Objective: To investigate (a) whether a susceptibility to illusional perceptions is intrinsic to Parkinson’s disease even before the onset of hallucinations or cognitive impairment and (b) whether there is a role for dopaminergic neurotransmission in their formation.

Background: Parkinson’s disease associated psychosis is disabling and difficult to treat. Hallucinations and delusions may be preceded by minor symptoms and illusions but we do not know if there is a predisposition to illusional perception occurring even earlier. In addition, the role of dopaminergic therapy in triggering hallucinations is unclear.

Methods: We used the Rubber Hand Illusion to measure susceptibility to somatic illusions in 21 Parkinson’s disease patients without psychotic symptoms or cognitive deficits (in both on- and off-medication states) and 21 controls. In this experiment, synchronous stroking of a rubber hand and the subject’s hidden real hand results in the illusional experience of ownership of the rubber hand, and proprioceptive mis-localisation of their real hand towards the rubber hand. The illusion is attenuated by asynchronous stroking. Multivariable linear mixed-effects regression with Tukey’s post-hoc test was employed for the analysis.

Results: Rubber Hand Illusion rating scores were similar in patients and controls in the illusion-promoting synchronous condition (mean: patients = 9.16, 95% CI [8.13, 10.18]; controls = 9.94, 95% CI [8.91, 10.97], p = .96). However patients rated the illusion more strongly than controls in the illusion-attenuating asynchronous condition (mean: patients = 3.75, 95% CI [2.73, 4.78]; controls = 1.04, 95% CI [0.01, 2.06], p < .011). Additionally, Parkinson’s disease independently predicted greater mis-localisation of the subject’s real hand towards the location of the rubber hand (β = 1.50, 95% CI [0.58, 2.43], p = .002, r = .46). There was no effect of medication state on either outcome.

Conclusions: Parkinson’s disease patients without hallucinations or cognitive deficits remain susceptible to illusional experiences under conditions that attenuate illusions in controls. This suggests an inability to synthesize multi-modal sensory inputs into a rejection of illusional perceptions and links it to known sensory deficits of Parkinson’s disease for the first time. It may inform future treatment approaches to this disabling clinical problem.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/illusional-perception-in-parkinsons-disease/