Surface perception in pictures

Subjects adjusted a local gauge figure such as to perceptually “fit” the apparent surfaces of objects depicted in photographs. We obtained a few hundred data points per session, covering the picture according to a uniform lattice. Settings were repeated 3 times for each of 3 subjects. Almost all of the variability resided in the slant; the relative spread in the slant was about 25% (Weber fraction). The tilt was reproduced with a typical spread of about 10?. The rank correlation of the slant settings of different observers was high, thus the slant settings of different subjects were monotonically related. The variability could be predicted from the scatter in repeated settings by the individual observers. Although repeated settings by a single observer agreed within 5%, observers did not agree on the value of the slant, even on the average. Scaling factors of a doubling in the depth dimension were encountered between different subjects. The data conformed quite well to some hypothetical fiducial global surface, the orientation of which was “probed” by the subject’s local settings. The variability was completely accounted for by singleobserver scatter. These conclusions are based upon an analysis of the internal structure of the local settings. We did not address the problem of veridicality, that is, conformity to some “real object.”     

Aging and the perception of slant from optical texture, motion parallax, and binocular disparity

The ability of younger and older observers to perceive surface slant was investigated in four experiments. The surfaces possessed slants of 20°, 35°, 50°, and 65°, relative to the frontoparallel plane. The observers judged the slants using either a palm board (Experiments 1, 3, and 4) or magnitude estimation (Experiment 2). In Experiments 1–3, physically slanted surfaces were used (the surfaces possessed marble, granite, pebble, and circle textures), whereas computer-generated 3-D surfaces (defined by motion parallax and binocular disparity) were utilized in Experiment 4. The results showed that the younger and older observers' performance was essentially identical with regard to accuracy. The younger and older age groups, however, differed in terms of precision in Experiments 1 and 2: The judgments of the older observers were more variable across repeated trials. When taken as a whole, the results demonstrate that older observers (at least through the age of 83 years) can effectively extract information about slant in depth from optical patterns containing texture, motion parallax, or binocular disparity.     

The visual contour in depth

We asked subjects to match points on the surface of a smooth three-dimensional (3-D) shape with points on the surface of another object that was geometrically identical to the first object but was placed in a different pose, was differently textured, and was differently shaded. In all cases, the fiducial point was on the rim of one of the objects (i.e., the boundary of the visible region of the surface), whereas the matching point was well within the silhouette of the other object. This allowed us to draw (preliminary) conclusions concerning the way monocular human observers are able to handle the neighborhood of the rim, where the local slant assumes arbitrarily high values. All experiments were done in real space with real objects (no computer-simulated scenes), the points being indicated with laser beam illumination. The subject was given control over the direction of the laser beams and was thus able to perform the task by adjustment from the vantage position. We studied both consistency (whether the subject’s judgments were invariant against changes of relative pose) and veridicality (whether the depth of the visual contour as calculated from the settings agreed with the true distance as measured by mechanical means). Subjects caught much of the 3-D structure of the contour but did deviate appreciably and apparently idiosyncratically from the true geometry.     

The stereoscopic anisotropy: individual differences and underlying mechanisms

Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms.     

Sex and age modulate the visual perception of distance

An experiment was conducted to evaluate the ability of 28 younger and older adults to visually bisect distances in depth both indoors and outdoors; half of the observers were male and half were female. Observers viewed 15-m and 30-m distance extents in four different environmental settings (two outdoor grassy fields and an indoor hallway and atrium) and were required to adjust the position of a marker to place it at the midpoint of each stimulus distance interval. Overall, the observers’ judgments were more accurate indoors than outdoors. In outdoor environments, many individual observers exhibited perceptual compression of farther distances (e.g., these observers placed the marker closer than the actual physical midpoints of the stimulus distance intervals). There were significant modulatory effects of both age and sex upon the accuracy and precision of the observers’ judgments. The judgments of the male observers were more accurate than those of the female observers and they were less influenced by environmental context. In addition, the accuracies of the younger observers’ judgments were less influenced by context than those of the older observers. With regard to the precision of the observers’ judgments, the older females exhibited much more variability across repeated judgments than the other groups of observers (younger males, younger females, and older males). The results of our study demonstrate that age and sex are important variables that significantly affect the visual perception of distance.     

