The inversion-induced size illusion in letters and the role of the contour-based centroid

<p>Inverting a visual stimulus can alter its perceived size. Recent studies have shown that upright letters are perceived as larger than inverted letters, an effect opposite in direction to the inversion-induced size illusion reported for faces. However, previous studies have primarily examined the average illusion across letters, leaving unclear whether the effect depends on letter-specific processing and whether it varies systematically with visual properties of individual letters. In Experiment 1, we measured the point of subjective equality (PSE) for 17 Latin letters. Upright letters were perceived as larger than inverted letters on average, replicating the previously reported inversion-induced size illusion, although the magnitude and direction of the effect varied across letters. A post hoc exploratory analysis further revealed a strong positive association between the illusion effect and the vertical position of the contour-based centroid. In Experiment 2, we therefore examined non-letter polygons corresponding to the convex hulls of the letters from Experiment 1, thereby preserving their contour-based centroids while eliminating letter identity. The polygons showed no inversion-induced size illusion on average, but the strong association between contour-based centroid position and illusion magnitude was replicated. Across both stimulus categories, stimuli with higher contour-based centroids tended to produce larger positive illusion effects. These findings suggest that the contour-based centroid is associated with perceived size across different visual shapes but cannot fully account for the average inversion-induced size illusion observed for letters. The illusion in letters may therefore depend on interactions between orientation-dependent shape properties and other visual or orthographic characteristics.</p>