Pre-stimulus low-alpha frontal networks are associated with pareidolias in Parkinson's disease.

Background: Pareidolias are visual phenomena wherein ambiguous, abstract forms or shapes appear meaningful due to incorrect perception. In Parkinson's disease (PD), patients susceptible to visual hallucinations experience visuo-perceptual deficits in the form of pareidolias. While pareidolias necessitate top-down modulation of visual processing, the cortical dynamics of internally generated perceptual priors on these visual misperceptions is unknown. Objectives: To study pre-stimulus related EEG spectral and network abnormalities in PD patients experiencing pareidolias. Methods: 21 PD in-patients and 10 age-matched healthy controls were evaluated. Neuropsychological assessments included tests for cognition, attention and executive functions. To evoke and quantify pareidolias, participants performed the noise pareidolia test (NPT) with simultaneous EEG recording. PD patients were sub-divided into two groups - those with high pareidolia counts (N=10) and those without (N=11). EEG was analyzed 1000ms before stimulus presentation in the spectral domain (theta, low-alpha and high-alpha frequencies) with corresponding graph networks that evaluated small-world properties, efficiency and centrality measures. Statistical analysis included ANOVA and multiple regression to evaluate the differences. Results: PD patients with high pareidolia counts were older with lower scores on neuropsychological tests. Their pre-stimulus EEG low-alpha band showed a tendency towards higher frontal activity (p=0.07). Graph networks showed increased normalized clustering coefficient (p=0.05), higher local parietal cortex efficiency (p=0.049) and lower frontal degree centrality (p=0.005). These network indices correlated positively to patients' pareidolia scores. Discussion: We suggest that pareidolias in PD are a consequence of an abnormal top-down modulation of visual processing which are defined by their frontal low-alpha spectral and network alterations in the pre-stimulus phase due to a dissonance between patients' internally generated mental-processing with external stimuli.