When illusions merge

Recently, we found very weak correlations between the magnitudes of visual illusions. However, we found strong correlations between 19 variants of the Ebbinghaus illusion which differed in color, shape or texture, suggesting that different illusions make up their own factors (e.g., an “Ebbinghaus factor”). Here, we asked whether the magnitude of a combination of two illusions is predicted by the basic illusions of the combined illusion. For example, we tested the Müller-Lyer and Ponzo illusions, as well as a combined version of both of them by adding inward and outward fins to the horizontal lines of the Ponzo illusion. A large sample (N = 100) with age ranging from 8 to 81 years was tested with an adjustment task. A regression model showed that the combined illusions were strongly predicted by the basic illusions. In addition, age does not seem to influence the illusory effects. We suggest that in the combined illusions no factor is lost, no factor is created, factors are not even transformed but just additive.