[narrator] Some moments feel so important,
we believe there is a perfect recording of them
etched in our minds.
[Neil Armstrong] That's one small step for man...
Tear down this wall!
[clamoring]
Diana, Princess of Wales, is dead.
[narrator] For many, 9/11 was one of those moments.
[woman 1] I was getting ready to go to class, and I put on the TV--
[woman 2] And the newscaster stopped and said, "This just in."
[man 1] Two planes hit the Twin Towers.
[woman 3] Every single channel had a building that was on fire.
[man 2] This businessman was covered in dust.
[woman 4] And I saw the big hole in the side of the building.
[man 3] Yeah, it was sort of surreal.
[woman 5] Very confusing and disorienting.
[woman 2] I felt a sense of dread.
[woman 6] I remember my mom was working in the city and I remember
smoke billowing out over the water of the Long Island Sound
behind the building where I went to elementary school.
I was just talking with my parents about it one day and my mom goes,
"No, you know in 2001,
I was working in Connecticut."
[narrator] And there were other problems with Melanie's memory of that day.
Her classroom windows didn't look out over the water,
the World Trade Center was over 40 miles away,
and the smoke was drifting in the opposite direction.
How could I possibly have seen the smoke billowing from over the water?
Like, how would I see that?
[woman] Your memories for 9/11 are probably not as accurate as you think.
We know about 50% of the details of that memory change in a year
even though most people are convinced they're a hundred percent right.
[narrator] They might correctly remember the gist of the day,
but not details like who they were with,
what they were doing when they heard, and what exactly they saw.
Even our most significant memories,
the ones that form the foundation of our life story,
aren't perfect recordings.
They can shift and warp over time.
It feels like the whole purpose of memory should be to preserve the past.
So why are memories so unreliable?
How exactly does remembering work?
Wait a second now, I do remember, you're... Um...
Memory, that everybody has,
is a gold mine of unexplored and untapped potential.
[man 1] Our memory just mediates our interaction with the world.
[man 2] Memory is one of our most fundamental activities,
and it is only when it fails us that we think about it at all.
[narrator] Yanjaa Wintersoul is a grand master of memory.
[man] Alright, here we go.
[narrator] Five years ago, she discovered the world of memory competitions.
[Wintersoul] When I first started,
it was mostly, like, a bunch of white guys from Europe
in very, like, sad-looking competition rooms.
[narrator] They memorize decks of cards in seconds,
thousands of digits in an hour.
It looks like we're all taking very speedy SATs.
I have three world records:
one for images, one for names and faces, and one for words.
[narrator] And she's demonstrated her skills on TV shows around the world.
Okay, page 38.
Starts mid-sentence.
"Information effectively by using humor."
[applause]
[narrator] We gave her ten minutes to memorize these 500 numbers, and...
Five, three, nine...
one, six, six...
-nine, seven... -...four, seven, eight...
-...seven, six, five, eight... -...four, seven, seven, nine...
...one, seven... two...
-...two, two, five... -...nine, zero, two...
-...six, four, eight, two... -...one, six, one, one, two.
[narrator] How does she do that?
-[applause from film crew] -Yay!
It all comes down to the peculiar way our brains store memories.
And perhaps no brain has taught us more about memory than this one.
It belonged to a man named Henry Molaison.
When Henry was 27, he had brain surgery to treat epilepsy
and the surgeon removed this little piece of his brain.
The surgeon noted that the procedure resulted in no marked behavioral changes,
with the one exception of a very grave recent memory loss.
It was so severe, it prevented Henry from navigating his own house
and recognizing his doctors.
But Henry still had other types of memory,
habits that don't require conscious thought,
like how to ride a bike,
so-called "implicit memories."
He also kept some conscious, or "explicit," memories.
He discussed historical events with his doctor
in this recording from the early '90s.
What happened in 1929?
-The stock market crashed. -It sure did.
