When ISIS terrorists attacked Paris on Nov. 13, they took 130 lives and wounded hundreds more. But their bloody acts may have injured far more people -- both survivors and the countless numbers who experienced the event vicariously through horrific media images -- in an insidious way, by causing potentially harmful changes in their brains.
Indeed, terrorism's effect on the brain is so powerful that those who survive attacks have significantly higher rates of post-traumatic stress disorder than those who make it through accidents or natural disasters. But terrorism also is linked to long-term problems such as anxiety and alcohol abuse in people who only have secondhand exposure to the event by watching it on television.
Recent research shows fear of future terrorist attacks can alter brain chemistry in a way that increases your risk of dying eventually from a heart attack or other ailments that might seem unrelated to the violence.
The latter study, published in 2014 in Proceedings of the National Academy of Sciences, looked at more than 17,000 Israelis, who live in a country where terrorist attacks are a frequent occurrence.
The subjects who had the most fear of terrorism tended to have resting heart rates that were 10 to 20 beats per minute faster than the norm, an indicator of increased risk for heart attacks and strokes. The elevated heart rate was linked to a change in brain chemistry. Blood tests revealed a decline in the function of acetylcholine, a neurotransmitter involved in responses to stress and which acts as a brake to the inflammatory response.
"Our brain reacts to acute stress situations by a rapid burst of acetylcholine release," explained study co-author Hermona Soreq, a professor of molecular neuroscience at the Edmond and Lily Safra Center for Brain Sciences at Hebrew University in Jersusalem.
"The brain sends the acetylcholine to body tissues through the vagus nerve; Since acetylcholine blocks inflammation responses, too much of it weakens the immune system. We found lower prospects to survive in patients after heart attack whose blood tests show weakened capacity to destroy acetylcholine."
Soreq said that terrorism-induced anxiety also causes the production of small molecules called microRNAs, which block the function of numerous other genes, and can alter regulation of the nervous system.
Terrorism triggers mechanisms hard-wired into the nervous system by evolution, which enabled our ancient ancestors to escape from animal predators and rival human clans. The human brain and vision system is fine-tuned to spot things that we should be afraid of, and then react to them in a flash -- even without consciously realizing it.
In a 2012 study, for example, researchers from the University of Edinburgh and New York University trained subjects to fear certain pictures by giving them mild shocks. Some subjects were allowed to look directly at the images, while others only got to glimpse them in one eye while researchers flashed a colorful image in the other eye to interfere with conscious perception.
Even with that hindrance, the subjects who got the one-eyed glimpse actually developed a fear response more quickly.
When a person spots danger -- say, a gunman who bursts into a theater -- the alarm is sounded in the amygdala, a sort of biological alarm system that triggers the body's response. It directs glands to release an array of chemicals such as adrenalin and cortisol, which shift the heart, lungs and muscles into high gear. The senses shift into narrowly focused hyper-awareness of information that might aid in survival.
A study published in Psychological Science in 2009, for example, found that fear made subjects see coarser lines, which help the brain to evaluate movement and distance, more clearly, while they couldn't make out fine lines as well as usual.
After a threat is over, in many cases, a survivor's brain gradually can shift back into normal operating mode. But not always. Studies have shown that between 28 and 33 percent of survivors of mass shootings and 34 percent of bombing survivors develop post-traumatic stress disorder -- far higher rates than people involved in terrifying accidents or natural disasters.
Survivors continue to relive the trauma and often suffer from symptoms such as irritability, difficulty concentrating, hypervigilant watchfulness, and an exaggerated startle response, and sometimes feelings of numbness.
Recent research by German scientists suggests that cortisol, one of the chemicals that the body releases in an effort to survive a terror attack, not only helps burn the event more vividly into a person's memory, but also helps to keep it vivid even after it is retrieved repeatedly.
But it's not only immediate survivors whose brains may be affected by terrorism. A study published in the New England Journal of Medicine in 2001 found that after the 9-11 attacks, 44 percent of U.S. adults and 35 percent experienced one or more substantial symptoms of stress, such as difficulty concentrating, repeated memories or dreams of the event, irritation and angry outbursts. The level of stress that people experienced was linked to the extent that they had watched TV coverage of the attacks.
Those effects sometimes last for years. Another study, published in 2008 in Journal of the American Public Health Association, looked at the effect of 9-11 on the mental health of office workers in Chicago, far away from the actual site of the attacks in New York and Washington.
University of Illinois at Chicago researchers found that subjects who had fears and beliefs shaped by the attacks in 2003 tended to have problems such as increased depression, anxiety, hostility, post-traumatic stress and drinking in a followup survey two years later.
Judith A. Richman, a UIC professor of epidemiology in psychiatry who led the study, said she suspects that the recent attacks in France could be having similar effects upon Americans' mental health.
"For people watching the news, and reading about the threats to Washington and New York that followed the Paris attacks," she said, "this all may have revived their anxiety."