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Abigail Lee, Robert Allison, Laurie Wilcox; Depth perception from successive occlusion. Journal of Vision 2021;21(9):1963. doi: https://doi.org/10.1167/jov.21.9.1963.
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Occlusion of one object by another is one of the strongest and best-known pictorial cues to depth. However, it has been suggested that, in addition to a cumulative sense of depth, successive occlusions of previous objects by newly presented objects can give rise to illusory motion in depth (Engel, Remus & Sainath, 2006). Engel and colleagues (2006) found that a stacking disk stimulus, where each disk occludes a previous disk in a pile, generates a strong sensation of the stack moving towards the observer. While the perceived motion associated with this illusion has been studied, the resultant depth percept has not. To investigate if the successive introduction of occluding objects affected the perceived depth of a stacked disk stimulus, we compared two conditions. In one, participants were presented with two static piles of disks, while in the other, participants viewed one static and one stacking pile of disks. In both conditions, we presented 20 disks in one pile and a range of disks in the other using a method of constant stimuli. Participants indicated which pile appeared taller. The proportion of ‘taller’ responses were fit with cumulative normal psychometric functions from which we calculated points of subjective equality for the number of disks in each pile. We found static piles with the same number of disks appeared approximately equal in height. In contrast, the successive presentation of disks in the stacking condition appeared to enhance the perceived height of the stack - fewer disks were needed to match the static pile. Surprisingly, we also found just-noticeable differences varied between conditions: the task was easier when participants compared stacking vs. static piles of disks. Our results suggest that successive occlusions generate a greater sense of height than occlusion alone, and that dynamic occlusion may be an underappreciated source of depth information.
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