MONDAY, Dec. 18, 2006 (HealthDay News) -- Most Americans remember where they were on the morning of Sept. 11, 2001.
But a new brain-scan study suggests that not all those memories were created equally.
"If you were near the World Trade Center, your memories are qualitatively different from other people -- even those who were elsewhere in Manhattan," said lead researcher Elizabeth Phelps, a professor of psychology at New York University.
Specifically, people who were within about two miles of Ground Zero on that day now retain especially vivid, detailed recollections of the scenes and events of that morning -- a kind of recall that experts call "flashbulb memories."
Brain imaging suggests that these memories are especially strong because the amygdala -- a brain area focused on fear and memory -- kicked into high gear as these people watched that morning's catastrophic events unfold.
Nearly all of the study participants who had been in lower Manhattan on 9/11 said they experienced first-hand the sights, sounds and smells of that day. And many said they feared for their own safety.
All of that may have played a role in imprinting these highly potent memories in their brains, Phelps said.
"This isn't unique to 9/11," she added, noting that flashbulb memories can be laid down in any kind of traumatic event, be it personal or very public.
But while other researchers have studied these trauma-linked memories before, "what's different about this is that we could actually look at the brain mechanism behind it," Phelps said.
Her team published its findings in this week's issue of the Proceedings of the National Academy of Sciences.
The findings don't mean that only people who were close to Ground Zero can have stark, detailed memories of that day.
"I think that you can have these memories even if you weren't at the event and not physically threatened yourself," said Dr. David Spiegel, a professor and chairman of psychiatry and behavioral sciences at Stanford University School of Medicine. "But I think what Phelps is showing us is the most extreme and physiologically identifiable version -- it's a matter of degree."
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.
In fact, studies conducted not long after 9/11 showed that the risk for PTSD rose with an individual's proximity to Ground Zero.
The subdued parahippocampal function seen in the Downtown group might play a role in PTSD, Phelps said.
"The amygdala helps you form a very strong memory," she explained. But in the normal brain, the hippocampus acts as a counterweight, "giving you the ability to keep it all in the right context."
With PSTD, the hippocampus' ability to reign in frightening memories may get lost. "We know that there are differences in the hippocampus in people that will go on to develop PTSD and those who will not," Phelps said.
The new findings do not minimize the profound, lasting memories of 9/11 that millions who were outside Lower Manhattan on the day carry with them, the experts said.
"It was upsetting for everyone -- I was on the other side of the country, and it was very upsetting," said Spiegel. "Part of it was that there was that strange sense of discontinuity. Everyone can remember what a clear, beautiful, quiet day it was -- and then it happened."
Find out more about brain imaging and how the brain works at Harvard University.