Seeing without eyes: Relation between visual mental imagery and visual hallucinations

Visual mental imagery (VMI) refers to the ability to "see with the mind's eye", in the absence of adequate sensory input, while visual hallucinations (VH) involve perceiving stimuli in their absence. This review investigates the relationship between VMI and VH across neurodegenerative and psychiatric conditions, focusing on Parkinson's disease (PD), Alzheimer's disease (AD), Dementia with Lewy bodies (DLB), and schizophrenia (SCZ). A relationship between VMI and VH was identified, with greater VMI vividness associated with increased VH occurrence in all conditions, except for DLB, for which no studies were found. Moreover, PD and SCZ patients with VH showed reality monitoring deficits. Neuroimaging studies highlighted the role of temporo-occipital regions in both phenomena, with additional disruptions in frontal connectivity in SCZ. Furthermore, patients with VH have stronger VMI, suggesting a prevalence of top-down processes leading to generate vivid internal images, which might be due to a condition of hyper-functioning of visual associative areas. In psychiatric conditions, reality monitoring deficits and thought disorder exacerbate this tendency, leading to the misattribution of bizarre internal contents to external events.