Abstract
Faces convey highly important information for social interactions. Previous studies have separately established that both configural processing and facial contour are crucial to face perception. Previous studies primarily focused on the modulation of physical contours on the fat face illusion, in which two identical face images are vertically aligned, the bottom face is perceived as wider than the top one. However, it remains unclear how psychological contours (configural processing) interact with physical contours to modulate this illusion. To address this question, we conducted six experiments using line-drawing faces and geometric figures. Experiment 1 replicated the illusion using upright and inverted line-drawing face images. Experiments 2a and 2b manipulated configural relationships of internal facial features to assess the effect of psychological contours on this illusion. Experiments 3a and 3b simultaneously manipulated external physical contours (upright and inverted trapezoids and a rectangle) and psychological contours (configural relationships of internal facial features) to investigate their interactions in this illusion. Experiment 4 used geometric figures to test whether this interaction on this illusion is specific to faces. We observed a stable fat face illusion with upright line-drawing faces, and the magnitude of this illusion was primarily driven by physical contour, with psychological contour playing a secondary, modulatory role. Importantly, the results in Experiment 4 showed that this modulation of the illusion is not specific to faces. This work highlights the dominant role of physical contour in the fat face illusion, suggesting a domain-general mechanism underlying contour-based modulation of face illusions. (PsycInfo Database Record (c) 2026 APA, all rights reserved).