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TEDGlobal 2012

Sarah-Jayne Blakemore: The mysterious workings of the adolescent brain

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Why do teenagers seem so much more impulsive, so much less self-aware than grown-ups? Cognitive neuroscientist Sarah-Jayne Blakemore compares the prefrontal cortex in adolescents to that of adults, to show us how typically “teenage” behavior is caused by the growing and developing brain.

- Cognitive Neuroscientist
Sarah-Jayne Blakemore studies the social brain -- the network of brain regions involved in understanding other people -- and how it develops in adolescents. Full bio

Fifteen years ago, it was widely assumed
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that the vast majority of brain development
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takes place in the first few years of life.
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Back then, 15 years ago, we didn't have the ability
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to look inside the living human brain
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and track development across the lifespan.
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In the past decade or so, mainly due to advances
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in brain imaging technology
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such as magnetic resonance imaging, or MRI,
00:37
neuroscientists have started to look inside the living
00:40
human brain of all ages, and to track changes
00:43
in brain structure and brain function,
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so we use structural MRI if you'd like to take a snapshot,
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a photograph, at really high resolution of the inside
00:52
of the living human brain, and we can ask questions like,
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how much gray matter does the brain contain,
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and how does that change with age?
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And we also use functional MRI, called fMRI,
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to take a video, a movie, of brain activity
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when participants are taking part in some kind of task
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like thinking or feeling or perceiving something.
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So many labs around the world are involved in this kind
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of research, and we now have a really rich
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and detailed picture of how the living human brain develops,
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and this picture has radically changed the way
01:24
we think about human brain development
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by revealing that it's not all over in early childhood,
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and instead, the brain continues to develop
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right throughout adolescence and into the '20s and '30s.
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So adolescence is defined as the period of life that starts
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with the biological, hormonal, physical changes of puberty
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and ends at the age at which an individual attains
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a stable, independent role in society.
01:51
(Laughter)
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It can go on a long time. (Laughter)
01:56
One of the brain regions that changes most dramatically
01:59
during adolescence is called prefrontal cortex.
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So this is a model of the human brain,
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and this is prefrontal cortex, right at the front.
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Prefrontal cortex is an interesting brain area.
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It's proportionally much bigger in humans than
02:14
in any other species, and it's involved in a whole range of
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high level cognitive functions, things like decision-making,
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planning, planning what you're going to do tomorrow
02:24
or next week or next year, inhibiting
02:26
inappropriate behavior, so stopping yourself saying
02:29
something really rude or doing something really stupid.
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It's also involved in social interaction,
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understanding other people, and self-awareness.
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So MRI studies looking at the development of this region
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have shown that it really undergoes dramatic development
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during the period of adolescence.
02:45
So if you look at gray matter volume, for example,
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gray matter volume across age from age four to 22 years
02:51
increases during childhood, which is what you can see
02:55
on this graph. It peaks in early adolescence.
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The arrows indicate peak gray matter volume
03:02
in prefrontal cortex. You can see that that peak happens
03:04
a couple of years later in boys relative to girls,
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and that's probably because boys go through puberty
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a couple of years later than girls on average,
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and then during adolescence, there's a significant decline
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in gray matter volume in prefrontal cortex.
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Now that might sound bad, but actually this is
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a really important developmental process, because
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gray matter contains cell bodies and connections
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between cells, the synapses, and this decline
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in gray matter volume during prefrontal cortex
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is thought to correspond to synaptic pruning,
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the elimination of unwanted synapses.
03:40
This is a really important process. It's partly dependent
03:42
on the environment that the animal or the human is in,
03:45
and the synapses that are being used are strengthened,
03:48
and synapses that aren't being used
03:52
in that particular environment are pruned away.
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You can think of it a bit like pruning a rosebush.
03:56
You prune away the weaker branches so that
03:58
the remaining, important branches, can grow stronger,
04:01