The impact of visual exploration of judgments of whether a gap is crossable

When observers look down into a gap in the ground plane, their judgments of the widest gap they can step across (gapmax) decrease as gap depth increases (Y. Jiang & L. S. Mark, 1994). This study investigated the possibility that Jiang and Mark's viewing conditions did not afford observers a sufficient opportunity to perform exploratory movements needed to detect information about gap width. Experiment 1 showed that the gap depth by gaze interaction disappeared only when restrictions were not imposed on observers' exploratory activities (eye, head, and body movements). Experiment 2 showed that observers tended to see the vertical surface as slanted away from them, which made the bottom of the surface appear farther away from them than the top. Only when observers were able to view the gap binocularly under conditions that did not restrict exploratory activity did their slant perception improve and their gapmax judgments no longer covary with gap depth. The data indicate that the exploratory movements of prospective actors are essential for the pickup of information about their action capabilities.     

Shape-slant relations under reduction conditions

Two experiments are reported which attempted to test implications of the shape-slant invariance hypothesis. Both experiments employed an apparatus in which variations in the slant and in the width settings of a rectangle were highly ambiguous and subject to instructional sets. In the first experiment, the stimulus was varied in both its width and its slant to achieve matches to four standard angles; the resultant width settings were found to be close to the projected widths of the obtained angles of slant. In the second experiment, width and slant were varied to match four standard widths; the projected widths of the obtained angles of slant did not differ from the obtained width settings.     

Correspondence in pictorial space

ATR Human Information Processing Research Laboratories, Kyoto, Japan We have investigated psychophysically determined image correspondences between pairs of photographs of a single three-dimensional (3-D) object in various poses. These correspondences were obtained by presenting the pictures simultaneously, side by side, and letting the subject match a marker in one picture with a marker (under manual control) in the other picture. Between poses, the object was rotated about a fixed vertical axis; thus, the shifts of the veridical correspondences (with respect to the surface of the object) were very nearly horizontal. In fact, the subjects produced appreciable scatter in both horizontal and vertical directions. The scatter in repeated sessions and between data depends on the local (landmarks) and global (interpolation) structure of the pictures. Since the object was fairly smooth (white semigloss finish) and nontextured, the only way to establish the correspondence is by way of the “pictorial relief.” The relief is some largely unknown function of the image structure and the observer. Apparently, more immediate entities (e.g., the shading or the contour) cannot be used as such, since they vary with the pose. We compare these data with results obtained with a surface attitude probe on a single picture. We studied various measures of consistency both within a single method and between methods. We found that subjects were confident in establishing correspondences, but results scattered appreciably in a way that depended on both global and local image structure. Correspondence results for various pose angles were mutually very consistent, but only to a minor extent with results of attitude measurements. The main finding was that subjects could establish correspondence on the basis of their 3-D interpretation (pictorial relief), even if the 2-D graytone distributions are quite different.     

Local and global visual mechanisms underlying individual differences in the rod-and-frame illusion

The role of local and global visual mechanisms in individual differences in the rod-and-frame (RF) effect was investigated. Field-dependent and field-independent observers, selected on the basis of Witkin and Asch’s (1948) classical procedure, were submitted to the small RF test (Coren & Hoy, 1986). Four frame tilts and two gap sizes were used. As expected, direct effects (i.e., rod settings in the direction of frame tilt) were observed at small degrees of frame tilt, while indirect effects (i.e., rod settings in the direction opposite that of frame tilt) were observed at larger frame tilts. Fielddependent observers showed larger direct effects in the case of the small gap. Indirect effects were comparable in both field-dependent and field-independent subjects, regardless of gap size. Following the model proposed by Wenderoth and Johnstone (1987), these findings indicate that low-level visual mechanisms, responsible for local orientation interactions, have a different gain in fielddependent and field-independent individuals. In contrast, global visual mechanisms, presumably acting     

