background Our knowledge about the neurobiological correlates of positive visual perceptual disorders is increasing rapidly. If we are to understand and integrate this emerging knowledge, the nomenclature needs to be clear and unambiguous.
aim To provide an overview of the different classifications of visual hallucinations to reach a better understanding of new neurobiological views in these phenomena.
method Relevant data were obtained from books, PubMed, Embase, and the Cochrane Library.
results The results are presented in the form of various classifications of visual hallucinations, grouped according to the following guiding principles: shape, size, content, relation to the sleepwake cycle, co-occurrence with percepts in any of the other sensory modalities, and association with neurobiological correlates. In addition, a classification system for visual illusions and distortions is presented.
conclusion The overview emphasizes how important it is to reappraise the concepts of positive visual perceptual disorders that were developed during the era of classical psychiatry. By becoming familiar with these concepts we will find it easier to design, execute and interpret neurobiological studies of these frequently occurring phenomena.