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Brennan Klein, Zhi Li, Durgin Frank; Which way is up in the horizontal-vertical illusion?. Journal of Vision 2014;14(10):149. doi: 10.1167/14.10.149.
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Very large horizontal-vertical illusions (HVI) may be observed in outdoor scenes, such that horizontal extents must be made as much as 25% longer to seem equal to vertical extents (Chapanis & Mankin, 1967). Here we ask whether these effects are referenced to the orientation of the observer or the world and whether they are affected by the extent of one's horizontal field of view (FOV). Forty-eight participants viewed poles, 3-9 m in height, from a distance of 15 m. Half viewed the poles while lying on their side at eye-level. Half stood upright. In each group, half wore a patch over one eye to reduce their FOV. The task was to instruct the experimenter to adjust the horizontal distance to a second pole until that distance matched the height of the observed pole. Adjustments were made from close and far starting positions, and the average matches were analyzed. In the upright condition we replicated very large HVI matches (1.25) for objects of 6 m or more. Across all pole heights, HVI matching ratios were larger for upright observers (1.2) than for sideways observers (1.1), p <.01, but remained primarily yoked to the world vertical rather than the bodily vertical. Across both viewing orientations, HVIs differed by pole height, p <.0001, increasing from about 1.1 for shorter poles to 1.2 for taller ones. There were no effects of FOV (binocularity). In a follow-up study simulating a similar outdoor scene in immersive VR, we rotated the world instead of the observer, and this produced essentially the same results as rotating the observer in the real world. These rotation results can be modeled by assuming a small (e.g., 5%) HVI illusion tied to the body and a larger (e.g., 15%) HVI illusion tied to a ground-plane-defined world orientation.
Meeting abstract presented at VSS 2014
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