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Evan M. Palmer, Philip J. Kellman; Underestimation of velocity after occlusion causes the aperture-capture illusion. Journal of Vision 2002;2(7):477. doi: 10.1167/2.7.477.
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© ARVO (1962-2015); The Authors (2016-present)
PURPOSE: In dynamic occlusion displays with an aligned rod that moves behind two misaligned windows, participants perceive the rod as being misaligned in the direction of the windows (the Aperture-Capture Illusion [ACI], VSS 2001; Hecht, 1924). Last year we presented evidence that the ACI is not caused by a misperception of velocity in the leading aperture. Here we present evidence that the ACI is at least in part caused by an underestimation of the velocity of the occluded portion of the rod. METHODS: In the first series of experiments, subjects viewed a horizontally translating vertical bar, whose top and bottom appeared sequentially through vertically separated and horizontally offset apertures. We manipulated the size and number of the windows to gauge their effect on the ACI. In the second series of experiments, subjects saw a horizontally translating dot move through an aperture and then disappear behind an occluder (cf. De Valois, VSS 2001). A vertical line was placed outside of the aperture along the dot's trajectory. After the dot disappeared, the line flashed and a beep sounded then subjects made a 2AFC decision about whether the dot would have been to the left of or to the right of the line at the time of the beep. RESULTS: In the moving rod experiments, the ACI was almost entirely eliminated by having many small apertures rather than two large ones. In the moving dot experiments, subjects consistently underestimated the velocity of the dot after occlusion. CONCLUSIONS: The ACI may arise from an underestimation of velocity after occlusion. This misperception of velocity after occlusion causes the portion of the rod seen through the leading window to appear closer to the aperture than it really is. In the moving rod experiments, having several small apertures allows for re-sampling of the moving rod's motion, improving perception of the rod's spatial position.
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