and this process, which effectively fine-tunes brain tissue
04:04
according to the species-specific environment,
04:08
is happening in prefrontal cortex and in other brain regions
04:11
during the period of human adolescence.
04:14
So a second line of inquiry that we use to track changes
04:17
in the adolescent brain is using functional MRI
04:21
to look at changes in brain activity across age.
04:24
So I'll just give you an example from my lab.
04:28
So in my lab, we're interested in the social brain, that is
04:29
the network of brain regions that we use to understand
04:33
other people and to interact with other people.
04:36
So I like to show a photograph of a soccer game
04:39
to illustrate two aspects of how your social brains work.
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So this is a soccer game. (Laughter)
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Michael Owen has just missed a goal, and he's lying
04:48
on the ground, and the first aspect of the social brain
04:51
that this picture really nicely illustrates is how automatic
04:54
and instinctive social emotional responses are,
04:57
so within a split second of Michael Owen missing this goal,
05:00
everyone is doing the same thing with their arms
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and the same thing with their face, even Michael Owen
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as he slides along the grass, is doing the same thing
05:06
with his arms, and presumably has a similar
05:08
facial expression, and the only people who don't
05:11
are the guys in yellow at the back — (Laughs) —
05:13
and I think they're on the wrong end of the stadium,
05:16
and they're doing another social emotional response
05:19
that we all instantly recognize, and that's the second aspect
05:21
of the social brain that this picture really nicely illustrates,
05:23
how good we are at reading other people's behavior,
05:27
their actions, their gestures, their facial expressions,
05:30
in terms of their underlying emotions and mental states.
05:33
So you don't have to ask any of these guys.
05:37
You have a pretty good idea of what they're feeling
05:39
and thinking at this precise moment in time.
05:41
So that's what we're interested in looking at in my lab.
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So in my lab, we bring adolescents and adults into the lab
05:46
to have a brain scan, we give them some kind of task
05:49
that involves thinking about other people, their minds,
05:52
their mental states, their emotions, and one of the findings
05:55
that we've found several times now, as have other labs
05:58
around the world, is part of the prefrontal cortex called
06:01
medial prefrontal cortex, which is shown in blue on the slide,
06:04
and it's right in the middle of prefrontal cortex
06:07
in the midline of your head.
06:10
This region is more active in adolescents when they make
06:12
these social decisions and think about other people
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than it is in adults, and this is actually a meta-analysis
06:18
of nine different studies in this area from labs around
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the world, and they all show the same thing, that activity
06:24
in this medial prefrontal cortex area decreases
06:26
during the period of adolescence.
06:30
And we think that might be because adolescents and adults
06:32
use a different mental approach, a different
06:35
cognitive strategy, to make social decisions,
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and one way of looking at that is to do behavioral studies
06:41
whereby we bring people into the lab and we give them
06:44
some kind of behavioral task, and I'll just give you
06:46
another example of the kind of task that we use in my lab.
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So imagine that you're the participant in one of our
06:52
experiments. You come into the lab,
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you see this computerized task.
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In this task, you see a set of shelves.
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Now, there are objects on these shelves, on some of them,
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and you'll notice there's a guy standing behind the set
07:05
of shelves, and there are some objects that he can't see.
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They're occluded, from his point of view, with a kind of
07:11
gray piece of wood.
07:13
This is the same set of shelves from his point of view.
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Notice that there are only some objects that he can see,
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whereas there are many more objects that you can see.
07:23
Now your task is to move objects around.
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The director, standing behind the set of shelves,
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is going to direct you to move objects around,
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but remember, he's not going to ask you to move objects
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that he can't see. This introduces a really interesting
07:35
condition whereby there's a kind of conflict
07:38
between your perspective and the director's perspective.
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So imagine he tells you to move the top truck left.
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There are three trucks there. You're going to instinctively
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go for the white truck, because that's the top truck
07:49
from your perspective, but then you have to remember,
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"Oh, he can't see that truck, so he must mean
07:54
me to move the blue truck," which is the top truck
07:56
from his perspective. Now believe it or not,
07:59
normal, healthy, intelligent adults like you make errors
08:01
about 50 percent of the time on that kind of trial.
08:04
They move the white truck instead of the blue truck.
08:08
So we give this kind of task to adolescents and adults,
08:10
and we also have a control condition
08:14
where there's no director and instead we give people a rule.
08:15
We tell them, okay, we're going to do exactly the same thing
08:19
but this time there's no director. Instead you've got to
08:21
ignore objects with the dark gray background.
08:24
You'll see that this is exactly the same condition, only
08:27
in the no-director condition they just have to remember
08:30
to apply this somewhat arbitrary rule, whereas
08:33
in the director condition, they have to remember
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to take into account the director's perspective
08:38
in order to guide their ongoing behavior.
08:41
Okay, so if I just show you the percentage errors
08:45
in a large developmental study we did,
08:48
this is in a study ranging from age seven to adulthood,
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and what you're going to see is the percentage errors
08:54
in the adult group in both conditions,
08:55
so the gray is the director condition, and you see
08:57
that our intelligent adults are making errors about 50 percent
09:01
of the time, whereas they make far fewer errors
