Introduction. Psychosis and hallucinations occur in 20-30% of patients with Parkinson's disease (PD). In the current study, we investigate cognitive functions in relation to the occurrence of psychosis in PD patients.
Methods. We tested three groups of subjects-PD with psychosis, PD without psychosis and healthy controls-on working memory, learning and transitive inference tasks, which are known to assess prefrontal, basal ganglia and hippocampal functions.
Results. In the working memory task, results show that patients with and without psychosis were more impaired than the healthy control group. In the transitive inference task, we did not find any difference among the groups in the learning phase performance. Importantly, PD patients with psychosis were more impaired than both PD patients without psychosis and controls at transitive inference. We also found that the severity of psychotic symptoms in PD patients [as measured by the Unified Parkinson Disease Rating Scale Thought Disorder (UPDRS TD) item] is directly associated with the severity of cognitive impairment [as measured by the mini-mental status exam (MMSE)], sleep disturbance [as measured by the Scales for Outcome in Parkinson Disease (SCOPA) sleep scale] and transitive inference (although the latter did not reach significance).
Conclusions. Although hypothetical, our data may suggest that the hippocampus is a neural substrate underlying the occurrence of psychosis, sleep disturbance and cognitive impairment in PD patients.