Research shows that changes in the brain appear before the first symptoms of depression appear. The discovery helps predict how the brain's internal communication changes before symptoms of depression appear. Using a brain scan called fMRI, researchers found that depression affects an important part of the brain related to motivation and attention.
The changes were detected before symptoms of depression developed, meaning the researchers were able to predict who would develop the illness and who would not.
“The key finding is that there is an expansion of the cortical area occupied by the brain network. Until now it was not known that the clinical conditions of depression can expand brain networks,” said neuroscientist and psychiatrist Jonathan Royser, an expert at University College London, United Kingdom, who was not involved in the study.
The study, published in the scientific journal Nature in early September, examined the brain activity of 141 patients with depression and 37 people without the disease. The goal was to discover how the disease changes the way brain regions communicate with each other.
“We typically look at the brain in terms of how its parts communicate with each other, as if they were all on a group phone call. The question is which region you're talking about and which network you're a part of.” —Flügge, cognitive neuroscientist at the University of Oxford in the United Kingdom, which was not involved in the study.
The researchers found that the so-called “frontostriatal salience network” was expanded in participants with depression compared to a group without the illness. This network is important for directing attention, focusing on relevant stimuli entering the brain, and regulating emotional responses.
“It is not yet known exactly what this network does, but it is already known to be important for psychiatric symptoms, including depression and anxiety,” Roeser said.
Salience network expansion predicts depression
The evidence of a greater network salience found in the study was considered a strong indicator that could predict whether people would develop depression later in life. The researchers found that this network had already expanded in a group of 10- to 12-year-old children who later developed depression in adolescence.
The discovery is “exciting and extremely rare,” Klein-Flück said. For the study, researchers measured participants' brain activity over a long period of time, both when they were healthy and when they were sick.
The study also showed that the strength of the salience network was associated with some symptoms of depression, particularly symptoms related to loss of pleasure and motivation.
According to Emily Hird, a neuroscientist at University College London, it is not yet possible to infer from the study data whether the changes in the salience network are related to any specific psychological experience or depressive thoughts.
The study did not compare brain activity with participants' symptoms or thoughts, it only looked at the “resting state.”
Instead, salience network remapping is “a trait, a risk marker that can help identify people who are vulnerable to developing depression in the future,” Hurd said.
Brain networks are remodeled in depression
But if the salience network expands in people with depression, how exactly does it expand? According to Roiser, the network has been remodeled to include areas of the brain that are not normally involved in the normal network, including areas important for depression.
“They show that the salience network permeates other regions of the brain, including one that we know plays an important role in effort,” Roeser said. “This is very interesting because we know that among people with depression there is a reluctance to perform tasks that require effort.”
Roiser and Hird believe that ongoing research indicates that the known antidepressant effects of exercise may be related to altered activity in this effort network. “Exercise is very effective in depression, at least as effective as antidepressants or psychotherapy,” says Roiser.
Klein-Flück was surprised he didn't talk about a part of the brain called the amygdala, which is important for processing emotions. “This area of the brain has been at the center of depression research for decades. “This may not seem important, but we know from previous work that it is very important in depression.”
A new “biomarker” for depression?
Because the expansion of the salience network is so stable and predictable in people with depression, Klein-Flück suggested that it could be used as a potential new “biomarker” for the disease in the future.
A biomarker is a way doctors can measure to diagnose a disease or disorder in patients, like an antigen test for Covid. If so, the size or “expansion” of saliency networks measured in brain scans could one day be a biomarker of depression.
“Larger samples and replications of this work are needed to determine whether it can be used reliably to predict the likelihood of an individual developing depression,” Klein-Flück said.
However, Rosser is more skeptical. He doesn't think scientists will ever find biomarkers for depression. “I don't think depression is a homogeneous entity from a neurological perspective, so there won't be a biomarker for it.”
Instead, Roiser thinks of depressive symptoms as manifestations of different brain states. “How did the doctors think the swelling in the legs was dropsy? “We now know that dropsy is not a disease, but a manifestation of many different diseases,” he explains.
Roiser believes depression is similar. “Depressive symptoms can arise from complex interactions between different brain circuits that govern how we think, feel and behave, with different circuits generating symptoms in different people.”
