A Dutch study of soldiers who have experienced combat stress has found severe stress reduces midbrain activity and has adverse effects on concentration, but most combat-related changes in that region normalize with time, according to the results published online Sept. 3 in Proceedings of the National Academy of Sciences.

Despite the recovery of midbrain structure and function, a reduction in functional connectivity between the midbrain and prefrontal cortex persisted at long-term follow-up, according to authors Guido A. van Wingen, PhD, of Radboud University, Nijmegen, the Netherlands, and colleagues.

“Taken together, these results suggest that the human brain can largely recover from the adverse effects of stress, supporting the view that neural plasticity in response to prolonged stress is adaptive,” they wrote. “However, the results also reveal long-term changes within the mesofrontal network that may increase the vulnerability to subsequent stressors and lead to long-lasting cognitive deficits.”

Findings were based on a prospective longitudinal study involving 33 healthy NATO soldiers who underwent a combination of functional MRI (fMRI), diffusion tensor imaging (DTI) and neuropsychological testing before and after a four-month deployment in Afghanistan. Most were evaluated a total of three times: a baseline test before deployment, short-term follow-up 1.5 months after returning from deployment, and a long-term follow-up an average of 1.6 years after deployment. The authors noted that nine participants in the combat group did not complete long-term follow-up.

In addition to the combat group, 26 healthy soldiers who were never deployed were evaluated at similar times and served as a control group.

“Combat stress reduced midbrain activity and integrity, which was associated to compromised sustained attention,” wrote the authors. These alterations then normalized within 1.5 years in soldiers without psychiatric complaints, but midbrain-prefrontal cortex coupling alterations remained. The authors also noticed a similar pattern with regard to amygdala reactivity, which also normalized by long-term follow-up while changes in amygdala connectivity with the dorsal anterior cingulate cortex persisted.

“This finding could imply that local subcortical networks are highly plastic and readily adapt to ongoing environmental changes, whereas alterations in long-ranging cortical-subcortical networks are resistant to recovery.”

The persistent changes in mesofrontal connectivity could make soldiers more vulnerable to subsequent stressors and “promote later development of difficulties with cognitive, social, and occupational functioning,” such as those observed in patients with post-traumatic stress disorder, according to the authors.