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Loes C J Dam, Raymond Ee; Bistability in stereoscopically perceived slant about a horizontal axis. Journal of Vision 2003;3(9):467. doi: 10.1167/3.9.467.
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We exposed the visual system to an ambiguous 3D stimulus consisting of a grid for which monocular and binocular cues could specify different slants about a horizontal axis. Observers were asked to estimate the slant of the grid in 3D space. Monocular and binocular specified slants were varied independently across stimulus presentations. When monocular and binocular cues specified similar slants (no conflict) observers perceived a single orientation. However when the difference between the specified slants was large (cue-conflict), observers were able to select either a monocular or a binocular dominated percept. This result corroborates what van Ee, van Dam and Erkelens (Journal of Vision, 2002, 2, 597–607) reported for slants about a vertical axis.
The slant estimates were described by a Bayesian model in which the different cues were implemented as different likelihoods. To describe the bifurcation in the data, two different modes were used in our model. One in which the monocular likelihood is dominant and the other in which the binocular likelihood is dominant.
Further, observers were able to flip between percepts more or less at will, although spontaneous flips occurred frequently. Eye movement measurements revealed that (1) the flipping between percepts can occur under strict fixation and (2) when eye movements are allowed, there is no clear correlation between eye movements and perceptual flips: flips occur during long fixations as well as just after saccades or blinks. Therefore we suggest that perceptual flipping of the bistable grid stimulus is a central process in which eye movements are not necessarily involved.
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