Abstract
We report results from an experiment showing that a tall pillar with a triangular base evokes radically different three-dimensional (3D) percepts depending on the vantage point from which it is observed. The base of the pillar is an isosceles right triangle, but the pillar is perceived as just a thin plane when viewed from some vantage points. Viewed from other vantage points, the perceived 3D shape of the pillar corresponds to a square or rectangular base. In general, our results suggest that the visual system uses a preference for rectangularity (or symmetry) to determine the 3D shape of objects. The amodal impressions of the invisible backside of the pillar are often quite compelling, and the corresponding illusions persist even when the observers know the true shape of the pillar. Interestingly, though, the compellingness and definiteness of the amodal impression of the pillar’s backside depends on the vantage point. This is reflected in corresponding differences in the interobserver variability of the 3D shape judgments. We also discuss how variants of this illusion are used as a powerful tool in the art of magic.
Highlights
When we look at a real-world scene, visual input at the retina is produced only by those parts of the objects in the scene that happen to (a) face toward us and (b) not be occluded by other objects in the foreground
A face mask can correspond to all the visible parts of a human head as viewed from a certain vantage point
The results of our experiment show that the very same triangular pillar can be perceived as having radically different 3D shapes depending on the position from which it is viewed: It can be perceived as a very thin plane or as having another rectangular, square, or triangular cross section
Summary
When we look at a real-world scene, visual input at the retina is produced only by those parts of the objects in the scene that happen to (a) face toward us and (b) not be occluded by other objects in the foreground. A semispherical shell viewed from the convex side will continue to look like a complete ball even when the observer knows that it is just a hollow shell (Ekroll, Sayim, & Wagemans, 2013; Ekroll et al, 2016). This kind of cognitive impenetrability is often considered to be a hallmark of perceptual processes at large (Firestone & Scholl, 2015; Leslie, 1988; Pylyshyn, 1999)
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