Recollections of that day have given researchers a unique window into how the brain forms memories of shocking events. "It's as if a flashbulb goes off and you take a mental picture of your surroundings," says psychologist William Hirst of the New York School for Social Research.

Flashbulb memories, as they are known, are tricky to study as people are seldom keen to talk to researchers just after hearing or seeing emotionally charged news. It can also be difficult to know how accurate a person's memory of the event is, since there is usually no way to be sure what actually happened.

Elizabeth Phelps of New York University was in Manhattan on 9/11 and saw the attack. When fellow neuroscientist John Gabrieli called to check on her they "decided to put together a consortium of memory researchers, and started collecting data within a week".

Public tragedy

"It struck us that 9/11 was probably one of the best examples of a public tragedy that we could use [to study flashbulb memory]," says Hirst.

The study involved surveying over 3000 Americans from seven different cities, including New York, within two to three days of the attack. Participants were asked to note all the details they could remember of the day itself, their personal circumstances at the time, and how they felt. To find out whether the memories formed would be lasting ones, the group sent out the same survey to the same participants again 11 months later, and once more 35 months after the attacks.

"People were about 60 per cent right about the details of the event after about a year, and this dropped to 50 per cent after three years," suggesting flashbulb memories are no more accurate than other types of memory, says Phelps. But that didn't affect how vividly people recalled the day, or how much faith they had in the details of their memories .

But evidence that flashbulb memories might be processed differently to other types of memory was provided by Patrick Davidson's team at the University of Arizona in Tucson, who investigated whether memories for 9/11 degraded in the same way as other memories in older individuals. The group found that while older people with frontal lobe damage struggled to remember other personal events, they remembered 9/11 as well as young people, suggesting that flashbulb memories might be uniquely impervious to ageing .

Malleable memories

These vivid yet inaccurate memories might well be the brain's best way of dealing with an emotional, life-changing event, says Phelps. Keeping the memory malleable to some extent can have benefits: "If a group arrives at a memory of shared suffering, it could be a positive thing in terms of group spirit and identity, and could pull the community together," says Hirst.

When Phelps reassessed the survey responses recently, she found that people's memories of where they were on 9/11 have stayed consistent, though how they felt is harder to recall. "People tended to have around 80 per cent accuracy in their place memories, but were only 40 per cent accurate at remembering their emotions," says Phelps, who is writing up the finding. A weak emotional memory could also be useful in getting over tragic events.

In another study three years after the attacks, Phelps used functional MRI to monitor the brain activity of New Yorkers as they recalled the events of 9/11, along with other personal, life-changing events from the same period. She found that when recalling 9/11 those who had been within 4 kilometres of the Twin Towers during the attack had relatively higher activity in the amygdala – a region of the brain involved in emotion – than those who had been further away, suggesting that flashbulb memories may be encoded differently in the brains of those closest to the attack .

This theory has been backed by surveys by Olivier Luminet and Antonietta Curci, at the Catholic University of Louvain in Belgium. The pair found that, while both US and non-US citizens developed flashbulb memories of the event, they appeared to have done so in different ways. US citizens who had rated the attack as more surprising had more consistent memories, but non-US citizens were better able to remember the event when they were more emotionally affected by it .

A decade on, can you be sure what you were doing when you heard of the attacks? Phelps and Hirst are conducting a 10-year follow-up of their original participants that will, for the first time, show how flashbulb memories survive in the long term.