Can we edit memories?

Memory is such an everyday thing
that we almost take it for granted.

We all remember what we had
for breakfast this morning,

or what we did last weekend.

It's only when memory starts to fail

that we appreciate just how amazing it is,

and how much we allow
our past experiences to define us.

But memory is not always a good thing.

As the American poet and clergyman
John Lancaster Spalding once said,

"As memory may be a paradise
from which we cannot be driven,

it may also be a hell
from which we cannot escape."

Many of us experience
chapters of our lives

that we would prefer
to never have happened.

It is estimated that
nearly 90 percent of us

will experience some sort of
traumatic event during our lifetimes.

Many of us will suffer acutely
following these events, and then recover,

maybe even become better people
because of those experiences.

But some events are so extreme

that many, up to half of those
who survive sexual violence, for example,

will go on to develop
post-traumatic stress disorder,

or PTSD.

PTSD is a debilitating
mental health condition

characterized by symptoms
such as intense fear and anxiety

and flashbacks of the traumatic event.

These symptoms have a huge impact
on a person's quality of life

and are often triggered
by particular situations

or cues in that person's environment.

The responses to those cues may have been
adaptive when they were first learned --

fear and diving for cover
in a war zone, for example --

but in PTSD,

they continue to control behavior
when it's no longer appropriate.

If a combat veteran returns home
and is diving for cover

when he or she hears a car backfiring,
or can't leave their own home

because of intense anxiety,
then the response to those cues,

those memories,

have become what we
would refer to as maladaptive.

In this way, we can think of PTSD
as being a disorder of maladaptive memory.

Now I should stop myself here,

because I'm talking about memory
as if it's a single thing.

It isn't.

There are many different types of memory,

and these depend upon different circuits
and regions within the brain.

As you can see, there are two
major distinctions in our types of memory.

There are those memories
that we're consciously aware of,

where we know we know

and that we can pass on in words.

This would include memories
for facts and events.

Because we can declare these memories,

we refer to these as declarative memories.

The other type of memory
is non-declarative.

These are memories where we often
don't have conscious access

to the content of those memories

and that we can't pass on in words.

The classic example
of a non-declarative memory

is the motor skill for riding a bike.

Now, this being Cambridge,
the odds are that you can ride a bike.

You know what you're doing on two wheels.

But if I asked you to write me
a list of instructions

that would teach me how to ride a bike,

as my four-year old son did
when we bought him a bike

for his last birthday,

you would really struggle to do that.

How should you sit on the bike
so you're balanced?

How fast do you need to pedal
so you're stable?

If a gust of wind comes at you,

which muscles should you tense
and by how much

so that you don't get blown off?

I'll be staggered if you can give
the answers to those questions.

But if you can ride a bike,
you do have the answers,

you're just not consciously aware of them.

Getting back to PTSD,

another type of non-declarative memory

is emotional memory.

Now this has a specific
meaning in psychology

and refers to our ability
to learn about cues in our environment

and their emotional
and motivational significance.

What do I mean by that?

Well, think of a cue like a smell of baking bread, or a more abstract cue like a 20-pound note. Because these cues have been pegged with good things in the past, we like them and we approach them. Other cues, like the buzzing of a wasp, elicit very negative emotions and quite dramatic avoidance behavior in some people. Now, I hate wasps. I can tell you that fact. But what I can't give you are the non-declarative emotional memories for how I react when there's a wasp nearby. I can't give you the racing heart, the sweaty palms, that sense of rising panic. I can describe them to you, but I can't give them to you. Now importantly, from the perspective of PTSD, stress has very different effects on declarative and non-declarative memories and the brain circuits and regions supporting them. Emotional memory is supported by a small almond-shaped structure called the amygdala and its connections