Cite as:
Tsirlin-Zaharescu I, Wilcox L M, Allison R S, 2006, "Perceptual asymmetry in stereoscopic transparency" Perception 35 ECVP Abstract Supplement

Perceptual asymmetry in stereoscopic transparency

I Tsirlin-Zaharescu, L M Wilcox, R S Allison

Surface segregation in random-dot stereograms depicting overlapping surfaces depends on the relative density of elements in the two planes. The percept of transparency appears stronger when more elements are placed in the foreground than when they are located in the background. To quantify this perceptual asymmetry, we measured the minimum disparity required for a coherent percept of transparency between two overlaid fields of sparse texture elements using a method of adjustment. While maintaining a fixed total element density, we varied the distribution of the elements between the front and back surfaces from 10:90 to 90:10. A significantly larger separation in disparity-defined depth was required to perceive transparency when the density in the background surface was larger than in the foreground surface. This dependence on the distribution of element density accounts for results that Gepshtein and Cooperman (1998 Vision Research 38 2913 - 2932) attributed to crossed vs uncrossed disparities. We are conducting further experiments to test the hypothesis that our results reflect an asymmetry in the degree of inhibition between the two surfaces caused by assignment of blank regions to the background plane.
[Supported by CRESTech and Premier's Research Excellence Award from the Province of Ontario.]

The influence of perceived surface slant of an ambiguous stimulus on vergenceD A Wismeijer, R van Ee, C J Erkelens (Helmholtz Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands; e-mail: d.a.wismeijer@phys.uu.nl)

We investigated the influence of perceived surface slant on vergence accompanying a saccadic movement. Using an ambiguous bistable slant rivalry stimulus, the subjects experienced one of two possible surface slants with opposite sign; one based on perspective foreshortening and the other one based on binocular disparity. The perspective cue specified a surface slant of ±70°, while the slant specified by the disparity cue was (oppositely) \mp50°°. A key aspect for our study is that the perceived sign of the surface slant is under voluntary control of the observer. Subjects were instructed to hold the slant percept specified by the perspective foreshortening. After a 4 s period of strict central fixation, subjects were instructed to make a saccade to the indicated edge of the surface at 10° of version. Subjects were able to comply with the instruction to hold the perspective-specified slant. Although the vergence at the landing position closely approximated the depth specified by the disparity cue, we conclude that, under the current conditions, there is no convincing evidence that voluntarily controlled perceived surface slant influences vergence.

An indispensable factor for 3-D illusory surface perception in binocular viewingH Zhang, M Idesawa (Graduate School of Information Systems, University of Electro-Commu-nications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan; e-mail: hzhang@hi.is.uec.ac.jp)

We designed stereograms composed of binocularly paired random dots and binocularly unpaired random dots which were immediately flanking each other; then we observed them for three cases: including both the paired and the unpaired dots, including only the paired dots, and including only the unpaired dots. We found that illusory surfaces could be perceived in the first case, even though subjective contours could not be seen from each eye's view; in contrast they could not be observed in the second case, although the subjective contours could be seen. These perceptions demonstrate that the binocularly unpaired dots, but not the monocular subjective contours, are essential for perceiving illusory surfaces: which is consistent with Mather and Idesawa (1989 Vision Research 29 143 - 146; 1991 Japanese Journal of Applied Physics 30 L751 - L754). In addition, we found that the flanking position of the unpaired dots could change opaque and transparent properties of the illusory surfaces. In the third case, binocular rivalry occurred. Surprisingly, the unstable unpaired dots became quite stable when a suitable illusory surface was superimposed on them. We conclude that binocularly unpaired stimuli are indispensable for the formation of illusory surfaces and the visual system produces illusory surfaces in order to reduce binocular instability.

Observers perceptual states modulate ERP components differently in orientation and colour rivalryS Veser, U Roeber, E Schröger (Institute of Psychology I, Leipzig University, Seeburgstrasse 14 - 20, D 04103 Leipzig, Germany; e-mail: veser@uni-leipzig.de)

When dissimilar stimuli are presented to the two eyes of an observer, these stimuli alternate. This phenomenon is known as binocular rivalry. In this study, orientation rivalry and colour rivalry were induced with isoluminant red/black and green/black gratings slanted to the left and right. We investigated electrophysiological responses following transitions from rivalling stimulation to non-rivalling stimulation. Depending on the prevailing percept reported by the observer, this transition could concern the eye which the stimulus was dominating (incompatible change) or suppressed (compatible change). For orientation rivalry, ERP differences between incompatible and compatible changes were found in P1, where percept-incompatible changes elicit a larger P1 peak than percept-compatible changes, whereas colour rivalry induced a percept-dependent modulation in the later ERP component, N1. Here, also percept-incompatible changes elicit larger amplitude than compatible changes. These results show that orientation rivalry yields earlier ERP differences than colour rivalry. Thus the solution of binocular rivalry depends on the properties of the stimulus as assumed in hierarchical models.

Fröhlich effect occurs beyond the level of binocular integrationA Najafian¶, M Sanayei¶§, B Noudoost§ (¶ Neuroscience Research Group, Isfahan University of Medical Sciences, Isfahan, Iran; § School of Cognitive Sciences, Institute for Studies in Theoretical Physics and Mathematics [IPM], Tehran, Iran; e-mail: najafian@med.mui.ac.ir)

The initial position of a moving object is perceived ahead in its trajectory, which is known as Fröhlich effect (FE). FE is absent for a single rotating dot. Adding another similar rotating dot on the opposite side of its circular path restores the illusion. We used a dichoptic display to present stimuli to each eye separately and compared the determined FE for a single rotating dot and two rotating dots in different conditions. When a single dot was presented to both eyes simultaneously or to one eye, FE was absent. When two rotating dots were presented to both eyes simultaneously and FE was 11.24°±0.76° (mean±SE). In another condition, we presented one rotating dot to the right eye and another dot to the left eye. Subjects now perceived two rotating dots although single dots were represented in the left and right early visual pathway. Measured FE in this condition was 13.47°\ pm0.73°, similar to previous condition (Mann - Whitney, p>0.05). Our results demonstrated that segregation of visual signals in the form of moving dots in the early visual pathway did not reduce FE. We conclude that activation of neurons responding to the information from both eyes is sufficient for FE to occur.

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