Default mode and primary visual network coupling is associated with increased mind-wandering frequency in Parkinson\'s disease with visual hallucinations

Background: An imbalance between top-down expectations and incoming sensory information is implicated in the genesis of hallucinations across neuropsychiatric disorders. In Parkinson9s disease (PD) with visual hallucinations, over-activity in the default mode network (DMN) is a hypothesised source of excessive top-down influence that can dominate visual perception. Methods: In 38 PD patients (18 with hallucinations; 20 without) and 20 age-matched controls, we administered a validated thought-sampling task to establish whether PD hallucinators experienced increased mind-wandering - a form of spontaneous thought strongly associated with DMN activity. Neural correlates of the behavioural differences in mind-wandering frequency were established via resting-state functional connectivity and voxel-based morphometry analyses. Results: PD hallucinators (PD+VH) exhibited significantly higher mind-wandering frequencies compared to non-hallucinators (PD-VH), who showed reduced levels relative to controls. Inter-network connectivity and seed-to-voxel analyses confirmed increased mind-wandering in PD+VH was associated with greater coupling between the DMN and primary visual cortex (V1). In contrast, reduced mind-wandering was associated with grey matter loss in the left posterior parietal cortex in PD-VH.Conclusions: Elevated mind-wandering associated with DMN-visual network coupling emerged as a distinguishing feature of PD hallucinators. We propose that increased mind-wandering reflects excessive spontaneous thought and mental imagery, which can furnish the content of visual hallucinations. DMN-V1 coupling may provide a neural substrate by which regions of the DMN exert disproportionate influence over ongoing visual perception. Our findings refine current models of visual hallucinations by identifying a specific cognitive feature and neural substrate consistent with the excessive top-down influences over perception that lead to hallucinations.