Depression has been shown to alter the structure of the brain's white matter, which contains the circuitry that allows brain cells to communicate with each other, and which underpins brain function.
So concludes a major study from the University of Edinburgh and the University of Glasgow, both in the United Kingdom, that used cutting-edge imaging technology to map the structure of white matter in the brains of more than 3,000 people.
In the journal Scientific Reports, the researchers describe how they found that white matter integrity - that is, a measure of the quality of the brain's white matter - was reduced in participants who reported symptoms of depression, compared with unaffected participants.
They suggest that the findings should help us to better understand the biology of depression and improve its diagnosis and treatment.
Previous studies have linked disruption in white matter to problems with thinking and emotion processing.
Depression is not the same as the mood changes and short-lived emotional reactions that we have to everyday challenges. It is a common illness that can endure, cause great suffering, and damage people's lives at school, at work, and at home.
Globally, depression affects more than 300 million people. It is the leading cause of disability worldwide, and the second leading cause of death in people aged 15 to 29.
Reduced white matter integrity
White matter, which makes up half of the human brain, comprises millions of bundles of nerve fibers, or axons, that link together neurons in different brain regions. In this sense, it can be viewed as containing the circuits that underlie the various functions of the brain.
As imaging technologies advance, scientists are discovering more and more about the role of white matter and how its alteration might affect normal brain function and contribute to psychological disorders.
For the new study, the researchers used a cutting-edge imaging technology called diffusion tensor imaging (DTI) to map white matter structure in the brains of 3,461 participants from the UK Biobank, a nationwide project that is gathering health data on over half a million people.
DTI is a type of MRI that is showing promise as a way to characterize tiny changes in brain structure to track disease progression and treatment. DTI creates a 3-D map as it follows the diffusion of water in brain tissue.
The DTI results showed that the participants with depression had reduced white matter integrity when compared with the participants without depression.
This reduction was seen globally as well as in certain "tracts," or bundles of axons, namely "in two of the three categories of tracts," and also "in individual tracts."
In their paper, the researchers note that previous investigations into links between depression and gray and white matter structure have produced inconsistent results.
Researcher Dr. Heather Whalley, senior research fellow in the Division of Psychiatry at the University of Edinburgh, says that their study uses "data from the largest single sample published to date and shows that people with depression have changes in the white matter wiring of their brain."
A study with such a large sample would suggest that the findings are likely to be regarded as very robust.
This is supported by meta-analyses that have pooled data from previous studies that have found links between depression and "reduced white matter integrity in several brain regions."
However, the researchers note that a limitation of their study is the fact that the participants were screened for depression based on self-reported symptoms and not in an interview using formal criteria.
"There is an urgent need to provide treatment for depression and an improved understanding of it[s] mechanisms will give us a better chance of developing new and more effective methods of treatment. Our next steps will be to look at how the absence of changes in the brain relates to better protection from distress and low mood."
Dr. Heather Whalley