Differentiating bipolar disorder from schizophrenia: The role of the induced Roelofs illusion

Visual illusions serve as a window into the altered perceptual experiences of individuals with psychiatric disorders. One such is the induced Roelofs effect, where the perceived position of a small stimulus is shifted toward the midline of a distractor frame surrounding the stimulus, as the shift is more pronounced in individuals with schizophrenia. The present work extends the induced Roelofs effect assessment to bipolar disorder. Healthy controls, patients with bipolar disorder, and patients with schizophrenia were shown a dot stimulus placed inside a frame whose horizontal position could vary, and they were asked to determine whether the dot position was to their left or to their right. The induced Roelofs effect was minimal in healthy controls, large in schizophrenia, and intermediate in bipolar disorder. No association between the effect size and psychotic symptomatology or cognitive abilities was found. Yet, a nonlinear, concave association was found between the total pharmacological load and the illusion size. The results suggest that the illusion is related to alterations in sensory processing, which are common to both bipolar disorder and schizophrenia, hence showing their potential utility in clinical evaluations, which currently tend to overlook the sensory domain.