No correlations between the strength of visual illusions

In cognition, audition, and somatosensation, performance correlates strongly between different paradigms suggesting the existence of common factors. Surprisingly, this does not hold true for vision. For example, performance in line bisection and visual acuity (FrACT) correlate very weakly (r2 = 0.001). Here, we show similar results for visual illusions. For 143 participants (69 females), aged from 8 to 81, we measured the strength of six illusions using the method of adjustment. Correlations were very low and mostly non-significant. For example, the correlation between the Ebbinghaus and Ponzo illusion was r2 = 0.08, i.e., only 8% of the variability in the Ebbinghaus illusion is explained by variability in the Ponzo illusion. Results for males and females did not differ significantly. Interestingly, illusion magnitude decreased with age for the Ebbinghaus, Ponzo, and Tilt illusions. Our null results are supported by good test-retest reliability and a Bayesian analysis. Factorial analysis revealed no common factor.