Percept-percept couplings.
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Depth Perception↗
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The grass is always greener: an ecological analysis of an old aphorism.
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Aphorisms and Proverbs as Topic↗
Measuring attention using induced motion.
Attention was measured by means of its effect upon induced motion. Perceived horizontal motion was induced in a vertically moving test spot by the physical horizontal motion of inducing objects. All stimuli were in a frontoparallel plane. The induced motion vectored with the physical motion to produce a clockwise or counterclockwise tilt in the apparent path of motion of the test spot. Either a single inducing object or two inducing objects moving in opposite directions were used. Twelve observers were instructed to attend to or to ignore the single inducing object while fixating the test object and, when the two opposing inducing objects were present, to attend to one inducing object while ignoring the other. Tracking of the test spot was visually monitored. The tilt of the path of apparent motion of the test spot was measured by tactile adjustment of a comparison rod. It was found that the measured tilt was substantially larger when the single inducing object was attended rather than ignored. For the two inducing objects, attending to one while ignoring the other clearly increased the effectiveness of the attended inducing object. The results are analyzed in terms of the distinction between voluntary and involuntary attention. The advantages of measuring attention by its effect on induced motion as compared with the use of a precueing procedure, and a hypothesis regarding the role of attention in modifying perceived spatial characteristics are discussed.
Adult↗
Wallpaper illusion: cause of disorientation and falls on escalators.
The wallpaper illusion, first described over a century ago, can occur when a person with normal binocular vision views a pattern that is periodic in the horizontal meridian of the visual field. Escalator trends present such a pattern. Evidence is presented favoring the view that disorientation experienced by escalator riders is caused by this illusion. Possibly some of the estimated 60,000 escalator falls occurring in the United States each year are linked to it.
Accidental Falls↗
The ceiling of the Church of St Ignatius and the perception of concave surfaces.
This research describes two hitherto unobserved phenomena in the frescoes of the seventeenth century architect and painter, Andrea Pozzo, painted on the vaulted ceiling of the central nave of the Church of St Ignatius in Rome. The present research also reports the results of two experimental studies on the problem of the perception of shapes projected on concave surfaces. A quantitative evaluation of the phenomena perceived from various points of observation is made by means of stimuli projected at various angles on a semicylindrical surface. The validity of the assumption of invariance, and in particular of the projective invariant called the cross-ratio, is discussed within the framework of ecological theories on perception.
Architecture↗
Sighting-down and the Poggendorff illusion: a mystery within a mystery.
As reported before, sighting-down one of the diagonal lines on a typical Poggendorff pattern will reduce the illusion even if that situation is only pictorial. But here it is also argued that sighting-down itself requires further understanding.
Depth Perception↗
Grouping based on phenomenal similarity of achromatic color.
It is widely acknowledged that a precondition for the perception of the world of objects and events is an early process of organization, and it has generally been assumed that such organization is based on the Gestalt laws of grouping. However, the stage at which such grouping occurs, whether early or late, is an empirical question. It is demonstrated in two experiments that grouping by similarity of neutral color is based not on similarity of absolute luminance at the level of the proximal stimulus, but on phenomenal similarity of lightness resulting from the achievement of lightness constancy. An alternative explanation of such grouping based on the equivalence of luminance ratios between elements and background is ruled out by appropriate control conditions.
Adult↗
Apparent relative size and depth of moving objects.
The effects of the relative velocities of moving objects on their apparent relative size and depth were investigated with the aid of square patterns generated on a CRT display by a microcomputer. The observer matched the apparent sizes of squares, arranged in two or more rows that moved with different velocities, and made judgments of the apparent relative depth of the rows. In many conditions, squares moving more slowly were perceived as larger in size than those moving faster, regardless of the kind of depth responses, but in some conditions which contained strong depth cues the size responses seemed to be affected by perceived depth. The size-change effect of moving objects is discussed from the viewpoint of size-depth relationship.
Acceleration↗
New depth to the Müller-Lyer illusion.
It is demonstrated that when the two halves of the Müller-Lyer illusion are shown separately to the two eyes this can result in a perception of the binocularly viewed shaft as slanted in depth. For short fin lengths, the magnitude of this slant can be predicted by the extent of the ordinary two-dimensional illusion. This result is discussed in relation to Julesz's claim that stereoscopic matching of the images in the two eyes must precede whatever process is responsible for the Müller-Lyer illusion (whereas the reported illusion seems to imply the reverse). The cyclopean Müller-Lyer demonstration on which Julesz's argument is based is reexamined. It is suggested that the matching of coarse-scale features in the images in the two eyes may help to explain both the cyclopean and the reported three-dimensional Müller-Lyer illusion.
Depth Perception↗
Stereoscopic slant reversals: a new kind of 'induced' effect.
