Visual object processing, reality monitoring, reasoning and visual hallucinations in Parkinson's disease

Up to 40% of patients with Parkinson’s disease will develop visual hallucinations at some point in their illness. Although medication, depression, duration of illness, and peripheral visual impairment have been identified as risk factors for hallucinations, more specific changes in cognitive function may also play a role. This pilot study evaluated 9 PD patients with visual hallucinations and 9 nonvisually-hallucinating PD patients on tests of imagery, recognition of objects, reasoning processes and reality monitoring. The reasoning processes tapped ability to derive a set of rules based on feedback, application of a strategy in relation to goal attainment and concept formation. Reality monitoring is defined as the normal process by which perceived and imagined events are discriminated in memory. In healthy volunteers, memories originating from experienced events have more contextual, perceptual and meaningful information than memories derived from internally generated events such as imagery processes, dreams and fantasies. However, if perceptual qualities of imagined events are unusually vivid, they may be more difficult to discriminate from perceived events. Our research revealed that visually hallucinating PD patients have greater difficulty with recognising visual objects viewed as either silhouettes (U=20.00, p=0.07) or when key identifying information which is hidden from view (U=22.00, p=0.05). Reasoning and reality monitoring processes were also deficient (both U=15.00, p=0.02). Errors in the reality monitoring task where most likely to occur for imagined items which were misattributed to perceived items. In contrast spatial processing, spatial imagery and visual object imagery showed higher levels of preservation. The findings from this study raise the possibility that visual hallucinations in PD could stem from a combination of impairments in visual object processing, particularly when key visual attributes of an item are obscured, reasoning and reality monitoring processes. Acknowledgements This work was supported by a Fasttrack grant from the Parkinson’s Disease Society