[narrator] That's an example of semantic memory:
facts, dates, numbers, words, the kinds of things that memory athletes memorize.
The real damage was to Henry's episodic memory,
his memory for personal experiences.
When his doctor asked, "Do you know what you did yesterday,"
he replied...
No, I don't.
How about this morning?
I don't even remember that.
[narrator] Without this one small part of his brain,
Henry had trouble forming new memories.
But that doesn't mean memories are stored in one specific place.
[cello playing]
[narrator] When you have an experience, say performing at a recital,
sensory information is processed to many different parts of your brain.
The sound of the cello.
The feeling of the strings under your fingers.
The face of your friend in the audience.
The pang of stage fright.
[cello continues]
And the part of the brain that pulls all of these elements together,
the part that Henry's surgery badly damaged,
is the medial temporal lobe,
which includes an important structure called the hippocampus.
When you relive that moment later,
the medial temporal lobe helps combine those elements once again.
Your life story is all the moments like this that you can relive.
In this graph is the life story of a typical 70-year-old.
There are lots of memories from the recent past,
but as you move backward in time, they start to fall off.
There are only a few memories from childhood,
and nothing before around three,
but there's this surprising bump in our teens and 20s.
When you're getting through high school, you're having a lot of momentous occasions
in that stage of your life.
And when we think about our life stories,
those change moments are the ones that stand out
as the ones that kind of define us
and define our lives going forward.
[narrator] Some people have more memories than others,
and you can improve your memory by just living a healthier and more active life.
I try to, like, not drink as much, sleep a lot, and eat well.
The one thing that I've seen in every single study
that's like "this is gonna work" is honestly meditation.
[narrator] Undergraduates were able to increase their score on the verbal GREs
from 460 to 520, just by taking a mindfulness meditation class.
Probably because meditation improves focus and focus improves memory.
And when it comes to personal experiences,
there are certain features that make us remember some better than others.
First, emotion.
If you show a person a string of faces,
they'll remember the most emotional ones best.
When we have an emotional experience, our amygdala,
the emotional center of the brain, which sits right next to the hippocampus,
actually up-regulates the hippocampus
and allows it to form a more detailed and stronger memory.
One of the things we wanted to do after 9/11
was look into the brains of individuals who were in New York that day.
So about half the people were, on average, around Midtown,
and the other half of people were much closer to the World Trade Center.
[narrator] Three years after the attacks,
they asked those people to remember their experiences.
[Addis] The individuals who were closer to the World Trade Center that day,
the 9/11 memories were more vivid and we saw more activity in the amygdala.
[narrator] Memories are also connected to a sense of place.
One of the things we know from our study of 9/11 memories is that
the thing that people were most consistent about was where they were.
-[man 1] I had just gotten home. -[man 2] Gym class.
[man 3] I was living in London at the time.
[woman] In New Smyrna Beach, Florida.
[man 4] The Upper East Side.
So we think place has a particularly strong role in memory.
And if you actually look in the hippocampus,
there seem to be cells that are specifically responsive to time and place.
[narrator] Here's a representation of these "place" cells
in the hippocampus of one particular rat.
And here's a video of that rat moving along a simple track.
His head is in this green circle, and here's his tail.
Each place cell is associated with a particular location along the track
and these cells have been color-coded by scientists.
When the rat is at the start of the maze, this green place cell fires.
But as it moves along, a different cell is activated,
and then another, and another.
When the rat pauses, the cells fire in rapid succession as he recalls his route.
London cabbies must navigate their own rat maze.
To get their licenses, they have to pass a century-old test called simply
"the knowledge."
They spend years memorizing London's 25,000 streets.
Scientists scanned the brains of would-be cabbies
before and after this process.
In the brains of people who didn't end up getting their licenses,
the size of the hippocampus didn't really change.
But those who passed, interestingly, their hippocampi actually grew.
[cello plays]
Finally, memories can be strengthened by story.
