Depth inversion illusion and its relationship to positive symptoms in clinical and non-clinical voice hearers

Among people with schizophrenia (PSZ), positive symptoms such as hallucinations and delusions are often conceptualised as resulting from abnormal top-down modulation of sensory information. PSZ often exhibit reduced susceptibility to visual illusions compared to healthy control subjects (HCS), suggesting that top-down impairments yield enhanced perception of stimuli that would otherwise be distorted by contextualising visual elements. However, it remains unknown whether resistance to illusions is uniquely associated with positive symptoms, or if it is associated with some other aspect of serious mental illness. To examine this question, 77 PSZ, 50 HCS, and 40 individuals who hear voices and hold unusual beliefs but do not have a psychiatric illness (nonclinical voice hearers; NCVH) completed a hollow mask illusion task. HCS reported experiencing the illusion significantly more often than PSZ and more often than NCVH at the trend level, whereas the latter two groups did not differ from one another. Additionally, there was no consistent association between illusion perception and symptom severity for either PSZ or NCVH. We interpret these results to indicate that resistance to visual illusions may mark a vulnerability for experiencing voices and holding unusual beliefs; however, it may not be associated with the severity of these symptoms.