Visual hallucinations are perceptions without a physical stimulus to relate this percept too. It affects millions of people, yet surprisingly little is known about what’s happening in the brain during visual hallucinations. Marouska van Ommen and co-authors published a paper in the journal “Schizophrenia Bulletin” that describes a possible mechanism: a complete absence of, or strongly reduced, activity in the primary visual cortex, while the remainder of the vision-related brain networks remain active and communicating with each other. This lack of information from the primary visual cortex leads conscious perception away from reality and towards “images” generated by the brain itself giving rise to hallucinations.
Van Ommen and colleagues came across this mechanism by using an MRI scanner to measure the brain activity of patients known to suffer from frequent visual hallucinations. Whenever they experienced a visual hallucination, the patients pressed a button. This allowed the researchers to determine which brain areas were more active or (unexpectedly) quiet during the visual hallucinations. Besides providing fundamental insights, the work has clinical relevance: visual hallucinations could be treated by enhancing the signals in the primary visual cortex to ensure they reach the vision networks. As such, increasing light levels or watching videos or pictures may diminish visual hallucinations. Want to know more? Find the article here.
The van Ommen-model for the cause of visual hallucinations: a disconnected V1. In this model, complex visual hallucinations are the result of a dissociation of higher-order visual processing areas from the primary visual cortex (V1). This dissociation manifests itself by an absence of activity in the primary visual cortex. Simultaneously, there is a looping of brain activity across the (other) vision-related functional networks on memory and attention*. Consequently, this biases conscious visual perception away from information derived from the outer world and towards internally generated percepts, i.e. visual hallucinations.
*These networks include the outside-world focused Dorsal Attention Network (DAN), the inner-world focused and memory-related Default Mode Network (DMN), and the saliency-focused Ventral Attention Network (VAN), which functions as a switch between DAN and DMN. Figure created with BioRender.com. Image credit for the featured image of the post: Bruce Rolff/Shutterstock.com.