deal with space perception. In particular, the brain is not relying on some sort of simplified physical model of the world, for the shadows produced by the copycat effect would correspond to a more complicated physical situation than the one encountered in real life.     

The development of sensitivity to cast and attached shadows in pictures as information for the direction of the source of illumination

The development of the perception of cast and attached shadows as information in pictures for the direction of the light source was studied with children in kindergarten, third and sixth grades, and adults in college. Subjects viewed photographs of objects under four different positions of illumination, with either cast or attached shadows alone, or with both present. Error in angular displacement from the correct position of the light source was 76.5° for kindergarten children, 40.5° for those in the third grade, 34.9° for those in the sixth, and 18.0° for college students. Neither position of light nor type of object affected performance. Results are discussed in the context of a developmental hypothesis of changing interpretation of the relations between pictorial and nonpictorial space from childhood to adolescence.     

Picture perception is array-specific: viewing angle versus apparent orientation

The amount that a pictured lattice appears to be swung from its frame changes with subjects' viewpoint, in contrast with previous experiments using insufficient range of rotation. Also, this effect is shown to be independent of the physical slant of the picture surface, though not necessarily of border-shape information. Analysis of depicted headings which rotate with viewpoint, with respect to the picture plane, although the angle between these headings in virtual space hardly changes, relates these data to specific geometric properties of the perspective array as seen from the different viewpoints. Data show differential rotation of differently oriented features within a single depicted object, in a way that is dependent on task instructions. It is proposed that in picture perception, contradictory cues are resolved by combination.     

Mach bands change asymmetrically during solar eclipses

Observations made during two partial eclipses of the Sun show that the Mach bands on shadows cast by the Sun disappear and reappear asymmetrically as an eclipse progresses. These changes can be explained as due to changes in the shape of the penumbras of shadows as the visible portion of the Sun forms crescents of different orientation.     

Size From Shadow: Some Informational Paths Less Traveled

This article deals with informational structure in cast shadows. Most research on cast shadows has focused on retrieving various geometric properties of the scene from shadow patterns, the shape of objects and distances between objects being the foremost features. Size appears to be a rather neglected aspect. However, shadows contain a wealth of information about size. I explore first a simple heuristic (approximate equivalence of size of shadow and size of object) and some complicating factors: elongated shadows, shadows from pinholes, and penumbra. Finally, I discuss the case of relative size of objects in multiobject scenes with shadows.     

Shadows as sources of cues for distance of shadow-casting objects

Shadows are neglected sources of information about shadow-casting objects' distance (location). In an experiment using real shadows, participants judged the distance of two rods, either with shadows (illumination from the left) or without shadows. Without shadows, a thin rod raised slightly above the surface was incorrectly judged to be more distant than a thick rod, but with shadows the thin rod was accurately judged to be closer. For judgments to be correct, shadows had to overwhelm competing height and stimulus-size cues. Exp. II involved nonoverlapping shadows produced by a light placed in front of the rods. Again, without shadows the thin-elevated rod was incorrectly judged to be more distant, but with shadows it was correctly judged to be closer. Exp. III involved two sets of thick and thin elevated rods and frontal lighting. Judgments were accurate when shadows were present but inaccurate when shadows were absent. Experiments conducted through the American Psychological Society Web-research site replicated results of Exp. I, II, and III. A final Web experiment replicated results of the others, but with shadows produced by illumination from the right. Three properties of shadows--angles, interposition, and positioning--are possible sources of distance information associated with cast shadows.     

Development of sensitivity to static pictorial depth information

Sensitivity to the pictorial depth cues of shading, linear perspective, and position on the picture plane were investigated with children from 3 to 7 years of age as well as with adults. Using a discriminative learning method, all Ss were found to be sensitive to shading information for depth when the display was oriented vertically, but not when it was oriented horizontally. In addition, binocular view did not decrease sensitivity relative to monocular view. Linear perspective was found to be effective in controlling 3-year-olds’ size discriminations of equal-area figures, while position on the picture plane was ineffective in the absence of other information for depth.     

Haptic perception of spatial relations

There are some indications that haptic space like visual space is not Euclidean (e.g. Blumenfeld, 1937 Acta Psychologica 2 125-174). In a series of experiments, we investigated the haptic perception of spatial relations in a systematic way. We restricted ourselves to a horizontal plane at waist height. Blindfolded subjects were asked to perform three tasks with their right hand: (i) a reference bar was presented under four different orientations and subjects were asked to rotate a test bar such that it felt to be parallel to the reference bar; (ii) subjects had to rotate two test bars in such a way that they felt collinear; (iii) subjects had to point a test bar in the direction of a marker. Bars and marker could appear at nine different locations. In all experiments large systematic deviations (up to 40 degrees) were made. The deviations strongly correlated with horizontal (right-left) but not with vertical (forward-backward) distance. Subjects showed qualitatively identical trends but the size of the deviations was strongly subject-dependent. In addition, a significant haptic oblique effect was found. These results provide strong evidence that haptic space in non-Euclidean.     

Development of ability to perceive and produce pictorial depth cue of overlapping.

The perception of the pictorial depth cue of overlapping was studied in children 3, 5, and 7 yr. old. Both a sequential and a simultaneous picture/object-matching task were used to test sensitivity. All age groups successfully perceived the depth relation information provided by pictorial overlapping. Height on the picture plane, which projectively covaries with overlapping, was not consistently used as a depth cue by any age group. Children's drawings were also analyzed for the presence of distance information. The drawings of the 3- and 5-yr. old children contained no overlapping cues and indicated a general lack of understanding of the third demension behind the picture plane. Seven-yr.-old children showed the beginnings of this understanding through their use of size perspective and height on the picture plane as depth cues. For all ages the production of the overlapping cue lags behind its perception.