This exploratory, cross-sectional study compared three age-matched groups—Parkinson’s disease (PD) with complex hallucinations (PD_Hal), PD without hallucinations (PD_NonHal), and healthy controls—while excluding ocular disease and cognitive impairment (Montreal Cognitive Assessment ≥ 24). Participants completed two computer-based illusion tasks: the Delboeuf size illusion and an amodal completion “shrinkage” illusion, each yielding a point of subjective equality as an index of illusion susceptibility. A subset of cortical regions was also assessed with resting FDG-PET in PD groups.
PD_NonHal showed paradoxically better performance than controls on the Delboeuf task (reduced assimilation bias), a pattern not seen in PD_Hal. In contrast, PD_Hal performed worse than controls on the amodal completion task (greater shrinkage bias), with PD_NonHal intermediate, suggesting distinct vulnerabilities of higher-order visuoperceptual processes in hallucinating patients. FDG-PET revealed broadly similar posterior cortical hypometabolism across PD groups but a relative left medial prefrontal hypermetabolism in PD_Hal, consistent with altered top-down attentional modulation. Clinically, motor severity was similar between PD groups, but PD_Hal reported poorer quality of life and greater ADL impact, with a trend toward worse axial signs. Overall, brief illusion-based tasks appear feasible, noninvasive probes of perceptual mechanisms linked to hallucinations in PD and may complement standard scales for earlier detection and characterization.
Reference: Cucca A, Manara CV, Catalan M, et al. Using illusions to understand hallucinations: differences in perceptual performances on illusory figures may underscore specific visuoperceptual impairments in Parkinson’s disease. Front Neurosci. 2023;17:1256224. doi: 10.3389/fnins.2023.1256224.
