Principles proposed to explain geometrical illusions have often been assumed to apply across multiple illusions. However, it remains unclear whether these illusions rely on shared perceptual mechanisms. In the present study, we focused on five geometrical illusions linked to spatial perception and examined the structure of individual differences in illusion magnitude.
All illusions were measured repeatedly using the method of adjustment with balanced response series to improve measurement reliability. In addition, illusion-specific control conditions were used to derive residualized illusion magnitudes, allowing us to examine inter-illusion correlations after controlling for individual differences attributable to the adjustment task in general. The results revealed moderate positive correlations among the Railroad (size-constancy) illusion, Ponzo illusion, and Poggendorff illusion.
This pattern was supported by complementary exploratory analyses, including factor analysis, hierarchical clustering, and multidimensional scaling. By contrast, the Kanizsa shrinkage illusion appeared distinct from the others, and the Müller–Lyer illusion showed little relation to this structure despite its historical association with spatial perception. These findings suggest that individual-difference structures can reveal shared components among specific geometrical illusions and help constrain theories of visual illusions.
The author thanks Dr Frederick Kingdom for helpful discussions regarding the content of the manuscript and Shelley Martin for permission to reproduce the figure material. Supported by the Japan Society for the Promotion of Science (grant number: 25K06901). Hiyoshi Psychology Laboratory, Keio University, 4-1-1 Hiyoshi Kohoku-ku, Yokohama, 223-8521, Japan The authors declare no competing interests.
The study was approved by the research ethics committee of Keio University. Written informed consent was obtained from all participants. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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