The amygdala. (Photo: Shutterstock)

After the Boston Marathon bombings in April 2013, a group of psychology researchers was poised to make the best of a tragic situation. They had recently mapped teenagers’ brains and knew that their existing data, coupled with the attack, would allow them to make discoveries about PTSD.

The initial research—the part that happened before the attack—put 40 teenagers between the ages of 14 and 19 (the sample was two-thirds female) through an MRI scan that measured their brains’ responses to neutral images, like those of a button or chair, as well as to violent ones. The researchers measured the blood flow to the subjects’ amygdalae, and the teenagers also reported how they felt when they saw the various images. The researchers chose to examine the brains of adolescents, they wrote, because young people are in the “developmental period of highest risk for trauma exposure.”

After the bombings, researchers surveyed 15 of the original study participants, who had media exposure on the day of the attack, and examined their responses for indications of PTSD.

Even after controlling for known risk factors like age, sex, anxiety, depression, and prior trauma, the relationship between an active amygdala and PTSD persisted.

The results were published last month in Depression and Anxiety. The scientists found that people with a highly reactive amygdala—the brain’s fear center—were more at risk for PTSD following the traumatic event.

“We were not surprised,” says Katie McLaughlin, an assistant psychology professor who led the study out of the University of Washington, Boston University, and Harvard. She knew that people with PTSD show heightened amygdala activity when exposed to distressing stimuli. However, it has been difficult for experts to figure out “whether amygdala reactivity is a risk factor for PTSD or a consequence of trauma exposure.”

What did surprise her was how robust the association is between a reactive amygdala and PTSD symptoms. Even after controlling for known risk factors like age, sex, anxiety, depression, and prior trauma, the relationship between an active amygdala and PTSD persisted, explaining the majority of the study’s cases that developed PTSD symptoms after the Boston Marathon bombings.

Previous PTSD studies have imaged the brains of combat troops before they deployed, but this one took into account that terrorist attacks are more unpredictable than combat stress. Plus, military personnel tend to be a self-selected crew, so factors like genetics and personality might determine how trauma affects them. In short, studies of soldiers aren’t always applicable to the broader population. McLaughlin's study also controlled for prior exposure to violence and existing symptoms, which previous studies didn’t do.

More than two-thirds of Americans experience a traumatic event at some point in their lives, so it's good that experts are figuring out why only a minority get PTSD. That might lead toward figuring out how to prevent the disorder: If we understand people’s physical vulnerabilities to it, we can be better at intervening for those most at risk.

Rosie Spinks contributed reporting.