9/11 Study Offers Insight Into How Memories Are Formed

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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."

More information

Find out more about brain imaging and how the brain works at Harvard University.