Paradoxical fusion of two images and depth perception with a squinting eye

Abstract

Some strabismic patients with inconstant squint can fuse two images in a single eye, and experience lustre and depth. One of these images is foveal and the other extrafoveal. Depth perception was tested on 30 such subjects. Relief was perceived mostly on the fixated image. Camouflaged continuous surfaces (hemispheres, cylinders) were perceived as bumps or hollows, without detail. Camouflaged rectangles could not be separated in depth from the background, while their explicit counterparts could. Slanted bars were mostly interpreted as frontoparallel near or remote bars. Depth responses were more frequent with stimuli involving inward rather than outward disparities, and were then heavily biased towards "near" judgements. All monocular fusion effects were markedly reduced after the recovery of normal stereoscopic vision following an orthoptic treatment. The depth effects reported here may provide clues on what stereoscopic pathways may or may not accomplish with incomplete retinal and misleading vergence information.

Alternation Frequency Thresholds for Stereopsis as a Technique for Exploring Stereoscopic Difficulties

“…The four stereograms were used in our previous study on paradoxical fusion with a single eye (Rychkova and Ninio 2009). In this case there were clear-cut results.…”

“…The four stereograms in this series were used in our previous work (Rychkova and Ninio 2009). All patients could see a convex or concave shape in the B1 hemisphere, but only half of them (thirteen to fifteen out of twenty four) could detect convexity or concavity in the three other stimuli.…”

“…All subjects first practised with an easy stereogram representing a small central disk in front or behind a larger disk (Rychkova and Ninio 2009, figure 2e). The instructions to subjects are given below when we describe the typical sequence of percepts from alternating left–right stereo image presentations and the criterion that subjects were required to use in making their threshold judgments.…”

“…It was soon revealed that patients who were able to interpret a certain stereoscopic pair in three dimensions at a certain alternation frequency on the synoptophore could require a significantly higher alternation frequency for another stereo pair, independently of the disparity ranges used in the stereograms. Differences were found between quite simple stereograms—for instance, between the first two and the last two stereograms in figure 1, which had been designed for strabismic patients with inconstant squint (Rychkova and Ninio 2009). Subsequently, differences in alternation frequencies were also observed with subjects who had normal vision.…”

“…The first three blocks had already been used in our previous study, and were found to provide a nicely graded range of difficulties (Rychkova and Ninio 2009). The stereograms that were perceived with the greatest difficulty under static viewing conditions in our earlier work turned out to be those that required the highest alternation frequencies in the present work.…”