Researchers have discovered biomarkers that may explain why symptoms of post-traumatic stress disorder can be so severe for some people and not for others.
The study of combat veterans who have experienced intense events shows that those with severe symptoms of PTSD have distinct patterns of neurological and physiological responses affecting associative learning—the ability to distinguish between harmful and safe stimuli in the environment.
“We are shedding new light on how people learn fear and unlearn it,” says study coauthor Ilan Harpaz-Rotem, an associate professor of psychiatry at Yale University.
Some people who experience trauma exhibit few or limited side effects after the event. The research team wanted to explore why others suffer greatly.
“There were pronounced differences in the ‘learning rates’ of those with severe symptoms and those without symptoms…”
Researchers gave veterans who had experienced intense events during combat deployment fMRIs and tested them for physiological responses while presenting them with pictures of two different faces. In classic fear-conditioning tests, the subjects received slight electric shocks after viewing one of the faces, but not the other. Later, researchers switched the faces that accompanied the shock in an attempt have subjects “unlearn” original fear conditioning and test their ability to learn that something new in the environment is dangerous.
Using computational modeling, researchers found that two areas of the brain—the amygdala and striatum—were less able to track changes in threat level in those with severe PTSD symptoms.
“There were pronounced differences in the ‘learning rates’ of those with severe symptoms and those without symptoms,” Harpaz-Rotem says.
Highly symptomatic individuals tended to overreact to a mismatch between their expectations and what they actually experienced, he says. A garbage can in a war zone might contain an explosive device, he explains, but those with severe PTSD symptoms have a more difficult time unlearning the fear in civilian life than those with less severe symptoms.
“The findings of this study provide new and innovative understandings of the neurobiology of PTSD and a better understanding of learning processes in this population that might be useful in the future to refine treatment for the disorder,” says coauthor Ifat Levy, associate professor of comparative medicine and neuroscience.
The findings appear in the journal Nature Neuroscience. The National Institute of Mental Health and the National Center for PTSD primarily funded the study.
Source: Yale University