Our brains pay much closer attention to information
when it's in the form of a narrative.
In one study, 24 people were asked to memorize 12 lists of ten words.
Half the people studied and rehearsed the list,
and they remembered, on average, 13% of the words.
The other half wove the words into stories of their own invention,
and they remembered 93%.
The more that you can associate things you want to remember
with structures you already have in your mind,
the easier it's going to be to remember.
You know, you're creating a narrative.
When we go to retrieve that memory,
we have almost many multiple ways of getting into that memory.
[narrator] Story, place, and emotion are the foundation
of some of our strongest memories.
And those same features can be hijacked to help you, say,
memorize 500 random digits.
Yes, let's do it.
[narrator] Starting with the first three digits,
she converts numbers into sounds using her own personal code.
[Wintersoul] So 5 is an "s,"
3 is an "a," and 9 is a "g," 'cause-- just because of the shapes.
So then it's basically like you're reading something
instead of looking at all these numbers.
So 539 is SAG.
[narrator] And the next triplet, 166, becomes TBB.
[Wintersoul] And I think of the Middle Eastern dish of tabbouleh.
[narrator] She pairs the two words to create a striking scenario.
[Wintersoul] This saggy, half-naked person is covered in, like, tabbouleh rice,
and because it's disgusting, I remember it more.
Anything that has, like, visceral or, like, very emotional things,
your brain is like, ugh.
[narrator] She translates the rest of the digits the same way.
[Wintersoul] Gimli from Lord of the Rings, he is running for office.
Rami Malek buying boots.
My spleen turns into the Lourve.
[narrator] Next, Yanjaa harnesses the power of place
with an ancient technique called "the memory palace."
She imagines herself walking through a neighborhood she knows well,
adding surreal imagery along the route.
It helps in putting...
very random abstract things in order
when you attach it to something you already know.
So I come out of the High Street metro.
So a saggy-skinned person
is just covered in tabbouleh.
[narrator] And a little further on...
[Wintersoul] In that tunnel, 478, that's a reef,
and 468, that's ravioli,
so it's a tunnel that's now a reef and full of ravioli.
On the carousel, we'll have a big alpaca llama
and it's, like, eating this tube of, like, melted cheese.
That dull list of numbers became an epic travel log
full of surprising images that she could revisit later.
Memory athletes aren't necessarily smarter than everyday people
and they don't have bigger brains.
But they change the connections within their brains
by training with techniques like the memory palace.
We are more...
wired to remember that than to remember random sets of digits.
In general, we're like emotional and visual learners.
And storytellers.
Only a dozen people in the world have memorized more than 20,000 digits of Pi.
But lots and lots of people have played Hamlet
and memorized all his lines...
Words, words, words.
...which contain nearly 50,000 letters.
Remember thee!
Ay, thou poor ghost, while memory holds a seat in this distracted globe.
[narrator] But that's not the full story.
Some of the same things that strengthen our memories
can also warp them.
And typically with emotional memories,
we tend to remember the central aspects.
So, our attention kind of zooms in on the core of that experience,
so we might forget some of the peripheral details,
like, you know, perhaps what a perpetrator was wearing, but we'll remember the gun.
Emotional 9/11 memories are just as inaccurate as everyday memories.
They both deteriorate at the same rate.
[Phelps] What was different is that people were highly confident
their memories for 9/11 were correct.
[record needle scratches]
Memories aren't high fidelity recordings that we store away.
They're more like live performances,
created with input from different parts of the brain in the present moment.
We can't remember every single detail of every experience.
And so we use pre-existing knowledge,
such as semantic memory or facts that we have...
[narrator] Or our pre-existing biases and beliefs.
[Addis] ...to fill in those gaps.
That could explain the errors in Melanie's memory of 9/11.
My mom worked in New York City growing up all my life,
so, of course she was in the city.
[narrator] And after 9/11, maybe Melanie saw billowing smoke on TV,
and that's how it entered her memory.
