Roberta Cessa^1,2 (roberta.cessa@studenti.unipd.it), Martina Passaggi¹, Deniz Demirkapı¹, Md Jobayer¹, Nadja Kosovic¹, Esma Dilara Yavuz¹, Luca Battaglini¹, Giulio Contemori¹, Marco Bertamini¹; ¹University of Padua, ²University of Rochester

Visual perception is shaped by noise: external noise (variability in physical stimuli) and internal noise (spontaneous neural fluctuations within the brain's processing systems). Stochastic resonance captures how these two sources interact, showing that adding moderate external noise enhances detectability of subthreshold signals. Internal noise varies across individuals, and people within the schizophrenia spectrum exhibit elevated neural variability. Whether similar alterations are already present in subclinical schizotypy, and whether they influence global perceptual organization, remains unresolved. We addressed this question through two experiments on symmetry perception. In a first threshold-based study, we measured the symmetry signal strength required for reliable detection across individuals, providing an index of perceptual organization. We then examined whether trait differences also influence how the visual system handles noise by testing the stochastic-resonance profile for symmetry detection. In this experiment, 100 participants completed a two-interval forced-choice task in which one random-dot pattern contained vertical symmetry and the other was random. Symmetry coherence was fixed at 65%, while dot density varied across 14 levels to manipulate external noise. All participants completed the O-LIFE and CAPS questionnaires. Global schizotypy scores showed mild positive correlation with symmetry thresholds, indicating reduced perceptual organization as scores increase (global: r=0.15, p<0.001, UE: r=0.25, p<0.001). Accuracy produced the expected inverted-U, as captured by a quadratic generalized linear-mixed model. However, higher schizotypy, particularly Unusual Experiences, produced a flatter curve and an earlier peak (global: d≈0.22, p=0.017; UE: d≈0.33, p<0.001), showing that performance improved only under lower noise conditions. Together, the elevated symmetry thresholds and altered stochastic resonance profiles demonstrate that schizotypy, particularly Unusual Experiences, impairs both perceptual organization and the optimal internal-external noise balance that supports it. These findings make symmetry perception a sensitive behavioral tool for discerning noise-related perceptual anomalies that differentiate symptom dimensions across the schizophrenia spectrum.

Acknowledgements: This work is part of the “InDi-ViDi” project, funded by the BIAL Foundation Grants Programme 2024/25 (Application No. 389/2024). Roberta Cessa is currently supported by a Fulbright Visiting Researcher Scholarship in the United States.