Voices in the Brain: Disruption of the Default Network Linked to Hallucinations

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Hearing voices that are not there was once thought to be a core symptom of schizophrenia and signify severe mental illness. It has been increasingly acknowledged that people with no other symptoms of schizophrenia or psychosis do hear voices [1]. There are several theories about what causes these auditory hallucinations. For example, there is good evidence that hearing voices can be linked to trauma [2]. Attempting to understand the phenomenon is not only scientifically interesting but also relates to how society views people with these experiences and if and how they may be treated. A search for how these experiences are associated with brain function is ongoing. New research published in the journal Human Brain Mapping suggests that the predisposition to hearing voices may relate to abnormal functioning of the brain whilst at rest [3].

The “default mode network” or DMN (also known as “default network” and “task-negative network”) refers to the activity of a network of brain regions that occurs during a wakeful resting state. When we are at rest we may be engaged in introspection, remembering, day-dreaming or other internal, unobservable activities, so it is difficult to know exactly what the DMN actually captures. However, in this state, activity is seen in several areas simultaneously, suggesting that these areas are connected. Research into DMN has been active for some years. Indeed, the first high-resolution structural connection map of the human cerebral cortex and DMN was published in 2008.

Which parts of the brain are in the DMN?

The DMN encompasses of a number of brain regions: the ventral medial prefrontal cortex, posterior cingulate, inferior parietal lobule, lateral temporal cortex, dorsal medial prefrontal cortex and hippocampal formation are all included [4]. This list of obscurely named areas of the brain covers a range of regions. Translated, the DMN includes a part of the front of the brain — the ventral medial prefrontal is at the lower part of the front of the brain towards the inside and is linked to decision making. The posterior cingulate is a back part of the limbic lobe, associated with autobiographical memory, which is memory concerned with the recollection of personally experienced past events. The inferior parietal lobule is towards the back of the brain towards the top of the head and is linked to interpretation of sensory information and emotions. The lateral temporal cortex is towards the front of the brain and is linked to knowledge about concepts. The dorsal (top) medial (in the middle of) prefrontal (at the front) cortex is linked to judging errors. The hippocampal formation is concerned with memory and attentional control and is found towards the middle of the brain, towards the base.

The DMN is not just about what areas are involved, the crucial issue is that these areas seem to have similar levels of activity and be in a network together. Neuroscience is interested not only in ideas about which parts of the brain do what functions but also in how the activity in the areas links up and how parts of the brain communicate with one another to create consciousness, behaviour, thoughts, feelings and so on.

The DMN refers to the connected activity of different subsystems within the brain, occurring “at rest”. The thing is, no-one is quite sure what is happening when the DMN is active; it isn’t physical movement or active task completion, but seems to include autobiographical memory, thinking about the future and thinking from others’ perspective. Research is showing that a disrupted DMN may be linked to experiences such as hearing voices.

Aberrant functioning of the DMN

Scientists in the Netherlands collected data from 29 healthy people who hear voices, termed auditory verbal hallucinations, and 29 control individuals who do not using functional magnetic resonance imaging(fMRI) brain scans. The people who hear voices showed greater activity in the temporal cortex regions and posterior cingulate — both parts of the DMN. The crucial element is that this greater level of brain activity in people who hear voices compared to those who do not was observed “at rest”. The authors suggest that a predisposition to hear voices may be linked to this different functioning of the DMN. The DMN appears more active in people who hear voices. The area of greater activity is the posterior cingulate. This area is thought to be the “hub” of the DMN. It joins together the parts of the network that are linked to memory processing and the parts of the network that are linked to self-referential thought. Researchers suggest that people hearing voices may have memories linked up with thoughts about themselves in a manner that creates the experience of a voice. Such voices are thought to be self-generated and the increased activity in the DMN suggests stronger links between these functional areas of the brain whilst doing tasks.

As always, there are limitations with the study. There are complex issues with these types of brain imaging techniques and the way in which the activity is inferred. Each computation of data requires a judgement of the best technique and no single way is perfect. The average connectedness between brain regions was several magnitudes greater in those who heard voices; however, the extent of real world meaning remains unclear. The study used people who hear voices but had few other symptoms, therefore any suggestion that this is what occurs for people who have psychosis are limited. There may be other brain differences in people with psychosis who have other experiences including low mood or anxiety about their safety. Finally, hearing voices and other such experiences may be caused by brain activity or may simply be played out in the brain. If I step on a pin, the activity in my brain will signal the sensation of pain, however my brain is not to blame for the pain. It is important that we remember that people’s experiences relate not only to their internal world but also the external world.

References

1. Johns and van Os. The continuity of psychotic experiences in the general population. Clin Psychol Rev. 2001 Nov;21(8):1125-41.
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2. Hardy et al. Trauma and hallucinatory experience in psychosis. J Nerv Ment Dis. 2005 Aug;193(8):501-7.
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3. Lutterveld et al. Network analysis of auditory hallucinations in nonpsychotic individuals. Hum Brain Mapp. 2013 Feb 21. doi: 10.1002/hbm.22264. [Epub ahead of print]
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4. Buckner et al. The brain’s default network: anatomy, function, and relevance to disease. Ann N Y Acad Sci. 2008 Mar;1124:1-38. doi: 10.1196/annals.1440.011.
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