Impaired visual processing in psychosis patients with a predisposition for visual hallucinations

Psychosis is frequently associated with the occurrence of visual hallucinations (VH), but their etiology remains largely unknown. While patients with psychosis show deficits on various behavioral visual and attentional tasks, previous studies have not specifically related these deficits to the presence of VH. This suggests that tasks used in these studies do not target the visual-cognitive neural mechanisms that mediate VH, which in turn limits the development of effective therapies. We therefore designed a study to target these mechanisms directly. In this case control study we asked patients with psychosis who had previously experienced VH to indicate when they recognized objects that were gradually emerging from dynamic visual noise, while scanning their brains using functional Magnetic Resonance Imaging. In a previous study, this recognition task was used to identify the neural basis of VH in patients with Parkinsons Disease. Based on this earlier work, we decided to test the following hypothesis: when compared to psychosis patients not experiencing VH and age-matched healthy controls, psychosis patients with VH show reduced occipital activity and frontal activity around the moment of recognition (known as pop-out). For all groups, neuroimaging revealed increased activity in all examined visual areas around pop-out. However, psychosis patients with VH showed reduced occipital responsiveness, especially in the inferior part of the bilateral lateral occipital complex, a region known to play a key role in object recognition. We did not observe altered frontal or prefrontal activity before pop-out in this group. A possible explanation is that the relatively sustained activation of the visual memory-related angular gyri around pop-out may have compensated for the impaired early visual processing in psychosis patients with VH. We discuss our results in terms of current theories of visual hallucinations, such as predictive coding and contextual modulation. Our study is the first to show that visual processing deficits contribute to the occurrence of VH in psychosis. These findings could be used to develop tests to identify the visual-cognitive mechanisms that mediate VH in this group.