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 Müller-Lyer illusion. Here, we aimed to investigate susceptibility to the Müller-Lyer visual illusion in the general population with different levels of schizotypy. Study Design: We assessed 312 participants from the general community on an online platform. In addition to basic demographics, participants completed the Müller-Lyer illusion, the Cardiff Anomalous Perceptions Scale (CAPS) to measure perceptual anomalies, and the Multidimensional Schizotypy Scale – Brief (MSS-B) for schizotypal 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 significant effect for anomalous perceptions alone, or for negative or disorganized schizotypy. Conclusions: The increased Müller-Lyer effect in positive schizotypy could be related to imbalances between bottom-up perceptual processing and substantial reliance on prior expectations. This association might be specific to delusion-like beliefs and magical ideation. Further research is needed to clarify the Müller-Lyer effect in schizophrenia-spectrum conditions from a Bayesian point of view.