To elicit powerful emotions and vivid memories, all it takes for many Americans is the mention of two numbers -- 9/11.
Nine years later, studies suggest, the terrorist attacks of September 11, 2001, continue to affect the way we think, remember and react to stressful situations. The actual trauma ended long ago, but for many people, measures of brain activity and body chemistry are different than they were before it happened.
While people who were closest to the attacks were probably affected most, the research suggests, the events of September 11 may have shaped the psyche of our nation in ways far more subtle than high-profile cases of post-traumatic stress disorder and other clinical disorders. Scientists are still trying to figure out how to interpret all the data they've collected.
"It makes sense that our experiences would affect our brains and bodies," said Barbara Ganzel, a neuroscientist who studies emotions at Cornell University in Ithaca, N.Y. "Now the question is: To what extent? It's something we need to know about so that we can find people who need help."
Plenty of attention has gone to PTSD and other medically classifiable reactions to war, terrorist attacks, and other traumatic events. But what about the people who suffer emotional trauma, yet still manage to cope and move on?
In one of her recent studies on reactions to 9/11, Ganzel and colleagues scanned the brains of people in this resilient group, between three and four years after they had experienced the attacks up close. When those people looked at pictures of emotion-filled faces, the researchers reported in 2008 in the journal NeuroImage, their brains looked different from the brains of people who were further away from the World Trade Center.
In particular, those who were nearby showed strong reactions in their amygdalas, the part of the brain that forms emotional memories. Their reaction to stress was also higher, with bigger spikes in levels of the stress hormone cortisol, the researchers reported in a different paper.
Yet another study, conducted seven years after the 1995 Oklahoma City bombing, found that survivors there had higher spikes in blood pressure and heart rate when remembering the attack compared to people who knew about it but weren't there. The survivors also had higher resting heart rates.
While it's not yet clear what all these findings mean, Ganzel suspects that physical differences in the bodies and brains of resilient survivors may affect how they respond to emotional events in their everyday lives, even if they never get diagnosed with a mental health disorder. She is currently looking into how well survivors of trauma regulate emotions and make decisions.
"We're not used to thinking about the long-term effects of stress in people who don't have a clinical disorder,” Ganzel said. "This is evidence that this may be indeed occurring. We need to know how, so we know how to help."
Psychiatric epidemiologist Judith Richman happened to be collecting data on the impacts of stress in the workplace in Chicago, when 9/11 happened. With three years of data under her belt, she saw the perfect opportunity to compare the mental health of people both before and after the attacks.
Her results, published in 2008 in the American Journal of Public Health, found that 9/11 continued to affect the mental health of our entire nation for at least four years. In particular, she and colleagues measured higher levels of anxiety and problematic drinking of alcohol in people after the event.
"I think what was particularly salient about 9/11 was that people were affected all over the nation even if they weren't personally affected," said Richman, of the University of Illinois at Chicago. "I'm in Chicago. One of my colleagues at the time said that when it happened, all he could do was stare at the Sears tower. He was expecting any minute that planes were going to crash into it."
Even if you sustain your mental health after an event like 9/11, your memories may be forever altered in unexpected -- and inaccurate ways, suggests the work of Elizabeth Phelps, a neuroscientist at New York University. She and colleagues followed more than 3,000 people in seven cities for up to five years after the attacks, asking them questions about what they remembered from that day.
Their results, published last year in the Journal of Experimental Psychology: General, showed, for one thing, that people were far more confident in their memories than they were accurate. Between the first week and the first year after the attacks, the study found, people altered nearly 40 percent of their "memories" about what happened that day. More emotional memories, like how they felt at the time, changed 60 percent of the time.
Rather than offering insight into trauma or stress, Phelps said, her study provides a real-life example of how our memories work -- and how they fail us, especially when emotions are running high. Decoding the nuances of how memories form and change after major life events, she added, could help not just with legal cases, but with our ability to understand history, on both a national and a personal level.
"We use our memories to rebuild our lives," she said. "If our beliefs in our memories are greater than their accuracy, that's useful to know."