Cross-modal slant and curvature matching of stereo- and motion-induced surfaces

In many laboratory setups and in many day-to-day situations, a unique solution of the structure-fromtwo-views problem is unobtainable. Yet, when the visual system is presented with two projections in a sequence, it nevertheless appears to generate a reasonably stable percept of structure. In the research reported here, we examined whether the same surface would be perceived when subjects were presented with a pair of views that alternated in time monocularly (two-frame motion) or were shown simultaneously to both eyes (stereo). In Experiment 1, we studiedslant perception: human observers were asked to match the slant of a motion-induced planar surface with its stereo-induced counterpart. In Experiment 2, the perceivedcurvature of parabolic surfaces was matched in a similar way. The results show that motion-induced slant is matched with ahigher value of the stereo-induced slant. However, the curvature experiment showed that motion-induced curvature is matched with alower stereo-induced curvature. One possible explanation may be that the slant and curvature are internally inconsistent in at least one of the modalities.     

Exocentric pointing in three-dimensional space

A study is reported of an exocentric pointing task in all three dimensions, in near space, with only two visible luminous objects--a pointer and a target. The task of the subject was to aim a pointer at a target. The results clearly show that visual space is not isotropic, since every set direction appeared to consist of two independent components--one in the projection onto a frontoparallel plane (tilt), the other in depth (slant). The tilt component shows a general trend across subjects, an oblique effect, and can be judged monocularly. The slant component is symmetrical in the mid-sagittal plane, requires the use of binocular information, and shows considerable differences between subjects. These differences seem to depend on the amount of binocular information used by each subject. There was a remarkably high level of consistency in the exocentric pointing, despite the absence of environmental cues. The within-subject consistency in the settings of the pointer corresponds to a consistency of about 1 min of arc in disparity of its tip, even though the pointer and target are separated by more than 5 deg.     

Escher in color space: individual-differences multidimensional scaling of color dissimilarities collected with a gestalt formation task.

The structure of color perception can be examined by collecting judgments about color dissimilarities. In the procedure used here, stimuli are presented three at a time on a computer monitor and the spontaneous grouping of most-similar stimuli into gestalts provides the dissimilarity comparisons. Analysis with multidimensional scaling allows such judgments to be pooled from a number of observers without obscuring the variations among them. The anomalous perceptions of color-deficient observers produce comparisons that are represented well by a geometric model of compressed individual color spaces, with different forms of deficiency distinguished by different directions of compression. The geometrical model is also capable of accommodating the normal spectrum of variation, so that there is greater variation in compression parameters between tests on normal subjects than in those between repeated tests on individual subjects. The method is sufficiently sensitive and the variations sufficiently large that they are not obscured by the use of a range of monitors, even under somewhat loosely controlled conditions.     

Escher in color space: Individual-differences multidimensional scaling of color dissimilarities collected with a gestalt formation task

The structure of color perception can be examined by collectingjudgments about color dissimilarities. In the procedure used here, stimuli are presented three at a time on a computer monitor and the spontaneous grouping of most-similar stimuli into gestalts provides the dissimilarity comparisons. Analysis with multidimensional scaling allows such judgments to be pooled from a number of observers without obscuring the variations among them. The anomalous perceptions of color-deficient observers produce comparisons that are represented well by a geometric model of compressed individual color spaces, with different forms of deficiency distinguished by different directions of compression. The geometrical model is also capable of accommodating the normal spectrum of variation, so that there is greater variation in compression parameters between tests on normal subjects than in those between repeated tests on individual subjects. The method is sufficiently sensitive and the variations sufficiently large that they are not obscured by the use of a range of monitors, even under somewhat loosely controlled conditions.     

Effects of context on a visual 3-D pointing task

We examined the effects of egocentric and contextual references on a 3-D exocentric pointing task. Large systematic deviations were found for the slant (angle in the horizontal plane). For most observers, the deviations were smaller when the veridical pointing direction was parallel to a wall. For some observers the size of the deviations was also dependent on whether the veridical pointing direction was frontoparallel or not. For the tilt (angle in the vertical plane), the deviations were smaller and less systematic. Hence, although observers show comparable systematic deviations, the way in which the presence of structure in an environment is used for judging positions of objects is observer-dependent.     

