Major depressive disorder (MDD) is accompanied by various physiological and functional changes in the human body and brain. Depression often starts in teenage years and affects young adults. While there are several treatments for this disorder, its complete causes and physical symptoms are not entirely understood. Now, a pilot study published in 2016 in BMC Psychiatry has found cortical thickness and emotion processing changes in depressed subjects compared to the control group.

The study, led by Bernice Fonseka from the University of Calgary, looked at 13 non-medicated participants and a control group of 14 participants. The active group contained adults with ages between 18 and 24 years with mild to moderate MDD. The premise of the study was that the emotional processing response observed in MDD may be caused by or correlated with changes in the thickness of the prefrontal cortex. This region is placed at the front of the brain, right behind the forehead.

The prefrontal cortex has many important functions, being involved in planning complex cognitive behavior, governing decision making and social behavior. In fact, it is responsible for putting our ideas into action and understanding the consequences of our activities. Moreover, it governs rule learning and abstract thinking.

Depressed people are known to have reduced accuracy in identifying facial expressions. Consequently, study participants were asked to look at black and white pictures showing faces with different expressions. They were supposed to answer by hitting a button whenever they identified an emotional expression or do nothing whenever the expression was neutral, and vice versa. The researchers measured the rate of false positives in the participants’ response to the stimulus. Then the researchers performed MRI brain scans on all participants to measure their cortical thickness.

Normally, our brains fire neurotransmitters and hormones to respond to stimuli, such as seeing faces. But this activity is also regulated by inhibitory or dampening substances that kick in when the stimulus is false. For example, when you confuse someone in the street with a friend, your brain responds by making you feeling happy or excited. As soon as the confusion is clarified, the excitement disappears, reaction which is given by neural inhibitors.

The study found that depressed participants were more likely to respond faster and more often than the control group when it came to sad, angry, or fearful faces, but had almost the same results in the case of happy faces.

Additionally, they thought more often than the control group that neutral faces were actually displaying some emotion, thus triggering false positives. Researchers also found that their left orbital inferior frontal gyrus was thicker than in the control group, along with eight other brain regions. The research suggests that depressed people process emotion differently and this is correlated with changes in the brain structure.