How Can We See Things That Are Not There?: Current Insights into Complex Visual Hallucinations

It is not at all uncommon to see people, animals, or objects that other people cannot perceive. Data from studies of pathological hallucinations suggests that distributed functional changes within visual and associated systems increases the risk of visual hallucinations, though how this occurs is not yet clear. Candidate theories developed in the context of neurodegenerative disorders, eye disease, and psychosis each emphasize specific aspects of dysfunction within visual systems. Outwith these, there are suggestions that mnemonic and executive systems may play a key role for some people; particularly in those without organic disease. Shifts within dynamic neural networks may explain why some people are at a raised risk of visual hallucinations, and why specific hallucinatory episodes occur. This hypothesis has highlighted limitations in methods for modelling and measuring dynamic brain function. Developments in functional imaging, novel interventional techniques such as transcranial magnetic stimulation, and new methods for analysing complex imaging data such as multi voxel pattern analysis and graph theory, together with advances in theoretical computational models of hallucinations, raise hopes for a better understanding of the brain changes associated with these experiences.