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.

Background: It is a widely held assumption that the brain performs perceptual inference by combining sensory information with prior expectations, all being weighted by their uncertainty. Perception of ambiguous stimuli is influenced by higher- and lower-level priors, which can be manipulated with associative learning and sensory priming, respectively. Here, we investigate the differential effect of auditory vs. visual associative cues on perception of illusory visual motion, and we also take lower-level priming effects into account. Furthermore, we examine the test-retest reliability of individual differences and their correlates. Methods: Healthy individuals (N=29; 13 men, mean (SD) age = 27.8 (9.6) years) performed a perceptual inference task (640 trials) twice with a one-week delay. They reported the perceived direction of (illusory) motion of dot pairs, which were preceded by a visuo-acoustic cue that probabilistically predicted the direction of the motion. In 30% of the trials, motion direction was ambiguous, and in half of these trials, the auditory vs. the visual cue predicted opposing directions. Cue-stimulus contingency could change every 40 trials. To explore the relationship with self-reported alterations in perception and thought, schizotypal traits were measured with the short Oxford–Liverpool Inventory of Feelings and Experiences questionnaire.Results: Importantly, on ambiguous trials where the visual and the auditory cue predicted conflicting directions of motion, participants made more decisions consistent with the prediction of the acoustic cue. Increased predictive processing under stimulus uncertainty was indicated by slower responses to ambiguous (vs. non-ambiguous) stimuli. Associative learning influenced perceptual decisions: we observed slower responses to less (vs. more) predictable non-ambiguous stimuli and an above-chance rate of cue-congruent decisions on ambiguous trials. Furthermore, priming effects were also observed in that perception of ambiguous stimuli was influenced by perceptual decisions on the previous ambiguous and unambiguous trials as well. Critically, all the above effects had substantial inter-individual variability which showed high test-retest reliability (intraclass correlation coefficient (ICC) > 0.78). Finally, indicators reflecting the dominance of associative learning vs. sensory priming (higher- vs. lower-level priors, respectively) clustered together, these sets of variables were inversely related to each other, and some task-derived indicators showed stable and meaningful associations with schizotypal traits .Discussion: Overall, higher-level priors based on auditory (as compared to visual) information seem to have greater influence on the perception of illusory visual motion, replicating the sensory conditioning effect. The influence of lower-level priors was evident as well, suggesting that sensory priming was also in action. Importantly, we observed large and stable differences in various aspects of task performance. Computational modelling combined with neuroimaging could allow testing hypotheses regarding the potential mechanisms causing these behavioral effects (e.g. characteristics of visual vs. auditory processing, perceptual organisation and attention, and uncertainty). The reliability of the behavioural differences implicates that such perceptual inference tasks could be valuable tools during large-scale biomarker and neuroimaging studies.