Depression has been linked to a larger-than-expected network of neurons that work together across parts of the brain, but not everyone is convinced that this network actually causes the condition.
Like many medical conditions, depression doesn鈥檛 have a single cause, but can be brought on by a traumatic life event, such as a bereavement, and has also been linked to .
鈥� such as the hippocampus, which is involved in memory, and the thalamus, which processes sensory information 鈥� than in people without the condition. But a review of 14 studies concluded that there was .
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Now, at Weill Cornell Medicine in New York and his colleagues have found that the condition may also involve a network of neurons across the brain that become activated together.
The researchers scanned the brains of six people who had been diagnosed with mild to severe depression for more than 10 hours across around 20 sessions. They found that the so-called salience network was twice as large in four of them compared with 37 people with no depression diagnosis, who were also scanned. Located in the cerebral cortex, the brain鈥檚 outermost layer, this network has been linked to attention and the ability to respond to stimuli.
Lynch and his team found similar results in three other groups, each made up of more than 40 adults, with and without depression. Finding the same results again has persuaded some other scientists that the link between a large salience network and depression is genuine. 鈥淚t鈥檚 quite convincing,鈥� says at the National University of Singapore. 鈥淚 think this is one of the best studies.鈥�
The researchers themselves acknowledged that not everyone with depression will have such an enlarged salience network, as demonstrated by two of the participants having a network that was similar to that of the controls. They are not currently able to explain this, says Lynch. The team didn鈥檛 find that depression severity, for example, was linked to a particularly large network. 鈥淲e are actively working on follow-up experiments to address this important issue,鈥� says Lynch.
For at the Australian National University in Canberra, a pertinent question isn鈥檛 just whether there is a link between variations in brain structure and depression, but also, among people with these traits, which came first? To get to the bottom of this, Lynch and his colleagues also analysed data from a previous study, made up of children who didn鈥檛 show signs of depressive symptoms when aged 10 or 12, but who went on to develop these symptoms within the next couple of years.
They found that even before these children developed these symptoms, they had a salience network that spread across 36 per cent more of their brain鈥檚 cerebral cortex, compared with children who hadn鈥檛 developed depression symptoms by the same age. 鈥淭his suggests to us that salience network expansion possibly predisposes somebody to depression, rather than being a consequence of depression itself,鈥� the researchers wrote in their paper.
鈥淚 think their evidence is really pretty strong that this is a pre-existing change that influences the risk of depression,鈥� says at the University of Pennsylvania. Reproducing the results in adults, combined with the analysis in children, makes this 鈥渁 very solid paper鈥�, she says.
But Espinoza Oyarce wants more research on larger groups of people before she is convinced. Researchers always want to establish whether one thing happens before another, but that can be difficult to do, she says. A third factor, such as stress, could also be involved, says Yeo. 鈥淪ocial factors might cause both an increase in the network size and depression,鈥� he says.
Getting to the bottom of this is important, as the size of someone鈥檚 salience network may one day help to predict their risk of depression, says Yeo. But Espinoza Oyarce and at the University of Sydney in Australia both say that measuring the network size is unlikely to ever be used to diagnose depression, given the cost and other impracticalities of brain scans.
However, all the scientists insist that research on the salience network should not be abandoned. For instance, we could one day show that the size of the network in someone with depression affects how they respond to treatment, says Hickie. 鈥淚 was certainly not expecting these results, and so it will I think make all of us scratch our heads and say, 鈥榳hat does this mean? Where do we go from here?鈥欌��, says Sheline.
Nature
