Seeing smaller: Occlusion, contour completion, and biases in spatial localization.

A quick glance around you will highlight the fact that the visual scene is often populated with partially occluded objects. Despite not being fully visible, we experience these objects as being intact, and our visual system maintains a representation of the not-visible parts of the objects. Here, we demonstrate a novel variant of a classic illusion called Phenomenological Contraction in which a partially occluded object can appear smaller than an unoccluded counterpart. In our version, which we call the Occluded Triangle Illusion, two equal-sized triangles partially overlap. When asked to identify where the vertex of the occluded triangle is, people consistently report a bias in where they perceive the vertex location. This bias is consistent with the fact that the occluded triangle appears smaller than the occluding triangle. In a series of eight experiments, we explored configural features of this illusion and identified several key features that influence that magnitude of mislocalization, including the level of occlusion, corner angle, and support ratio. We further replicated several experiments without explicit cues to occlusion and demonstrate a fundamental bias on how observers perceive the intersection point of two converging line segments. Taken together, the results of the demonstrations and experiments presented here build on an extensive literature on the mechanisms underlying process contour completion and the dynamic nature of size perception. (PsycInfo Database Record (c) 2026 APA, all rights reserved).