What were you doing when you first heard about what happened on September 11, 2001?

It seems as if everyone can describe that horrific moment on hearing about the first plane to crash into the World Trade Center and being glued to the television as events unfolded. They are part of a wide range of exceptionally vivid memories we seem to have concerning events that stay with us long after other memories fade with time.

Known as "flashbulb memories", they are a form of autobiographical that are especially powerful due to surprise and shock that ensures that the memory stays with us. It seems to be a central trait of flashback memories that we are as likely to recall where and when we first heard about the traumatic moment as the details of what actually happened. There is also a critical distinction between flashbulb memories and what psychologists refer to as event memories which focus on the details of the event that forms the flashbulb moment. We may all recall the events of 9/11, such as how many planes were involved for example, but it's the flashbulb aspects of those memories that stay with us for so long.

The term flashbulb memory was first coined in 1977 by psychologists Roger Brown and James Kulik. In describing how these memories work, they invoked the example of President John F. Kennedy's assassination on November 22, 1963. Certainly, the question "What were you doing when you heard that J.F.K. had been shot?" helped characterise an entire generation. It was only replaced as later "flashbulb moments" occurred such as the assassinations of Robert F. Kennedy, Martin Luther King, and John Lennon, the shooting of President Ronald Reagan, the Challenger shuttle disaster, and so on.

And not all flashbulb memories are going to involve tragic events. The fall of the Berlin Wall, the end of World War II, and other positive news stories that have generated public celebration can become flashbulb moments as well. Unfortunately, pleasant surprises tend not to be as common as the unpleasant kind so flashbulb memory research has been largely rooted in tragedy.

There are different possible explanations for how and why flashbulb memories occur but the key feature of these kind of memories is the emotional tie that goes with them. According to memory researcher William Hirst of the New School for Social Research, it is the publicly shared trauma that makes flashbulb events so memorable. "Every time there's a public trauma, psychologists run out in the street and capture people's memories of what happened," says Hirst. "They did it with the Challenger explosion. They did it with the death of Princess Diana... And we did it with 9/11."

Much like ordinary autobiographical memory, flashbulb memories are apparently formed based on:

the emotional intensity of the event

the event's importance, especially if it has long-term consequences

the degree to which the memory of the event is rehearsed, i.e., how often are people likely to recall the event?

the uniqueness or distinctiveness of the event

the level of surprise associated with the event

Ever since 9/11 occurred, research into flashbulb memories has helped form a better understanding about memory and forgetting. So far, there have been more than twenty studies looking at how well people recall the 9/11 attacks and the kind of memory errors that can occur. These include errors of omission (forgetting important details of what happened), and errors of commission ( involving event details that never really happened). Even people who are extremely confident that they are remembering important details can make critical errors.

Research studies comparing memories of the 9/11 attacks with ordinary autobiographical memory showed that even flashbulb memories can fade with time as key details are forgotten. Still, even with this forgetting that occurs, people questioned about their recall of events like 9/11 report being extremely confident in what they are recalling, despite those memories being flawed.

A new research study published in the Journal of Experimental Psychology: General represents one of the most comprehensive test of flashbulb memories linked to the 9/11 attacks. Conducted by William Hirst and a team of researchers from numerous universities, the study followed thousands of participants over a ten-year period to measure the accuracy of their 9/11 memories as well as their reported in what they were recalling.

Within a week of the 9/11 attacks, Hirst and his fellow researchers conducted the first survey at locations around the United States asking 3,245 participants about where and when they learned of the attacks as well as key details of the attacks (eg., how many planes were involved, etc.). The researchers then followed up the same group of respondents 11 months, 35 months, 119 months, and finally ten years afterward (always one month before the anniversary of the attacks). Though there were significant attrition over time, 202 respondents completed all four surveys.

Based on their findings over the years (which formed the basis of several intermediate studies), Hirst and his colleagues noticed that most of the forgetting occurred in the first year and leveled off afterward. Still, confidence in the accuracy of the 9/11 memories remained high even as the consistency of those memories declined with time.

Much like other research studies into flashbulb memories, the researchers focused on a) where the respondents were living at the time of the attacks, b) the level of emotional intensity of the memories, c) the personal loss or inconvenience experienced, d) the amount of media that the respondents watched immediately afterward and in the weeks that followed, and e) the amount of discussion with others about the attacks. Except for emotional intensity, all of these factors contributed to event memory (actual details of the attacks), but none of these different factors studied correlated with accuracy of flashbulb memories.

Despite changes in memory accuracy however, confidence remained high for respondents recalling the attacks even ten years later. Though media stories about the 9/11 attacks can help keep event memory alive, the flashbulb memories remained strong as well. In many ways, flashbulb memories appear similar to the emotionally charged memories that victims of trauma have about whatever traumatic event they experienced.

While traumatic memories involve events that are directly experienced though, flashbulb memories are as much about how we learn about emotionally charged events as about the events themselves. Also, they tend to be memories that are meant to be shared with others since they involve emotional events that affect everyone. It is this collective sharing that causes flashbulb memories to be repeated and rehearsed over time which ensures that they stay stable over time, even if those memories contain errors that persist as well.

Flashbulb memories about important events also make people feel that they are a part of history. Events such as the 9/11 attacks or the assassination of J.F.K. have a way of shaping our identities as members of a greater community. Memories of "what were you doing when you heard?" make us feel like we are participants rather than just passive observers. The fact that these memories often contain errors seems less important than the sharing with others which shapes our view of the world. That flashbulb memories can be affected by later influences, including 9/11 "truther" conspiracy theory as well as movies such as Fahrenheit 911 and United 93, demonstrates how vulnerable we are to having our important memories revised with time.

Though we believe that these memories are accurate, we are prone to make errors, either by forgetting important details or "remembering" things that never really happened. We are usually open-minded enough to correct inaccuracies in our memory but we also have an unfortunate tendency to only accept new evidence that confirms what we believe about the world. As we take in this new information, it becomes part of the process by which permanent memories are formed which become extremely resistant to change (a process psychologists call memory consolidation). As we rehearse these memories by sharing them with the people around us, we also hear other accounts of the same event which may seem more exciting and vivid than ours and which lead us to "revise" our own memories (which is known as reconsolidation).

These twin process of consolidation and reconsolidation help explain why inaccuracies can creep into memories of events that seem unforgettable. They also demonstrate how unreliable memories can be, no matter how confident we are that we are remembering things accurately. While most of the forgetting that occurs usually happens in the first year, we are always engaged in the active rehearsal of important memories as we relate to others what we think happened.

That active rehearsal is going to mean that memories are going to be vulnerable to change over time. We all need to accept that even flashbulb memories can be wrong, no matter how sure we are that what we remember is the truth.

So, what were you doing that day? And how certain are you that you are remembering things accurately?