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Memory is such an everyday thing
that we almost take it for granted.
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We all remember what we had
for breakfast this morning,
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or what we did last weekend.
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It's only when memory starts to fail
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that we appreciate just how amazing it is,
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and how much we allow
our past experiences to define us.
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But memory is not always a good thing.
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As the American poet and clergyman
John Lancaster Spalding once said,
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"As memory may be a paradise
from which we cannot be driven,
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it may also be a hell
from which we cannot escape."
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Many of us experience
chapters of our lives
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that we would prefer
to never have happened.
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It is estimated that
nearly 90 percent of us
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will experience some sort of
traumatic event during our lifetimes.
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Many of us will suffer acutely
following these events, and then recover,
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maybe even become better people
because of those experiences.
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But some events are so extreme
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that many, up to half of those
who survive sexual violence, for example,
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will go on to develop
post-traumatic stress disorder,
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or PTSD.
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PTSD is a debilitating
mental health condition
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characterized by symptoms
such as intense fear and anxiety
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and flashbacks of the traumatic event.
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These symptoms have a huge impact
on a person's quality of life
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and are often triggered
by particular situations
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or cues in that person's environment.
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The responses to those cues may have been
adaptive when they were first learned --
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fear and diving for cover
in a war zone, for example --
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but in PTSD,
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they continue to control behavior
when it's no longer appropriate.
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If a combat veteran returns home
and is diving for cover
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when he or she hears a car backfiring,
or can't leave their own home
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because of intense anxiety,
then the response to those cues,
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those memories,
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have become what we
would refer to as maladaptive.
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In this way, we can think of PTSD
as being a disorder of maladaptive memory.
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Now I should stop myself here,
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because I'm talking about memory
as if it's a single thing.
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It isn't.
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There are many different types of memory,
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and these depend upon different circuits
and regions within the brain.
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As you can see, there are two
major distinctions in our types of memory.
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There are those memories
that we're consciously aware of,
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where we know we know
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and that we can pass on in words.
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This would include memories
for facts and events.
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Because we can declare these memories,
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we refer to these as declarative memories.
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The other type of memory
is non-declarative.
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These are memories where we often
don't have conscious access
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to the content of those memories
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and that we can't pass on in words.
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The classic example
of a non-declarative memory
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is the motor skill for riding a bike.
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Now, this being Cambridge,
the odds are that you can ride a bike.
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You know what you're doing on two wheels.
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But if I asked you to write me
a list of instructions
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that would teach me how to ride a bike,
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as my four-year old son did
when we bought him a bike
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for his last birthday,
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you would really struggle to do that.
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How should you sit on the bike
so you're balanced?
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How fast do you need to pedal
so you're stable?
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If a gust of wind comes at you,
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which muscles should you tense
and by how much
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so that you don't get blown off?
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I'll be staggered if you can give
the answers to those questions.
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But if you can ride a bike,
you do have the answers,
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you're just not consciously aware of them.
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Getting back to PTSD,
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another type of non-declarative memory
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is emotional memory.
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Now this has a specific
meaning in psychology
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and refers to our ability
to learn about cues in our environment
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and their emotional
and motivational significance.
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What do I mean by that?
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Well, think of a cue
like a smell of baking bread,
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or a more abstract cue
like a 20-pound note.
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Because these cues have been pegged
with good things in the past,
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we like them and we approach them.
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Other cues, like the buzzing of a wasp,
elicit very negative emotions
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and quite dramatic
avoidance behavior in some people.
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Now, I hate wasps.
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I can tell you that fact.
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But what I can't give you
are the non-declarative emotional memories
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for how I react
when there's a wasp nearby.
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I can't give you the racing heart,
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the sweaty palms,
that sense of rising panic.
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I can describe them to you,
but I can't give them to you.
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Now importantly,
from the perspective of PTSD,
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stress has very different effects on
declarative and non-declarative memories
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and the brain circuits
and regions supporting them.
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Emotional memory is supported
by a small almond-shaped structure
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called the amygdala and its connections.
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Declarative memory, especially the what,
where and when of ?? memory,
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is supported by a seahorse-shaped
region of the brain
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called the hippocampus.
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The extreme levels of stress
experienced during trauma
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have very different effects
on these two structures.
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As you can see, as you increase
a person's level of stress
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from not stressful to slightly stressful,
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the hippocampus,
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acting to support the event memory,
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increases in its activity
and works better to support
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the storage of that declarative memory.
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But as you increase
to moderately stressful,
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intensely stressful,
and then extremely stressful,
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as would be found in trauma,
the hippocampus effectively shuts down.
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This means that under
the high levels of stress hormones
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that are experienced during trauma,
we are not storing the details,
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the specific details,
of what, where and when.
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Now, while stress is doing that
to the hippocampus,
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look at what it does to the amygdala,
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that structure important
for the emotional, non-declarative memory.
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Its activity gets stronger and stronger.
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So what this leads us with in PTSD
is an overly strong emotional,
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in this case fear, memory
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that is not tied
to a specific time or place
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because the hippocampus
is not storing what, where and when.
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In this way, these cues
can control behavior
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when it's no longer appropriate,
and that's how they become maladaptive.
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So if we know that PTSD
is due to maladaptive memories,
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can we use that knowledge
to improve treatment outcomes
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for patients with PTSD?
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A radical new approach being developed
to treat post-traumatic stress disorder
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aims to destroy those maladaptive
emotional memories
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that underly the disorder.
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This approach has only
been considered a possibility
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because of the profound changes
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in our understanding
of memory in recent years.
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Traditionally, it was thought
that making a memory
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was like writing in a notebook in pen.
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Once the ink had dried,
you couldn't change the information.