The Salience Brain Network
Image credit: o8t
The more we understand the mechanisms behind depression, the more effective our prevention and treatment strategies can become. Nature published a major 2024 study of the neuroscience of depression that really advances our understanding of the bases of the disorder. This is one of the more exciting papers I recently came across, as it pulls us away from the old one broken brain region heuristic toward something more nuanced.
The core finding is that the frontostriatal salience network, the system involved in detecting salient stimuli, switching attention, and integrating our internal thoughts with the outside world is basically double in size (expanded nearly twofold) in people with depression relative to age-matched controls. Importantly, this is not because of physical growth in brain tissue. It is instead a functional expansion; the boundaries in this network have shifted to overlap more with the neighbouring regions involved in thinking and attention.
What is particularly interesting is that this expansion is stable and "trait-like". It was even seen in children years before they started to show depressive symptoms, which indicates that this might be the biological risk factor rather than just a consequence of the illness.
Additionally, people with depression often present with disruptions involving the default mode network (self-referential thinking and rumination), the salience network (attention and switching), and the frontoparietal/executive control network (focusing and cognitive flexibility). Precision functional mapping research shows the salience network to be nearly 2X expanded in most individuals with depression, moving into shared territories of the default mode and frontoparietal networks.
Given the salience network's role in attention switching and coordinating other large-scale brain networks, this stable incursion across the borderland may explain why people get so rigid in their thinking, as well as having difficulty slipping away from the negative loops and having decreased cognitive flexibility.
They used precision functional MRI (dense, individualised scanning) and found that this effect was repeatable in several samples and caused by network border shifts with three different modes of encroachment for different individuals.
This study gives a lot of evidence that depression is a disorder of brain networks and communication as opposed to just a single brain region. Although more research is needed to know whether these network changes are causal, compensatory, or a marker of pre-existing vulnerability, some recent studies are already exploring this area.
I definitely find this network perspective promising for understanding and diagnosing depression, and what is really encouraging is that researchers are continuously discovering more about depression that helps people suffering from it get treated more effectively.
Reference
Lynch, C. et al. (2024). Frontostriatal salience network expansion in individuals with depression. Nature https://doi.org/10.1038/s41586-024-07805-2
