Where were you, on the morning of Sept. 11, 2001, when you learned that a passenger jet had slammed into the North Tower of the World Trade Center? What were you doing? Who were you with?

Memories of that day, which was deeply painful for so many people, might seem indelibly carved into your mind. There's a good chance, however, you retained far fewer details than you think.

Strongly negative or traumatic experiences are processed and encoded through a distinct neural pathway that filters out "peripheral details," says University of Waterloo cognitive psychologist Myra Fernandes.

The more fear or stress that's generated, the more amplified the filtering effect becomes, especially during experiences that threaten our survival.

The brain locks "on the central aspects of the experience at the expense of the peripheral details," she explains. "The gist is maintained but many of the details are lost.

"From an evolutionary point of view, this is totally plausible," because it emphasizes the parts of a recollection that might serve as a warning later, helping us avoid highly stressful situations in the first place.

'The essential details'

The phenomenon is seen specifically in episodic memories — those of lived experiences that are tied to emotions and context.

The reliability of episodic memory took a central role last week in the ongoing criminal trial of former CBC radio host Jian Ghomeshi, who has pleaded not guilty to four counts of sexual assault and one count of overcoming resistance by choking. The charges involve three women.

It's beneficial to our emotional well-being to remember the gist rather than every detail. - Myra Fernandes , cognitive psycholgist

It's a common tactic for lawyers to dial in on the minute details of a witness's testimony, pressing to expose any possible contradictions in various retellings of the same story and therefore, the theory goes, raise doubts about its truthfulness.

"By asking someone to repeat a story over and over again, essentially you start to see the story unravel," criminal defence lawyer Daniel Brown explained in an interview with CBC's Metro Morning last week.

Those who say they have lived through trauma, however, are sometimes unable to articulate a coherent narrative owing to the brain's tendency to zero-in on only the most essential elements of what happened.

"The person is overcome with all these essential details," says Lori Haskell, a clinical psychologist in private practice and assistant professor in psychiatry at the University of Toronto. Her research partly focuses on how sexual violence affects the neurobiology of survivors.

"So later on when they try to start creating a narrative, those details aren't accurate – they're not recalled with great detail," she adds.

'Durable' memories

Advanced-imaging studies have shown that traumatic experiences cause two relatively tiny areas deep within the brain, called the amygdala, to kick into overdrive. When the amygdala ramp up, there is a cascade effect through the brain.

Elizabeth Kensinger, a cognitive neuroscientist at Boston College, studies how emotion plays into memory formation.

The amygdala, shown here in red, become extremely active during highly stressful experiences, MRI studies have revealed. (Elizabeth Kensinger)

"The amygdala is activated, and that actually affects how the other regions that take part in processing memory are brought online and how they communicate with one another," she says.

Kensinger's research suggests that while extraneous details may be forgotten, the core components may be less susceptible to the various ways time can erode our recollections.

"There's some really interesting evidence that emotional memories tend to be more durable."

A game of 'telephone'

In general, our episodic memories are "prone to distortions" because they are, in essence, a "reconstruction" of events assembled from building blocks stored throughout the brain. The more we recall any single thing, the greater the chance becomes that we'll remove, or even insert, a block that's not supposed to be there.

Fernandes uses the analogy of the classic children's game "telephone."

"When we recall a memory, we relay it down through our brains … Every time the message is passed on, it changes slightly."

A variety of influences can increase the probability that a recollection will contain erroneous bits. Decades of research by renowned American cognitive psychologist Elizabeth Loftus, for example, has shown that simple, well-crafted linguistic prompts can easily lead someone to unknowingly insert or omit false details into the retelling of a story.

That's not to say all memories contain inaccuracies. In fact, generally speaking, the human brain does an extraordinary job of encoding countless experiences every day.

But it would be too overwhelming to retain all of the information we take in throughout our lives. Research suggests that while we sleep, our brains whittle down experiences — not just traumatic ones — into their most useful parts to make more room, like freeing-up space on a hard drive.

"It's beneficial to our emotional well-being to remember the gist rather than every detail," says Fernandes.

Are you sure?

So, how well do you really remember the details of your day on Sept. 11?

When doing research for The Invisible Gorilla — a book exploring the fallibility of the human mind — University of Illinois cognitive psychologist Daniel Simons wrote down his own answers.

He discovered two of the three people he thought he was with on that morning weren't with him at all, and he has no recollection of being with the person who was there. He's not alone.

More than a dozen universities participated in a survey that asked 2,100 Americans from across the U.S. about their memories of Sept. 11, one, three and 10 years after the attacks.

When all was said and done, 40 per cent of participants told stories notably different than the one that emerged from their original answers.

Interestingly as time passed, those whose answers changed significantly did not become less confident about the accuracy of their stories.

The study is part of a huge body of evidence pointing to the reality that memory is malleable, vulnerable to the curious nature of our own neurobiology.

That doesn't mean we should distrust it, says Simons, but rather, we should appreciate its limits.