The role of the retina in visual hallucinations: A review of the literature and implications for psychosis

Visual hallucinations are a psychotic symptom present in numerous clinical conditions such as eye disease, Parkinsonian syndromes, neurodegenerative disorders and psychosis. Alteration of low level visual processing is a common feature in these clinical conditions, and various stages of processing from the retina to visual cortices are involved. We undertook a literature review of abnormalities of the retina and their potential link with the occurrence of VHs in these clinical conditions of interest. We found that structural and functional abnormalities of the retina are frequently present. In Parkinson disease and eye disease, VHs have been related to dysfunctions of the retina. By contrast, in neurodegenerative disorders and psychosis, possible links have yet not been explored. We show that structural or functional abnormalities of the retina are given little consideration in cognitive models of VHs, which primarily postulate an alteration of sensory visual processing and a top-down attentional process. We conclude that contrast sensitivity measures and an exhaustive exploration of the retinal functions using the clinical electroretinography standards of the International Society for the Clinical Electrophysiology of Vision (ISCEV) are needed to explore retinal involvement in the occurrence of visual hallucinations.