Foreshortening and the perception of parallel projections

Does picture perception follow polar projective geometry? Parallel projection drawings, which are not produced by using rules of polar projection, are widely regarded as visually acceptable representations of three-dimensional (3-D) objects in free viewing. One explanation is that they are perceived by means of a system in which there is no foreshortening. If so, edges of a 3-D block in 1∶1 proportions should be denoted by lines in 1∶1 proportions on the picture surface. However, three experiments suggest that the perception of parallel projections of a block involves foreshortening. In Experiment 1, 90 subjects were shown a set of parallel projections of a cube, in which each drawing depicted three sides of the cube, drawn as a square with obliques—a frontal square with receding edges shown by parallel obliques of various lengths. The subjects preferred a drawing with a receding side length that was considerably foreshortened in relation to the front side. In Experiments 2 and 3, subjects viewed drawings of three blocks that differed in the ratios of the lengths of their receding edges to their frontal edges (1∶1, 1∶2, and 1∶0.65). In Experiment 2, the subjects were shown square-with-obliques drawings of the three blocks with receding edges shown by parallel obliques of various lengths. Again, the subjects preferred drawings with a receding side that was foreshortened. In Experiment 3, the drawings showed two sides of a block. The receding dimension was drawn with parallel or converging lines. The preferred foreshortening was not a fixed ratio of the dimensions of the 3-D blocks. We suggest that square-with-obliques parallel projections showing cubes are taken by vision to be approximations to projections using foreshortening. We suggest also that as the line showing the receding edge elongates, foreshortening becomes less of a factor.     

Constant errors donot occur in the matched reproduction of angles

MacRae and Loh (1981) reported that when observers attempt to match angles, whether simultaneously or successively, and regardless of whether the angles have the same orientation or not, acute angles generally are set too large and obtuse angles, too small. Without presenting data, they argued indirectly that starting-position effects in their adjustment procedure could not have accounted for the errors. In four experiments, we have demonstrated that effects similar to MacRae and Loh’s do occur when their starting positions are used and that no such effects occur when psychophysical methods that minimize or eradicate starting position effects are used.     

Effects of spelling errors on the perception of writers

The authors investigated the perceived relationship between spelling errors and cognitive abilities in a series of 3 experiments. Specifically, they examined whether college students' ratings of an author's intellectual ability, logical ability, and writing ability were affected by the presence of spelling errors. In the 1st experiment, the presence of 4 spelling errors in a short essay did not significantly affect the ratings. The spelling ability of college students, as measured by a standard oral dictation spelling test, was moderately conelated with a brief test of intelligence. In a 2nd experiment, college students rated the author of a short essay as having lower ability when there was a large number of spelling errors. The effect was more pronounced on the ratings of writing ability than it was on the ratings of logical ability or intellectual ability. This finding was replicated in a 3rd experiment, in which the essay contained misspellings actually made by writers. The results suggest that spelling errors can affect how people perceive writers, particularly when there are many spelling errors. College students appear to attribute spelling errors more to writing ability than they do to general cognitive abilities such as intelligence and logical ability.     

The perception of an optical flow projected on the ground surface

In most experiments in which the importance of visual control on postural stability is studied, optical stimuli attached to vertical surfaces are used. Analyses of long-term standing readjustments generally involve back-and-forth movements of a visual scene or its projection on vertical circular screens. In a natural environment, however, visual information is largely available from the ground. The aim of the experiment reported was to assess the effect of a flow pattern simulating an open outdoor setting on motion perception and postural control. Subjects were presented with an optical texture projected onto the ground. Periods of motionless texture alternated with equivalent durations of unidirectional flows. The change of position of the subject's centre of gravity over time was recorded on a force platform. Results show that the direction of body sway corresponded with that of texture motion. Important aftereffects, as shown in linear vection experiments, were also observed. However, the latency of postural responses was much shorter than with prolonged unidirectional flows produced in other locations of the visual environment. The hypothesis of an ecological specificity of the flows perceived on the ground during terrestrial displacements is discussed.     

The homogeneity effect on figure/ground perception in infancy

We examined whether the homogeneity of the two profiles of Rubin's goblet affects figure/ground perception in infants. We modified the two profiles of Rubin's goblet in order to compare figure/ground perception under four test patterns: (1) two profiles painted with horizontal lines (horizontal-line condition), (2) two profiles painted middle gray (uni-color condition), (3) one profile painted light gray and the other dark gray (two-color condition), and (4) a goblet painted with concentric circles (concentric-circles condition). In the horizontal-line condition the homogeneity of the profile was strengthened, and in the two-color condition the homogeneity of the profile was weakened compared to the uni-color condition, which was an original Rubin's goblet. In the concentric-circles condition the homogeneity of the reversed areas of the horizontal-line were strengthened.     

The ground dominance effect in the perception of 3-D layout

The relative effectiveness of the ground surface and other environmental surfaces (the ceiling and sidewalls) in determining perceived layout was investigated in five experiments and a real-world demonstration. In the first three experiments, two vertical or horizontal posts were positioned between two surfaces (ground and ceiling in all three experiments, left wall and right wall in Experiment 1), and optical contact was manipulated so that the two surfaces provided contradictory information about the relative distances of the posts. Observers judged which of the two posts appeared to be closer. In Experiment 4, to control the height on the posts at which the distance judgments were made, a blue dot was attached to both vertical posts at varying heights and observers judged which dot appeared closer. In Experiment 5, the posts were replaced by two gray ellipses to eliminate the effects of the regular shape and texture. Our findings were that (1) among all four surfaces tested, observers showed a preference to respond according to the optical contact information provided by the ground surface--a ground dominance effect, (2) this effect did not depend on the height of the posts in the image, (3) as the scene was tilted away from a ground/ceiling orientation, the ground dominance effect decreased, and (4) this effect was not due to the location of the judgment.     

