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Engaging in a firefight, along with other combat stresses, could lead to long-term changes in the connections between the midbrain and prefrontal cortex. Photo by U.S. Army Sgt. Matthew C. Moeller, 5th Mobile Public Affairs Detachment

Some soldiers who serve in Afghanistan or other war-torn countries return home with visible injuries: concussions, broken bones or amputated limbs. Many others, though, suffer from injuries we can’t visibly see. The daily strain of being exposed to armed combat, enemy fire and unpredictable explosions can lead to a range of behavioral symptoms, including fatigue, slower reaction times and a difficulty in connecting to one’s immediate surroundings.

A new study of soldiers returning home from Afghanistan, published today online in the Proceedings of the National Academy of Sciences, hints at the underlying cause for these behavioral changes. Researchers from the Netherlands and elsewhere used neurological exams and MRI scanning techniques to examine 33 soldiers before and after a four-month deployment in NATO’s International Security Assistance Force, and compared them to a control group of 26 soldiers who were never deployed.

The results were sobering—and indicate that a relatively short period of combat stress can alter an individual’s neurological circuitry for a long time.

As compared to the pre-deployment baseline tests and the control group, the returning soldiers’ brains showed distinct differences, despite the fact than none had suffered physical injuries and only one had exhibited enough symptoms to be clinically diagnosed with post-traumatic stress disorder. A pair of different techniques using MRI—diffusion tensor imaging, which measures the diffusion of water in the brain, indicating tissue density, and fMRI, which measures blood flow in various parts of the brain—revealed that the soldiers’ midbrains had reduced tissue integrity and showed less neuron activity during a working memory task.

Working memory is related to sustained attention, the researchers note, which could explain the results of the study’s neurological performance tests. As part of the tests, the soldiers were asked to complete a complex, mentally draining task known as a dot cancellation test. When compared to the other groups, those returning from combat committed more errors in the task over time, indicating a reduced ability to pay sustained attention. On an individual basis, participants with a greater reduction in midbrain activity were more likely to be error-prone in completing the dot cancellation test.

Both of these changes appeared when the soldiers were tested six weeks after combat, but mostly disappeared when they returned for a follow-up another 18 months later. However, a related change in the soldiers’ neurological makeup—a reduction in connections between their midbrain and prefrontal cortex—persisted in the follow-up, nearly two full years after exposure to combat was over. This is good reason, the researchers feel, to suggest that combat stress can alter the brain over the long term, and perhaps alter other areas of the brain as well.

“These results suggest that the human brain can largely recover from the adverse effects of stress,” they write in the study. “However, the results also reveal long-term changes that may increase vulnerability to subsequent stressors and lead to long-lasting cognitive deficits.”

Other researchers have examined how acute periods of stress can alter brain chemistry. Many believe that sudden bursts of hormones associated with stress, such as cortisol and norepinephrine, can permanently impair brain tissue.

Of course, lab studies can test returning soldiers’ ability to pay sustained attention to a task for several minutes, but whether combat has affected their ability to navigate social situations or make long-term decisions is another question entirely. The researchers involved, though, note that we should consider the possibility.

“The persistent changes in mesofrontal connectivity may increase the vulnerability to subsequent stressors and promote later development of difficulties with cognitive, social and occupational functioning,” they write. What soldiers see in combat, it seems, can stay with them when they come back home.