According to a new study, a key brain structure that regulates emotions - known as the amygdala - works differently in preschoolers with depression when compared with their healthy peers.

These differences were measured using functional magnetic resonance imaging (fMRI) - and study researchers believe that these clear changes in brain function of very young depressed children likely mark the beginnings of a lifelong problem.

For this study, scientists from Washington University's Early Emotional Development Program studied 54 children aged 4-6 years. Before the study began, 23 of these kids had been diagnosed with depression, but none of them had ever taken any antidepressant medication.

Study authors used fMRI to measure brain activity by monitoring blood flow in the brains of these children. While they were in the fMRI scanner, they were shown pictures of people whose facial expressions conveyed emotions such as happiness, sadness, fear or a neutral expression.

Although studies using fMRI to measure brain activity have been used for years, this is the first time that such scans have been attempted in children this young with depression.

The amygdala of depressed children showed increased activity compared with their healthy peers when they viewed any pictures of people's faces, regardless of the emotions they saw on the faces.

Not only that, greater amygdala activity in depressed preschoolers was also associated with their parents reporting more sadness and emotion regulation difficulties in their children.

Study authors believe this may be because depression affects the amygdala by exaggerating what, in other children, is a normal amygdala response to both positive and negative facial expressions.

But they agree that their results are preliminary and much more research will be needed to conclusively prove this theory.

This study suggests that the brains of depressed preschoolers may show an exaggerated version of a normal developmental emotional response at a very young age - one that can hopefully be reversed with proper prevention or treatment in the future.