This was the major finding of a study from Sichuan University in Chengdu in China, which features this week at the annual meeting of the Radiological Society of North America in Chicago, IL.

The researchers — Dr. Youjin Zhao and co-author Dr. Su Lui — came to their conclusions after examining high-resolution magnetic resonance imaging (MRI) brain scans of 37 people with major depressive disorder (MDD), 24 with social anxiety disorder (SAD), and 41 people in good health (the controls).

They looked for differences in the gray matter of the brain, focusing specifically on the thickness of the cerebral cortex, a thick outer layer of tissue that is densely packed with neurons and deals with most of the brain's information processing.

MDD, commonly referred to as depression, is a serious medical illness characterized by persistent sadness and irritability. It is a leading cause of disease and injury worldwide.

Depression causes people to lose interest in things that they used to enjoy and find fulfilling; in some cases, the illness is so severe that it can be an enormous struggle just to get out of bed. It damages not only the individuals affected but also their families, friends, and communities.

In the United States, depression is a significant public health issue that affects more than 16 million people at any one time and costs the nation some $210 billion a year.

Depression and anxiety often occur together

SAD is an intense fear or anxiety of being judged or rejected in a social situation. The symptoms can be so debilitating that normal life becomes very difficult.

People with SAD struggle to find and keep friends and partners. There are around 15 million adultswith SAD in the U.S.

It is not unusual for people diagnosed with depression to also have an anxiety disorder, and the other way around. Nearly half of people diagnosed with depression are also found to have an anxiety disorder.

What do the results mean?

The researchers say it is not clear what their findings mean. For instance, another area of the cortex that appears to be thicker in MDD and SAD patients is the anterior cingulate cortex, which is linked to emotion.

It is possible, says Dr. Zhao, "that a greater cortical thickness may reflect a compensatory mechanism that is related to inflammation or other aspects of the pathophysiology."

But there could be another reason, she explains: "Greater anterior cingulate cortical thickness could be the result of both the continuous coping efforts and emotion regulation attempts of MDD and SAD patients."

The researchers also found that patients with SAD had structural differences in their "fear circuitry," and that the MDD patients had differences in the region that contains the "visual recognition network."

Changes to the visual recognition network might explain why people with MDD sometimes have difficulty with selective attention and working memory, says Dr. Zhao.

"The visual recognition network is involved in emotional facial processing, which is crucial for social functioning. Depression has been associated with structural alterations in these regions."

--Prof. Paolo Fiorina

The researchers note that their findings offer a starting point for further research, with larger groups of patients and healthy controls, together with "machine learning analysis," to find out more about how structural MRI might help to diagnose and help decide treatments for MDD and SAD.