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Vebjørn Ekroll, Tom R. Scherzer; Enigmatic cases of modal amodal completion: What do modal and amodal percepts represent?. Journal of Vision 2014;14(10):891. doi: 10.1167/14.10.891.
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In the "occlusion illusion" an object hidden behind a static occluder is perceived as though it were less occluded than it actually is (Palmer, Brooks & Lai, 2007, Perception, 36(5), 650–669). We confirm and extend this finding using a stimulus with a moving occluder. In agreement with Palmer et al.'s findings and their partial-modal-completion hypothesis, we found that the illusion is indeed related to the sensory evidence for occlusion. Our experiments also confirm Palmer et al.'s speculation that the occlusion illusion involves an intriguing, seemingly paradoxical percept. In our experiments, subjects viewed an opaque disk with an open sector rotating in front of a background and indicated a) the perceived angular extent of the occluding disk sector and b) the perceived angular extent of the part of the background experienced as directly visible. While the perceived angular extent of the occluding disk sector corresponded to the physical extent of the stimulus, the perceived angular extent of the background region experienced as directly visible through the open sector in the occluder was clearly overestimated. Thus, the sectors of the circle experienced as directly visible and occluded sum to more than 360 degrees, which – much like Escher's well-known paintings – makes the total percept an "impossible figure". We argue that the key to resolving this paradox is to question the seemingly self-evident tacit assumption that occluded portions of a visual scene are represented by amodal percepts, while unoccluded portions of a scene are represented by modal percepts. Instead, we propose that visual percepts are experienced as modal whenever they are based on sufficiently conclusive sensory evidence. Conversely, they are experienced as amodal when this is not the case. Functionally, this perceptual representation of the conclusiveness of the sensory evidence underlying perceptual inferences might be more useful than estimates about optical visibility.
Meeting abstract presented at VSS 2014
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