Absolute distance perception to locations off the ground plane

Using vision, humans can accurately judge distances to locations on the ground surface up to distances of at least 20 m. Most theories of depth perception assume that this ability is associated with the fact that we live in a terrestrial world in which locations of interest often appear on the ground and for which feedback about distance is often available from nonvisual sources such as walking. Much less is known about the ability of humans to judge absolute distances to locations other than on or supported by the ground plane beyond a few meters, at which point binocular stereo provides at best limited information about distance scaling. We show that one commonly used action measure for probing absolute distance perception exhibits accurate performance, even for targets located on the ceiling of a large room. We follow this with evidence that distance to ceiling locations is recovered with a mechanism that depends, at least in part, on the angle from the line of sight to the target location and a gravity-based frame of reference.     

Constant errors occur in matched reproduction of angles even when likely biases are eliminated

The literature on the reproduction of visually perceived angles often seems to contain an explicit or implicit assumption that systematic errors are caused by misperception of the sizes of the angles. We argue that in the absence of procedural artifacts no consistent error should arise from purely perceptual causes. Accordingly, an experimental procedure was designed to eliminate or evaluate a number of artifacts that probably affected earlier experiments. A computer-controlled display generated both a standard angle and a variable one that could be adjusted by a subject to match the standard. The data analysis followed Cox’s (1970) procedure for the analysis of binary data to focus on the numbers of positive and negative errors rather than on their magnitudes and so eliminate the numerically distorting effects of asymmetrical error distributions. Nevertheless, the results showed what has usually been reported before: acute angles were matched with too large a comparison angle, and obtuse ones with too small an angle. Thus, there is a real effect to be explained, although the explanation must invoke response processes.     

The role of shape recognition in figure/ground perception in infancy

In this study we sought to determine whether infants, like adults, utilize previous experience to guide figure/ground processing. After familiarization to a shape, 5-month-olds preferentially attended to the side of an ambiguous figure/ground test stimulus corresponding to that shape, suggesting that they were viewing that portion as the figure. Infants’ failure to exhibit this preference in a control condition in which both sides of the test stimulus were displayed as figures indicated that the results in the experimental condition were not due to a preference between two figure shapes. These findings demonstrate for the first time that figure/ground processing in infancy is sensitive to top-down influence. Thus, a critical aspect of figure/ground processing is functional early in life.     

The ground dominance effect in the perception of relative distance in 3-D scenes is mainly due to characteristics of the ground surface

Previously, we (Bian, Braunstein, and& Andersen, 2005) reported a dominance effect of the ground plane over other environmental surfaces in determining the perceived relative distance of objects in 3-D scenes. In the present study, we conducted three experiments to investigate whether this ground dominance is due to inherent differences between ground and ceiling surfaces, or to the locations of these surfaces in the visual field. In Experiment 1, two vertical posts were positioned between a ground surface and a ceiling surface, and optical contact was manipulated so that the two surfaces provided contradictory information about the relative distances of the posts from the participant. The two surfaces were either both above, both below, or one above and one below fixation. In Experiment 2, only one surface was presented, either above, below, or at fixation. In Experiment 3, the posts were replaced by two red dots, and the eccentricity of the optical contact on the two surfaces was equated in each of five locations in the visual field. In all three experiments, participants judged which of the two objects appeared to be closer. Overall, we found a higher proportion of judgments consistent with a ground surface than with a ceiling surface in all locations, indicating that the ground dominance effect is mainly due to characteristics of the ground surface, with location in the visual field having only a minor effect.     

The Ouchi illusion: An anomaly in the perception of rigid motion for limited spatial frequencies and angles

The spatial parameters underlying a novel illusion of relative motion are characterized. A simple stimulus composed of two sine-wave gratings was sufficient to generate the illusion. We measured the response of subjects to rapid, small-amplitude oscillations of this stimulus behind a fixation point. The effect was clearly strongest for acute angles between the gratings, but only when spatial frequency was between 6 and 11 cpd. We surmise that activity in the grating cells of the primate visual cortex (von der Heydt, Peterhans, & Dursteler, 1992) might be the cause of the illusion. The illusion is potentially an important tool in understanding how higher cortical areas combine disparate motion signals.     

Misremembering pictured objects: people of all ages demonstrate the boundary extension illusion

In the boundary extension illusion, subjects recollect more of a photographed scene than was originally shown. In this study, first- and fifth-grade children, young adult college students, and older adults studied 4 one-object or 4 two-object picture stimuli for 15 s each. Immediately after each visual scene was shown, the subjects drew it from memory inside a rectangle that was the same size as the previous picture. This study demonstrated that all age groups, from young children to older adults, were susceptible to the boundary extension illusion. This finding is discussed in terms of Intraub's perceptual schema hypothesis and Johnson's source-monitoring hypothesis.