Combat stress produces partially persistent changes to midbrain-prefrontal cortical circuitry and cognition
While acute short-lasting stress can be beneficial, such as when pushing to meet an important and potentially rewarding deadline at work, prolonged strong stress is known to cause cognitive impairments such as memory deficits. The precise nature and loci of anatomical and functional alterations due to chronic stress, and the extent to which they are (ir)reversible, constitutes an important topic in cognitive neuroscience that is being increasingly investigated.
In their recent follow-up study, van Wingen et al. (2012) investigated 33 healthy soldiers, using neuropsychological tests as well as functional and diffusion magnetic resonance imaging, first before a four-month combat-zone deployment to Afghanistan as a part of the North Atlantic Treaty Organization International Security Assistance Force peacekeeping operation. Then, follow-up studies were conducted 1.5 months (short-term) and 1.6 years (long-term) after the deployment. As a control group, they investigated 26 healthy soldiers who were not deployed at similar time intervals. During deployment, the combat group was exposed to typical combat zone stressors, such as armed combat, combat patrols, exposure to enemy fire, as well as risk of exposure to improvised explosive devices.
At the 1.5-month short-term follow up, midbrain activity was reduced in the combat group, including area containing substantia nigra. Functional connectivity between the midbrain area and lateral prefrontal cortex was also weakened. Combat stress further reduced fractional anisotropy and increased mean diffusivity in the midbrain areas, suggesting weakening of anatomical connectivity. Notably, these measures correlated with reduced performance in a sustained attention task. At the time of the long-term 1.6-year follow-up, the other deficits had normalized, but the reduced functional connectivity between the midbrain and prefrontal cortical areas persisted. The authors conclude that these persistent changes may increase the vulnerability to subsequent stressors and promote later development of difficulties with cognitive, social, and occupational functioning. More generally, these findings also provide important information about neurocognitive deficits that may develop when an individual is exposed to severe chronic stress in other types of context.
Reference: van Wingen GA, Geuzed E, Caan MWA, Kozicza T, Olabarriagah SD, Denysb D, Vermettend E, Fernándeza G. Persistent and reversible consequences of combat stress on the mesofrontal circuit and cognition. Proc Natl Acad Sci USA (2012) advance online publication. http://dx.doi.org/10.1073/pnas.1206330109