You are sitting on the train, looking out of the window, caught up in deep thought. You are probably daydreaming or reflecting on yourself. At that moment, the Default Mode Network (DM-network) is active. This network activates when we do nothing or engage in self-reflection. It consists of several brain regions (including the medial prefrontal cortex, the posterior cingulate cortex, the precuneus, and the temporal lobes) that interact with each other.
Several studies have shown that the connections within the DM-network are weaker in people with schizophrenia than in other people. Schizophrenia is a mental illness involving delusions, hallucinations, disorganization, and cognitive problems. At first, the explanation for the weakened DM-network in these people was mainly sought in genetic predisposition, but recently, another factor has been found to play an important role.
Researchers investigated the connectivity of the DM-network by making fMRI scans of both schizophrenia patients and "healthy" people. What did they find? Not only did the people with schizophrenia show a weakened DM-network, but also, some of those without schizophrenia showed the same trend. It turned out that a large proportion of those with weakened connectivity had experienced trauma in childhood. So, a weakened DM-network does not seem to be specifically associated with schizophrenia, but with childhood trauma.
Why is that the case? As a child, your brain is still developing and therefore more susceptible to changes. If you experience traumatic events like abuse, this leads to hormonal reactions that affect the brain for the rest of your life. These permanent changes increase the risk of mental disorders such as schizophrenia.
The DM-network is associated with internal processes (daydreaming, thinking about yourself, etc.). When this network is disrupted, it can become more difficult for the brain to distinguish between internal processes and the environment. Because the boundary between the internal and the external world is less clear, internal cognition can sometimes be processed incorrectly by sensory association areas in the brain. This means that the brain processes this internal cognition as if it came from the external world. This causes hallucinations (perceiving things that are not there), a prevalent symptom of schizophrenia.
To summarize, changes in connectivity in the DM-network are not unique to people with schizophrenia or other mental disorders. An important underlying cause of a weakened DM-network is a traumatic childhood. But although most people with schizophrenia have had a traumatic childhood, most people with a traumatic childhood do not have schizophrenia. This shows that other factors also play a role in the development of schizophrenia. As is often the case in neuroscience, we do not yet know exactly all the ins and outs of the workings of the DM-network in relation to schizophrenia, but we keep getting closer.
Author: Pauline van Gils