Eighteen months after they have returned from a war zone, soldiers bear an unmistakable sign of emotional trauma deep inside their brains. But in most, a key node of the brain’s fear circuitry returns to normal, perhaps keeping mental illness such as post-traumatic stress disorder (PTSD) from developing, says a new study published Tuesday in the journal Molecular Psychiatry.

The study, a follow-up to an earlier brain-imaging study conducted by Dutch researchers, put two groups of Dutch soldiers into a brain scanner called a functional Magnetic Resonance Imaging scanner, and had them look at pictures of people expressing anger or fear. One group of 23 soldiers was scanned just after returning from Iraq or Afghanistan. A second group of participating soldiers had not been deployed to any war zone.

In the first study, immediately following the first group of soldiers’ return, the two groups showed very different brain patterns in response to the angry and fearful faces. In the post-deployment soldiers, the amygdala, an almond-shaped region deep in the brain where fear and other highly charged emotional reactions are processed, became highly active when they looked at faces demonstrating fright or anger. In the non-deployed soldiers, the pictures did not elicit strong activity in the amygdala.

When the amygdala of a healthy, normal individual becomes highly active, suggesting a strong emotion such as fear, imaging studies like these usually show a sudden drop in activity in the dorsal anterior cingulate cortex--a brain structure that has been linked to emotional regulation. Once the fear has passed, that structure kicks back in--keeping us, perhaps, on an emotional even keel.

This was the pattern Dutch researchers saw consistently in non-deployed soldiers.

But immediately after returning from the war zone, Dutch soldiers showed an altered pattern in the brain scan: the dorsal anterior cingulate cortex tended to stay online even when the amygdala was working overtime, suggesting an ongoing struggle for control.

Eighteen months after returning home from a war zone, according to the study published Tuesday, deployed soldiers showed markedly less activation in their amygdalae in response to the fearful and angry faces---a sign that suggests the “hypervigilance” that soldiers rely on in combat situations (and which persists in people with post-traumatic stress disorder) had receded.

But in one key way, their emotional responses did not return to a normal push-pull pattern. These soldiers’ emotional regulation response--high activity in the dorsal anterior cingulate cortex--still did not automatically go quiet when the amygdala became active, or kick back in when the amygdala returned to its resting state. The authors suggested that this disturbed state may be a remnant of their stress response to combat--a scar that may heal in some and reopen in others, possibly leading to post-traumatic stress disorder.