Oculomotor adjustments and size-distance perception

The relationship between perceived size and distance and oculomotor adjustments were assessed in two experiments. In both experiments, Ss were required to make scalar linear size, angular size, and distance judgments of stimuli subtending a constant retinal image size at different levels of convergence. The results of the first experiment indicate that the perceived linear size, angular size, and distance of the stimulus decreased with increased convergence, the decrease in perceived linear size being greater than that of perceived angular size. While again showing a decrease in perceived linear and angular size, the results of the second experiment also show that there was a smaller decrease in perceived distance with increased convergence when Ss continued to view the stimulus as convergence was changed than when they did not view the stimulus as convergence was changed. The implications these results have for size and distance perception are discussed.     

Seeing double and depth with Wheatstone's stereograms

Charles Wheatstone, in his classic paper on the invention of the stereoscope, concluded "... objects whose pictures do not fall on corresponding points of the two retinae may still appear single" (1838 Philosophical Transactions of the Royal Society of London 128 384). Soon after, Ernst Brücke, Alexandre Prévost, David Brewster, Joseph Towne, and Joseph LeConte all published disagreements with this conclusion. LeConte's objections were most frequent and most prolonged. To understand the basis of the disagreements, we conducted three experiments using Wheatstone's original stereograms and found that most stereograms produced depth perception with diplopia, which partially explains the consistency among his critics' conclusions. Most of the criticism at variance with Wheatstone's conclusion was based on research conducted outside Germany. We argue that LeConte's lack of knowledge of the German literature on vision research prevented him from considering investigating Wheatstone's experiment with a stereogram having a smaller disparity.     

An empirical test of formal equivalence between Emmert's law and the size-distance invariance hypothesis

Emmert's law and the size-distance invariance hypothesis have been said to be formally equivalent, provided that Emmert's law means that the perceived size of an afterimage is proportional to the perceived distance of the projected surface of the afterimage. However, there have been very few studies that have attempted to verify this formal equivalence empirically. We measured both the perceived size and distance of afterimages and real objects with the same proximal size. Nineteen participants projected afterimages of 1 deg in visual angle on the wall located at distances of 1 to 23 meters from the participants. They also observed real objects, disc-shaped and made from a sheet of Styrofoam board, with the same proximal size as that of the afterimages, which were located at the same physical distances as those of the wall on which the afterimages were projected. Each participant reproduced the apparent sizes of the afterimages and real objects using the reproduction method and estimated the apparent distances using the magnitude estimation method. When the mean apparent sizes of the afterimages and real objects, represented as a function of apparent distance, were fitted to a linear function, the slopes for the afterimages and real objects did not differ significantly. These results are interpreted as evidence for the formal equivalence of Emmert's law and the size-distance invariance hypothesis.     

Stereoscopic matching and the aperture problem

In order to perceive stereoscopic depth, the visual system must define binocular disparities. Consider an oblique line seen through an aperture formed by flanking occluders. Because the line is perceived behind the aperture, the line must have disparity relative to the aperture. What is the assigned disparity of the line in this aperture problem? To answer this question five observers adjusted the horizontal disparity of a probe until it was perceived at the same depth as the disparate line behind the aperture. The results show that, when both the horizontal and the vertical disparities of the occluders are well-defined, the probe must have the same horizontal disparity as the horizontal separation between the line half-images. However, when the horizontal and vertical disparities of the occluders are ill-defined, the intersections of the line and the occluder borders can determine the matching direction. In the latter case, the matching direction varies with the aperture orientation and there is considerable variability across observers.     

Hysteresis,cooperativity, and depth averaging in dynamic random-dot stereograms

Experiments were performed to assess the response of the human visual system to dynamic random-dot patterns composed of disparity mixtures. In Experiment 1, the perceived depth and relative stability of two patterns were compared; one pattern depicted two transparent layers of dots, and the other depicted a volume of dots. Two effects were found: (1) the volume pattern exhibited a large degree of disparity averaging; and (2) asymmetries were observed in the relative stability of these two patterns. Experiment 2 was designed to determine whether these findings could be attributed to spatially localized processes occurring-at the location of disparity discontinuities. This was accomplished by introducing unpaired noise points localized either along the disparity discontinuities or in the center of the layered and volume patterns. The amount of depth averaging and the direction of the asymmetry did not appear to depend on processes localized along the disparity discontinuities. Results of these experiments, taken in conjunction with those of previous studies, suggest that hysteresis is independent of cooperative persistence mechanisms.     

