Draining the pond and catching the fish: Uncovering the ecosystem of auditory verbal hallucinations

The various models proposed for the mediation of auditory verbal hallucinations (AVH) implicate a considerable number of brain areas and mechanisms. To establish which of those mechanisms are actually involved in the mediation of AVH, we developed a novel method to analyze functional MRI data, which allows for the detection of the full network of mutually interacting brain states, and the identification of those states that are relevant to the mediation of AVH, while applying a minimum number of preconceived assumptions. This method is comparable to the draining of a pond to lay bare the full ecosystem that affects the presence of a particular fish species. We used this model to analyze the fMRI data of 85 psychotic patients experiencing AVH. The data were decomposed into 98 independent components (ICs) representing all major functions active in the brain during scanning. ICs involved in mediating AVH were identified by associating their time series with the hallucination time series as provided by subjects within the scanner. Using graph theory, a network of interacting ICs was created, which was clustered into IC modules. We used causal reasoning software to determine the direction of links in this network, and discover the chain of events that leads to the conscious experience of hallucinations. Hallucinatory activity was linked to three of the seven IC clusters and 11 of the 98 ICs. ICs with the most influential roles in producing AVH-related activity were those within the so-called salience network (comprising the anterior cingulate gyrus, right insula, Broca's homologue, premotor cortex, and supramarginal gyrus). Broca's area and the cerebellar regions were significantly, but more distantly involved in the mediation of AVH. These results support the notion that AVH are largely mediated by the salience network. We therefore propose that the mediation of AVH in the context of schizophrenia spectrum disorders involves the attribution of an excess of negative salience by anterior-cingulate areas to linguistic input from Broca's right homologue, followed by subsequent processing errors in areas further 'downstream' the causal chain of events. We provide a detailed account of the origin of AVH for this patient group, and make suggestions for selective interventions directed at the most relevant brain areas.