Interindividual receptor variability of normal colour observers: analysis of the 2-deg Stiles and Burch data

Examination of the interobserver variability among the ten observers of the 2-deg Stiles and Burch colorimetric study reveals an interesting pattern: the chromaticities of the matches made by different normal subjects fall approximately on straight lines and these lines appear to converge. Because of a geometric relationship that holds between the normal, anomalous, and dichromatic colour spaces, it can be argued that part of the variability must be due to actual differences in the shape or spectral position of the receptor sensitivities of different normal observers.     

Searching for two things at once: Establishment of multiple attentional control settings on a trial-by-trial basis

Recent work has demonstrated that attention can be configured to multiple potential targets in spatial search. However, this previous work relied on a fixed set of targets across multiple trials, allowing observers to offload attentional control settings to longer-term representations. In the present experiments, we demonstrate multiple attentional control settings that operate independently of space (Experiments 1 and 2). More important, we show that observers can be cued to different control settings on a trial-by-trial basis (Experiment 3). The latter result suggests that observers were capable of maintaining multiple control settings when the demands of the task required an attentional search for specific feature values. Attention can be configured to extract multiple feature values in a goal-directed manner, and this configuration can be can be dynamically engaged on a trial-by-trial basis. These results support recent findings that reveal the high precision, complexity, and flexibility of attentional control settings.     

Perception of local orientation from shaded images

The perception of local orientation from shaded images was examined. In Experiment 1, subjects viewed a boundaryless Gaussian hill and judged local orientation using both a gauge figure and a pointing method. One subject reported an internally consistent surface which was incompatible with the judged light-source direction and model used to generate the image. The remaining subjects reported a surface similar to the generating one, and analysis of their results indicated a contour of zero difference between response and generating slants. This contour of zero slant difference was explored in three subsequent experiments using the pointing technique. These experiments investigated possible influences of luminance artifact (Experiment 2), perception of global orientation (Experiment 3), and self-occluding contours (Experiment 4). All three of these experiments yielded results similar to those of Experiment 1, with distinct contours of zero slant difference. This contour was explored for relationships with the simulated slant of the generating surface and the differential structure of image intensity. This analysis indicated that the contour of zero slant difference was approximately a line of constant slant which shared large regions of adjacency to the zero crossings of the second directional derivative of image intensity.     

Socially learned habituation to human observers in wild chimpanzees

Habituation to human observers is an essential tool in animal behaviour research. Habituation occurs when repeated and inconsequential exposure to a human observer gradually reduces an animal’s natural aversive response. Despite the importance of habituation, little is known about the psychological mechanisms facilitating it in wild animals. Although animal learning theory offers some account, the patterns are more complex in natural than in laboratory settings, especially in large social groups in which individual experiences vary and individuals influence each other. Here, we investigate the role of social learning during the habituation process of a wild chimpanzee group, the Waibira community of Budongo Forest, Uganda. Through post hoc hypothesis testing, we found that the immigration of two well-habituated, young females from the neighbouring Sonso community had a significant effect on the behaviour of non-habituated Waibira individuals towards human observers, suggesting that habituation is partially acquired via social learning.     

Foreshortened tiles in paths converging on an observer viewing a picture: Elevation and visual angle ratio determine perceived size

Observers were shown wide-angle pictures of tiles on a ground plane and were asked about the aspect ratios of the tiles. The observers viewed the pictures from a fixed center of projection. Some of the tiles were in a path coming straight toward the observer. In one picture, the path came from the center of the picture, and in two others the path came from the left side of the picture (one from 30° and one from 45° to the left of the center, from the observer’s point of view). The apparent aspect ratios were a function of the elevations of the tiles and the ratios of visual angles of the sides of the tiles. Judgments were identical for all three paths. The local slant of the picture surface was not a significant factor.