Data are presented from three experiments confirming an earlier finding that the stereoscopic slant perceived may be opposite to the geometrically predicted direction of slant (Gillam 1967). The stimulus for stereoscopic slant was created by imposing a disparity gradient on a frontal plane surface. Reversals are shown to occur readily for slants around a vertical axis but rarely for slant around a horizontal axis. Reversal frequency is greater for surfaces which have a regular pattern, providing good perspective information about slant. Cue conflict cannot explain reversals because adherence to perspective information predicts a perception of zero slant rather than reverse slant. A new explanation has been proposed attributing reversals to the ambiguity of horizontal disparity gradients and disambiguation of the disparity gradient by its relationship to the perspective gradient. It is shown that for any given disparity gradient there is a physical surface which would give rise to a slant reversed with respect to that normally predicted. Such a surface is eccentric in the field of view, with eccentricity given by the difference between the slants signalled by the disparity gradient and the perspective gradient. This explains why reversal responses to disparity gradients occur in the presence of perspective. It is proposed, on the basis of this analysis and the fact that reversals occur, that, like convergence and vertical disparity, perspective is a factor contributing to the correct scaling of disparity gradients in the horizontal meridian with respect to surface eccentricity.
Attention↗
The natural moon illusion: a multifactor angular account.
It is argued that the failure to explain the celestial illusion results from conceptual confusion about perceived size and from disregard of the observational evidence relating to the natural moon illusion. The evidence shows that the illusion consists of a perceived angular size enlargement of horizon objects, by a factor of about 1.5-2.0 in diameter in comparison with elevated objects. Most measurements of the illusion have been made in terms of angular size, although in some proposed explanations an illusion of linear size is assumed. The magnitude of the illusion varies, particularly with the detail of the horizon scene. The illusion can be explained as the sum of several factors that affect perceived angular size: size contrast, vergence commands and eye or head position, aerial perspective, and colour. The relative contributions of these factors are assessed.
Color Perception↗
Spatial-gradient limit on perception of multiple motion.
Motion is perceived whenever a subject is presented with an appropriate spatiotemporal visual pattern. Like many other visual tasks, motion perception involves both local and global processing, and thus might be subject to the well-known paradox that arises from the fact that local features and observations form the basis for global perception, but sometimes this global percept can not be easily derived from any single local observation, as is best exemplified by the aperture problem. Globally, dual (transparent) motion can be readily perceived. Spatial limits on the local ability to perceive multiple motion are sought. By using the framework of apparent motion, it is found that dual, orthogonally oriented motion can be perceived only when the dots that constitute the two motions are separated by some spatial limit. For short-range apparent motion, the limit is found to be comparable to D(max), and the visual system cannot perceive more than a single coherent motion in a local "patch" of radius D(max). It was also found that this spatial limit on local-motion perception is not constant, but depends linearly on the spatial organisation of the stimuli, and vanishes for stimuli having reverse contrast. The lower bound on the ability to perceive multiple motion is compared with some well-known bounds in stereopsis, and a cortical columnar architecture that might account for it is proposed.
Adult↗
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.
Computer Graphics↗
The time course of abstract visual representation.
Studies in change blindness re-enforce the suggestion that veridical, pictorial representations that survive multiple relocations of gaze are unlikely to be generated in the visual system. However, more abstract information may well be extracted and represented by the visual system. In this paper we study the types of information that are retained and the time courses over which these representations are constructed when participants view complex natural scenes. We find that such information is retained and that the resultant abstract representations encode a range of information. Different types of information are extracted and represented over different time courses. After several seconds of viewing natural scenes, our visual system is able to construct a complex information-rich representation.
Adolescent↗
The importance of a "cliff" in depth avoidance by human infants.
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Avoidance Learning↗
A developmental study of three-dimensional perception in Israeli children.
One hundred thirty-six 5- to 10-year-old Israeli male and female children were given three black and white photographs of a highway, a column of identical tanks, and a row of elephants, and were asked some questions on each one of the photographs in order to elicit responses of three-dimensional perception. The results showed a clear developmental pattern, quite similar to what has been found in other studies. It appears that the procedure used in the present study is an easier and probably more reliable way of assessing perspective responses than the original procedure of Piaget and Inhelder.
Child↗
Telescopic lenses and driving.
In some states, persons with significantly reduced visual acuity are being licensed to drive while wearing telescopic spectacle lenses (TSL). In order to evaluate possible visual field limitations present with these devices, the peripheral visual fields of a group of normally sighted subjects were measured while they wore TSL. Severely restricted central fields and sizeable ring scotomas were present with all units tested. These result indicate that driving with TSL should be discouraged.
Accidents, Traffic↗
Horopter shifts due to a magnification change.
Both the equidirection ( nonius ) and the equidistance horopter criteria were used on a group of subjects with and without the application of a 5% overall magnifier before the right eye. For three of six subjects, shifts in the equidirection horopter induced by this afocal lens could not be predicted on the basis of the magnification properties of the lens and the assumption that corresponding points are fixed. Equidistance horopter plots showed reasonably systematic reductions in slope between the predicted and the measured values for all subjects. Irregularities in all data were accounted for by shifts in the vergence angles during the experiment.
Aniseikonia↗