No correlations between visual illusion strength

In cognition, audition and somatosensation, performance correlates strongly between different tasks suggesting the existence of common factors. Surprisingly, this does not hold true for vision. For example, Vernier acuity and Gabor detection correlate very weakly (r2 = 0.003). Here, we show similar results for visual illusions. 143 participants, aged from 8 to 81, adjusted the strength of six illusions. Correlations were very low and mostly non-significant. For example, the correlation between the Ebbinghaus and the Ponzo illusion was r2 = 0.08, i.e., the two illusions have only 8% of variance in common. Results for males and females did not differ. Illusion magnitude decreased with age for the Ebbinghaus, Ponzo, and Tilt illusion. Our null results are supported by good test-retest reliability and a Bayesian analysis. We suggest that, contrary to cognition, audition and somatosensation, there is no general factor for vision.