Phenomenological Contraction does not depend on explicit cues to occlusion

Just as our visual system enables us to experience the things we are looking at, so too does it allow us to experience aspects of those things that are not visible, as is the case when an object is partially occluded from view. Our research focuses on understanding how this ‘seeing what is not there’ is accomplished. While amodal completion is a well-studied process that allows us to experience partially occluded objects as complete, a lesser known phenomenon is that amodally completed parts of objects tend to appear smaller than identically sized counterparts that are fully visible. This phenomenological contraction was first described by Kanizsa and has not been studied nearly as much as the mechanisms behind amodal completion itself. We developed a paradigm using a stimulus composed of two triangles, one partially occluding the other that allowed us to quantify phenomenological contraction by measuring the mislocalization of the occluded triangle’s vertex. In our previous work we found the contraction is independently influenced by the size of the occluded object, corner angle, and the level of occlusion. Our current research seeks to determine whether such mislocalization/contraction is dependent on the presence of explicit occlusion. Across three experiments our approach was to replicate our previous experiments while presenting only partial contours of a single triangle without an explicit occluder. We again found independent influences of corner angle and level of occlusion (how much contour was visible) even when the partial contour consisted of only two line segments (lacking the base of the triangle). Based on our data we conclude that the phenomenological contraction arises from mechanisms of interpolation and/or extrapolation that are largely independent of explicit cues to occlusion and as such be considered as separate from amodal completion.