Objective: Test the validity of a theoretical model that visual hallucinations in Parkinson’s arise from over-reliance on prior information.

Background: Parkinson’s hallucinations are common and a potentially distressing symptom. Gaining a better understanding of their causal mechanisms will improve patient management and quality of life. The perceptual expectation bias framework suggests hallucinations are driven by imbalance in integration of bottom-up and top-down information. We hypothesised that people with Parkinson’s hallucinations would over-rely on top-down prior knowledge in the face of direct (conflicting) sensory input, and designed a task to examine this.

Method: A visual decision-making task was completed by 52 Parkinson’s participants without dementia (mean age: 69 years, 68% male), grouped into 38 with hallucinations (assessed by PsycH-Q), 14 without hallucinations, and by 30 age-, sex- and education-similar controls. Participants judged the horizontal direction of a moving dot stimulus that varied in information from low to high certainty per trial. A directional cue (i.e., prior knowledge) could be informative in aiding the judgement, or uninformative, or irrelevant. We used mixed effects linear models to test whether Parkinson’s participants with hallucinations over-relied on the cue.

Results: The control and non-hallucination groups performed similarly. The hallucination group did not perform the task as accurately as controls (p<0.006). All three groups could adaptively use the informative cue to lift their performance when there was little directional information in the stimulus (p<0.0001). However, the uninformative cue reduced performance in all groups (p<.033). These differences remained when accounting for cognitive ability (MoCA).

Conclusion: People with Parkinson’s who experience hallucinations did not perform as well as controls on a task probing visual decision-making, but, in uncertain situations they used prior information in the same way to improve performance. Thus we did not find evidence to support the theoretical position that hallucinations are driven by over-reliance on prior knowledge. This does not invalidate the theory; a task that more directly relates to hallucinatory experiences (e.g. visual perception of natural scenes), rather than a general visuoperceptual decision-making task, may be an important next step to elucidate the mechanisms that underlie hallucinations in Parkinson’s disease.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/are-hallucinations-in-parkinsons-driven-by-over-reliance-on-prior-information/