Stereoscopic depth and the occlusion illusion

In the occlusion illusion, the visible portion of a partly occluded object appears larger than a physically identical nonoccluded region. Stereoscopic displays allowed for a direct test of the apparent-distance hypothesis. In Experiments 1A and 1B, we measured both the perceived size and the perceived depth of partly occluded targets when the binocular disparity of both targets and occluders was varied. Stereoscopic occlusion greatly increased perceived target size but not perceived target distance. A reduced illusion was still present when the target was stereoscopically in front of the abutting rectangle, however. Experiments 2A and 2B showed similar results, even when the occluding figures were illusory rectangles that formed no explicit T-junctions. Experiment 3 showed that an unexpected negative size illusion on control trials was primarily due to adaptation to the occlusion illusion on other trials. The present findings eliminate apparent-distance explanations of the occlusion illusion but are consistent with other hypotheses, such as partial modal completion and selective dimensional expansion.     

Hysteresis, cooperativity, and depth averaging in dynamic random-dot stereograms.

Experiments were performed to assess the response of the human visual system to dynamic random-dot patterns composed of disparity mixtures. In Experiment 1, the perceived depth and relative stability of two patterns were compared; one pattern depicted two transparent layers of dots, and the other depicted a volume of dots. Two effects were found: (1) the volume pattern exhibited a large degree of disparity averaging; and (2) asymmetries were observed in the relative stability of these two patterns. Experiment 2 was designed to determine whether these findings could be attributed to spatially localized processes occurring at the location of disparity discontinuities. This was accomplished by introducing unpaired noise points localized either along the disparity discontinuities or in the center of the layered and volume patterns. The amount of depth averaging and the direction of the asymmetry did not appear to depend on processes localized along the disparity discontinuities. Results of these experiments, taken in conjunction with those of previous studies, suggest that hysteresis is independent of cooperative persistence mechanisms.     

Random-dot stereograms of real objects: observation on stereo faces and moulds

A simple technique for producing random-dot stereograms of real objects is described. It is then shown that stereograms of inside-out faces (moulds) cease to be perceptually reversible only when the presentation is truly cyclopean. Further possible applications of the technique are outlined.     

Assessing anti-fat bias in preschoolers: a comparison of a computer generated line-drawn figure array and photographic figure array

This study examined differences in preschoolers' ratings of anti-fat bias and identification of current body size depending on the realism of the figure array used: computer generated line-drawn or photographic. Children reported strong anti-fat bias with both arrays. However, less extreme bias was elicited with the photographic figure array. In addition, children were inaccurate reporters of their current body size when both figure arrays were used. However, children were consistent in their selection of figures that were thinner than their actual body size. Children's consistent selection of thinner figures as representative of their current body size may be an additional indicator of anti-fat bias. Overall, these results suggest that some of the anti-fat bias observed in preschoolers is attributable to less realistic figure arrays. Therefore, photographic figure arrays are proposed as a better measurement tool in the assessment of anti-fat bias in preschoolers.     

Stereoscopic matching, eye position, and absolute depth

The viewing system parameters (in particular the mutual orientation of the coordinate frames fixed respectively to the left and right projection surface) probably have to be explicitly incorporated into solutions to the stereoscopic-matching problem in binocular vision. An algorithm for computing the relative orientation of the two directions of gaze from purely visual information without a prior solution to the correspondence problem is outlined. The availability of an algorithm of this sort seems to be a precondition for any stereoscopic-matching process that uses the epipolar constraint. It is further argued that fixation of objects in the near space requires integration of the results of stereoscopic matching over eye movements, and that this requirement puts rather tight restrictions on the possible form of representing the results of stereoscopic processing.     

Cleomedes (c. 1st century AD) on the celestial illusion, atmospheric enlargement, and size-distance invariance

Cleomedes (Kleomedes) is a little-known Greek author (c. 1st century AD) who produced what is probably the earliest extant statement of size-distance invariance. He supported the Stoic philosophy and was concerned to discredit the Epicurean position that we perceive objects as having their true size. He explained the celestial illusion (the apparent enlargement of the sun near the horizon) in two ways: partly as a refractive effect of the atmosphere similar to angular enlargement when looking into water; and partly as a linear enlargement due to increased apparent distance in a misty atmosphere. He is the earliest extant author to offer apparent distance as a clear explanation of the celestial illusion. He attributed these views to Posidonius (c. 135-50 BC). His explanations remained at the geometrical level, and he did not speculate on sensory mechanisms.     

Longitudinal transmission of conflict resolution styles from marital relationships to adolescent-parent relationships.

This study longitudinally investigated transmission: Can the way adolescents resolve conflicts with their parents be explained by the way parents resolve conflicts with each other? Questionnaires about conflict resolution styles were completed by 282 young adolescents (mean age = 13.2) and their parents. Path analyses with cross-lagged effects indicated that transmission of conflict resolution styles from marital relationships to adolescent-parent relationships occurs: Conflict engagement and positive problem solving in marital relationships were significantly related to, respectively, conflict engagement and positive problem solving in adolescent-parent relationships 2 years later. No significant longitudinal effects emerged with regard to withdrawal. Thus, the study shows that the way marital conflicts are handled affects how adolescents deal with conflicts.