<title>Correct 3-D but illusory 2-D percepts linked to binocularly unpaired regions</title>

Misconvergence to the stimulus plane causes apparent displacement of the stimulus elements seen monocularly1

“…Our initial interest in the illusion was the “novelty” of a stimulus appearing displaced toward the occluder: a result that contrasts recent discussions of Leonardo's constraint in which apparent displacement is away from the occluder (e.g., Ohtsuka & Ono, 2002; Ono et al., 2003). However, van Ee et al.…”

“…See Figure 2b,c for examples of no displacement and displacement away from the occluder, respectively.) The illusory displacements of monocularly seen stimuli that have been reported previously, however, are always away from an occluder (e.g., van Ee, Banks, & Backus, 1999;Ohtsuka & Ono, 2002;Ono, Lillakas, Grove, & Suzuki, 2003). For instance, van Ee et al (1999) reported that the monocularly seen segment of a vertical line (a line monocularly occluded in the center by a rectangle), appeared displaced away from the occluder.…”

Leonardo's constraint: Two opaque objects cannot be seen in the same direction.

“…Specifically, given the displacements reported in the present study when an occluder is fixated, one would expect that (a) the square, which is occluded in its midportion, would appear rectangular because of the outward displacement of its two vertical sides (i.e., horizontally elongated) and (b) the two oblique lines would not appear collinear because of the outward displacements of their abutted ends. Studies (Ohtsuka & Ono, 2002;Ono et al, 1998;van Ee & Erkelens, 2000) that measured both shape perception and visual direction, however, show that these two predicted distortions do not occur in three-dimensional perception. Taken together, these results suggest that visual-direction information and shape and alignment information are processed by two separate mechanisms and that the shape and alignment mechanism corrects for the expected distortions illustrated in Figure 6B.…”

“…It is similar to Gregory's (1963) hypothesis about the inappropriate constancy scaling as an explanation for the Mueller-Lyer illusion (see Day & Knuth, 1982;Robinson, 1972). Moreover, the ideas presented here address theoretical puzzles, namely, (a) why shape perception remains veridical in three-dimensional perceptual space despite visual direction being nonveridical (Ohtsuka & Ono, 2002;van Ee & Erkelens, 2000); (b) why in three-dimensional perceptual space, a square behind an occluder is seen as a square (Ohtsuka, 1995a;Ono et al, 1998) and an oblique line behind an occluder is seen aligned (Drobnis & Lawson, 1976;Gyoba, 1978;Liu & Kennedy, 1995;Ohtsuka, 1995b); and (c) why the end of an oblique line that abuts an occluder in a drawing appears more misaligned than does its far end (Wenderoth, 1980). To propose that the underlying mechanism for visual direction examined in this study is different from that for shape and alignment is plausible but ad hoc.…”

“…Also, when a background element is fixated, the edges of the occluder are displaced, and the monocularly seen areas are included in the cyclopean view. To address the fact that the cyclopean view cannot correctly include the entire visual angle subtended by surfaces, Ohtsuka (1995aOhtsuka ( , 1995b and his colleagues (Ohtsuka & Ono, 2002;Ohtsuka & Yano, 1994;Ono, Ohtsuka, & Lillakas, 1998) hypothesized that there is a perceptual compression of some part of the visual field accompanying the displacement. For example, when both edges of the occluder are displaced inward, compression must occur toward the midportion of the occluder, as in the stimulus situation considered in Experiment 2.…”