Can you believe your eyes? Positive schizotypy is associated with increased susceptibility to the Müller-Lyer illusion

Background and hypothesis: Visual illusions provide a unique opportunity to understand cognitive and perceptual alterations in schizophrenia-spectrum conditions. Schizophrenia patients often exhibit increased susceptibility to the Muller-Lyer illusion. Here, we investigate susceptibility to the Muller-Lyer visual illusion in the general population with different levels of schizotypy. Study design: We assessed a population-based convenience sample (N = 263) on an online platform. In addition to basic demographics, participants completed the Muller-Lyer illusion, the Cardiff Anomalous Perceptions Scale (CAPS) to measure perceptual anomalies, and the Multidimensional Schizotypy Scale - Brief (MSS-B) for schizotypic traits. To evaluate what predicts susceptibility to the illusion, we fitted a large set of multilevel logistic regression models and performed model averaging over the coefficients. Study results: We found support for increased illusion susceptibility among individuals with high positive schizotypy. However, we did not find a comparable effect for anomalous perceptions alone, or for negative or disorganized schizotypy. Conclusions: The increased Muller-Lyer effect in positive schizotypy might be specific to delusion-like beliefs and magical ideation. Further research is needed to clarify how a hierarchical Bayesian formulation of brain function (e.g. imbalances between bottom-up perceptual processing and substantial reliance on prior expectations) can account for the Muller-Lyer effect in schizophrenia-spectrum conditions.