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Ruben Moreno-Bote, Asya Shipiro, John Rinzel, Nava Rubin; Probabilities of perceptual depth ordering in transparent motion and the relative effect of different depth cues. Journal of Vision 2007;7(9):744. doi: 10.1167/7.9.744.
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© ARVO (1962-2015); The Authors (2016-present)
During ‘transparent motion’ perception of plaids, subjects often report bi-stable alternations in the depth ordering of the two constituent gratings. To study this we used plaids with large angles between the gratings' directions of motion, so that incoherent grating motion is perceived nearly 100% of the time. Observers were asked to indicate which of the two gratings was perceived as behind. This allowed us to examine the role of different depth cues on the segmentation of the scene into different surfaces. We varied the spatial frequency, the duty cycle and the speed of one of the rectangular wave gratings while keeping fixed the properties of the other grating. We found that the mean fraction of time that a grating appeared to be in the back increased gradually as its wavelength and/or duty cycle decreased. This suggest that the visual system treats higher spatial frequency and lower duty cycle as cues for a surface being more distant, consistent with the ecological properties of the visual world (distant objects produce smaller projected images on the retina). The effect of varying speed was to make the faster grating appear to be behind more often. This result (which some regard as counter-intuitive) can be explained by noting that when we fixate on a static nearby object as we move in the environment, static objects behind the fixated one produce faster retinal motion the further they are. These depth cues were strong enough to overcome conflicting disparity cues, again showing a graded change in the fraction of time that the stereoscopically-far grating was seen in the back. Our results suggest that the fraction of dominance time of each percept is a measure of the probability the brain assigns to it, and that these probabilities are determined based on prior knowledge about the visual world.
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