9/11 Study Offers Insight Into How Memories Are Formed

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He added that, based on the new 9/11 study, "we now have a physiological mechanism that can help explain why these shocking and emotionally arousing events might be associated with more intense recollection."

In the study, conducted in 2004, Phelps' team used functional MRI to observe the real-time brain activity of 24 adults who had been in Manhattan on Sept. 11, 2001.

The volunteers were divided into two groups: those in the "Downtown" group had been an average of two miles from the World Trade Center, while the "Midtown" group was farther away -- about 4.5 miles on average.

The researchers used specific word cues to elicit participants' memories, either of 9/11 or another event in 2001, such as a birthday or a summer vacation. Brain scans showed a major spike in amygdala activity for 83 percent of people in the Downtown group as they recalled 9/11. In contrast, just 40 percent of those in the Midtown group showed this kind of rise.

At the same time, the Downtown group showed a relative decrease in activity in another brain area -- called the posterior parahippocampal cortex.

The amygdala finding was expected, because psychologists and neuroscientists have long suspected that fear plays a key role in laying down traumatic memories.

"Remember, people who were downtown really thought they could be hurt," Spiegel said. "Now, of course, we know what the confines of the attack were, but the towers could have fallen over instead of imploding inward -- no one knew. We're talking about fear, a proximate life-threat, not just witnessing something."

Sensory input was probably key, too. While most Americans watched 9/11 on television, "individuals that were closest to the event had multi-sensory stimulation," noted Dr. Grant Mitchell, director of psychiatry at Northern Westchester Hospital Center in Mount Kisco, N.Y. "They experienced the event, they could feel it, see it, smell it, hear it. That's another factor involved in producing especially vivid memories."

Although none of the participants in the study had post-traumatic stress disorder (PTSD), the experts agreed that the findings could shed new light on that condition, which includes flashbacks and anxiety attacks triggered by reminders of a traumatic event.