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 responses 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 the 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.

Declarative memory, especially the what, where and when of event memory,

is supported by a seahorse-shaped region of the brain

called the hippocampus.

The extreme levels of stress experienced during trauma

have very different effects on these two structures.

As you can see, as you increase a person's level of stress

from not stressful to slightly stressful,

the hippocampus,

acting to support the event memory,

increases in its activity

and works better to support the storage of that declarative memory.

But as you increase to moderately stressful, intensely stressful

and then extremely stressful, as would be found in trauma,

the hippocampus effectively shuts down.

This means that under the high levels of stress hormones

that are experienced during trauma,

we are not storing the details,

the specific details of what, where and when.

Now, while stress is doing that to the hippocampus,

look at what it does to the amygdala,

that structure important for the emotional, non-declarative memory.

Its activity gets stronger and stronger.

So what this leaves us with in PTSD

is an overly strong emotional -- in this case fear -- memory

that is not tied to a specific time or place,

because the hippocampus is not storing what, where and when.

In this way, these cues can control behavior

when it's no longer appropriate,

and that's how they become maladaptive.

So if we know that PTSD is due to maladaptive memories,

can we use that knowledge to improve treatment outcomes

for patients with PTSD?

A radical new approach being developed to treat post-traumatic stress disorder

aims to destroy those maladaptive emotional memories

that underlie the disorder.

This approach has only been considered a possibility

because of the profound changes in our understanding of memory

in recent years.

Traditionally, it was thought that making a memory

was like writing in a notebook in pen:

once the ink had dried, you couldn't change the information.

It was thought that all those structural changes

that happen in the brain to support the storage of memory

were finished within about six hours,

and after that, they were permanent.

This is known as the consolidation view.

However, more recent research suggests that making a memory

is actually more like writing in a word processor.

We initially make the memory and then we save it or store it.

But under the right conditions, we can edit that memory.

This reconsolidation view suggests that those structural changes

that happen in the brain to support memory

can be undone,

even for old memories.

Now, this editing process isn't happening all the time.

It only happens under very specific conditions

of memory retrieval.

So let's consider memory retrieval as being recalling the memory

or, like, opening the file.

Quite often, we are simply retrieving the memory.

We're opening the file as read-only.

But under the right conditions,

we can open that file in edit mode,

and then we can change the information.

In theory, we could delete the content of that file,

and when we press save,

that is how the file -- the memory --

persists.

Not only does this reconsolidation view

allow us to account for some of the quirks of memory,

like how we all sometimes misremember the past,

it also gives us a way to destroy those maladaptive fear memories

that underlie PTSD.

All we would need would be two things:

a way of making the memory unstable -- opening that file in edit mode --

and a way to delete the information.

We've made the most progress