Hallucinations, the perception of things that are not there, are a core symptom of psychotic disorders. However, little is known about the underlying mechanisms of visual hallucinations (VH), which hinders the development of targeted therapies. Van Ommen’s doctoral research aimed to better understand these mechanisms. She concludes that visual hallucinations (VH) arise from a lack of communication between the brain region where visual signals from the external world enter (V1) and the brain networks that further process these signals. Van Ommen had patients with VH, patients without VH, and healthy controls perform various perception and attention tests. Additionally, she utilized functional Magnetic Resonance Imaging (fMRI) to investigate their brain activity. Approximately 37% of patients with a psychotic disorder experienced VH, often involving the perception of people, human-like beings, and animals. The content of the hallucinations ranged from “normal” to “bizarre.” Patients with a psychotic disorder exhibited poorer visual perception and attention compared to healthy controls. The fMRI results indicated reduced activity in the visual cortex of patients with VH during visual perception. Additionally, the brain networks involved in processing visual signals exhibited weaker connections. Remarkably, during VH, the visual areas were active, with the exception of V1. These findings suggest that VH arises from a lack of communication and cooperation between V1 and the brain networks that typically process these signals. Based on these findings, targeted treatments can be designed to reduce VH in this patient group.