So the fact that we reconstruct our episodic memory
so we piece them back together
means that our episodic memories are actually very flexible.
[narrator] Scientists have been able to exploit this flexibility
to plant false childhood memories of being left at a shopping mall,
taking a hot air balloon ride,
even having tea with Prince Charles.
In one study, young adults were asked to try to remember
a crime they had supposedly committed in their teens,
even though these crimes were completely fake,
made up by researchers.
After a couple of interviews full of leading questions,
70% of the subjects accepted that they had committed those crimes
and many came up with rich, detailed memories
-that were completely false. -[siren wails]
One of the places where this plays out
that is unfortunate is things like, um, eyewitness identifications.
They said they were gonna take you into a room, we're gonna have seven men,
and that if I saw the suspect,
I was to write his number on a piece of paper
and hand it over to the detective.
[narrator] More than two decades after she was raped,
Jennifer Thompson appeared on television with the man she had identified
as her attacker.
After I picked out Ronald Cotton's photograph,
that's when they said to me, "We thought that was him."
We can boost the confidence in a false memory
by confirming it or by at least repeating it multiple times.
By now, Ronald's image had completely contaminated, so to speak,
the original memory of that night, and so...
the face of my rapist had become Ronald Cotton,
so much so that seeing the actual perpetrator right there...
I didn't have one memory of it.
Years after Ronald was imprisoned,
DNA evidence proved that Jennifer had been raped by another man.
In the U.S., DNA has helped to overturn hundreds of convictions,
and 70% of those involved eyewitness testimony.
It's not just our memories of crimes that can become contaminated.
It's the memories that tell us who we are and where we came from.
Researchers interviewed a group of 14-year-olds,
and then, decades later, asked them to recall their teenage years.
What their relationship with their parents was like,
how they had felt about sex and religion,
what activities they had enjoyed.
Their memories, it turned out, were uniformly poor.
For most memories, no better than chance.
So this poses the question: Why would we have a memory system
that is so unreliable and error-prone if it was designed to remember the past?
[narrator] That's the big question.
And once again, those recordings of Henry Molaison
point to a possible answer.
What do you think you'll do tomorrow?
Whatever is beneficial.
[narrator] Henry often struggled to answer questions like this.
It seemed to scientists that he hadn't just lost his past,
he could no longer imagine the future.
Three decades after Henry's surgery,
another patient's medial temporal lobe was severely damaged
in a motorcycle accident.
In this interview from 1988, the patient was asked by his doctor...
[interviewer] Do you feel hopeful about the future?
I guess so. I don't really think much about the future.
[interviewer] You don't think much about the future?
[narrator] That same patient once described thinking about the future
as being asked to find a chair in an empty room.
The future and the past seem to be somehow linked in the mind.
[Addis] We decided to put people into the scanner
and have them remember past experiences
and imagine future experiences.
And...
we really didn't know what to expect.
When people remembered, a particular network lit up, and...
[Addis] That same network was engaged, pretty much identically
when people were having to imagine future events.
[narrator] When you let your mind wander, you switch back and forth all the time,
remembering and imagining.
Your mind is a time machine.
In Lewis Carroll's Through the Looking-Glass,
the Queen of Hearts remarks,
"It's a poor sort of memory that only works backwards."
It turns out... she's right.
The same machinery that brings all those pieces together to relive the past
can bring some of those pieces together with other pieces
to simulate possible futures.
Now, the flexibility that leads us to remember things that never happened,
that undermines the justice system,
that corrupts our most vivid memories,
it starts to look like a superpower,
the key to our success as a species.
It allows us to troubleshoot upcoming experiences,
to think through the ways in which events might unfold,
potential obstacles that might come up
and the ways in which we might deal with those obstacles.
[narrator] And some scientists say the simulation engine between your ears
does something even more profound:
It weaves together memories of the past and dreams of the future
to create your sense of self.