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Hiroyuki Mitsudo; Stereoscopic structure seen in flat patterns. Journal of Vision 2006;6(6):270. doi: 10.1167/6.6.270.
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© ARVO (1962-2015); The Authors (2016-present)
One of the ecological requirements imposed on human vision is the recovery of correct three-dimensional (3D) structure invariant across the directions of gaze. This structural invariance is never questioned because we rarely see incorrect 3D structure in ordinal situations. Here I present counterexamples to the belief by showing that binocularly fixated flat patterns produce illusory 3D structure, depending on the direction of vertical eccentric gaze. Stimuli were comprised of curved lines and were presented on a flat monitor almost perpendicular to the lines of sight. Observers were required to fixate the flat patterns at various angles of the vertical gaze direction, and to report their perceived depth. Experiment 1 revealed that (a) the flat patterns appeared three-dimensionally on a downward or upward eccentric gaze, and (b) the perceived 3D structure was reversed depending on whether the stimuli were viewed on a downward or upward gaze. The results are consistent with the idea that the illusory structure arises from a horizontal mismatch of the cyclodisparity induced by torsional binocular misalignment. Experiment 2 showed that the magnitude of the illusory depth rose steeply when gaze direction exceeded a particular angle (on downward gaze, approximately 40 deg). The illusions therefore suggest that ocular torsion is optimized for stereo vision in a wide but limited range of gaze direction.
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