09:03
when there's no director present, when they just have
09:06
to remember that rule of ignoring the gray background.
09:08
Developmentally, these two conditions develop
09:11
in exactly the same way. Between late childhood
09:14
and mid-adolescence, there's an improvement,
09:17
in other words a reduction of errors, in both of these trials,
09:19
in both of these conditions.
09:22
But it's when you compare the last two groups,
09:23
the mid-adolescent group and the adult group
09:26
where things get really interesting, because there, there is
09:28
no continued improvement in the no-director condition.
09:31
In other words, everything you need to do in order to
09:34
remember the rule and apply it seems to be fully developed
09:37
by mid-adolescence, whereas in contrast,
09:39
if you look at the last two gray bars, there's still
09:42
a significant improvement in the director condition
09:44
between mid-adolescence and adulthood, and what
09:47
this means is that the ability to take into account someone
09:50
else's perspective in order to guide ongoing behavior,
09:53
which is something, by the way, that we do in everyday life all
09:57
the time, is still developing in mid-to-late adolescence.
09:59
So if you have a teenage son or a daughter and you
10:03
sometimes think they have problems taking other people's
10:06
perspectives, you're right. They do. And this is why.
10:08
So we sometimes laugh about teenagers.
10:12
They're parodied, sometimes even demonized in the media
10:16
for their kind of typical teenage behavior. They take risks,
10:19
they're sometimes moody, they're very self-conscious.
10:23
I have a really nice anecdote from a friend of mine
10:26
who said that the thing he noticed most
10:28
about his teenage daughters before and after puberty
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was their level of embarrassment in front of him.
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So, he said, "Before puberty, if my two daughters
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were messing around in a shop, I'd say, 'Hey,
10:39
stop messing around and I'll sing your favorite song,'
10:41
and instantly they'd stop messing around and he'd sing
10:43
their favorite song. After puberty, that became the threat.
10:44
(Laughter)
10:48
The very notion of their dad singing in public
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was enough to make them behave.
10:53
So people often ask,
10:56
"Well, is adolescence a kind of recent phenomenon?
10:57
Is it something we've invented recently in the West?"
11:00
And actually, the answer is probably not. There are lots
11:02
of descriptions of adolescence in history that sound
11:04
very similar to the descriptions we use today.
11:08
So there's a famous quote by Shakespeare from "The Winter's Tale"
11:10
where he describes adolescence as follows:
11:14
"I would there were no age between ten and
11:17
three-and-twenty, or that youth would sleep out the rest;
11:20
for there is nothing in the between but getting wenches
11:23
with child, wronging the ancientry, stealing, fighting." (Laughter)
11:25
He then goes on to say, "Having said that, would any
11:31
but these boiled brains of nineteen and two-and-twenty
11:36
hunt in this weather?" (Laughter)
11:39
So almost 400 years ago, Shakespeare was portraying
11:42
adolescents in a very similar light to the light that we
11:44
portray them in today, but today we try to understand
11:47
their behavior in terms of the underlying changes
11:51
that are going on in their brain.
11:54
So for example, take risk-taking. We know that adolescents
11:56
have a tendency to take risks. They do.
11:59
They take more risks than children or adults,
12:02
and they are particularly prone to taking risks
12:04
when they're with their friends. There's an important drive
12:07
to become independent from one's parents
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and to impress one's friends in adolescence.
12:12
But now we try to understand that in terms of
12:15
the development of a part of their brain called the limbic system,
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so I'm going to show you the limbic system in red
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in the slide behind me, and also on this brain.
12:23
So the limbic system is right deep inside the brain,
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and it's involved in things like emotion processing
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and reward processing. It gives you the rewarding feeling
12:32
out of doing fun things, including taking risks.
12:35
It gives you the kick out of taking risks.
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And this region, the regions within the limbic system,
12:41
have been found to be hypersensitive to the rewarding
12:44
feeling of risk-taking in adolescents compared with adults,
12:47
and at the very same time, the prefrontal cortex,
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which you can see in blue in the slide here,
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which stops us taking excessive risks,
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is still very much in development in adolescents.
13:00
So brain research has shown that the adolescent brain
13:03
undergoes really quite profound development,
13:07
and this has implications for education, for rehabilitation,
13:10
and intervention. The environment, including teaching,
13:14
can and does shape the developing adolescent brain,
13:18
and yet it's only relatively recently that we have been
13:22
routinely educating teenagers in the West.
13:24
All four of my grandparents, for example, left school
13:27
in their early adolescence. They had no choice.
13:31
And that's still the case for many, many teenagers
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around the world today. Forty percent of teenagers
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don't have access to secondary school education.
13:40
And yet, this is a period of life where the brain is
13:45
particularly adaptable and malleable.
13:47
It's a fantastic opportunity for learning and creativity.
13:50
So what's sometimes seen as the problem
13:54
with adolescents — heightened risk-taking, poor impulse
13:56
control, self-consciousness — shouldn't be stigmatized.
13:59
It actually reflects changes in the brain that provide
14:02
an excellent opportunity for education
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and social development. Thank you. (Applause)
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(Applause)
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Translated by Joseph Geni
Reviewed by Morton Bast

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About the Speaker:

Sarah-Jayne Blakemore - Cognitive Neuroscientist
Sarah-Jayne Blakemore studies the social brain -- the network of brain regions involved in understanding other people -- and how it develops in adolescents.

Why you should listen

Remember being a teenager? Rocked internally with hormones, outwardly with social pressures, you sometimes wondered what was going on in your head. So does Sarah-Jayne Blakemore. And what she and others in her field are finding is: The adolescent brain really is different.

New brain imaging research and clever experiments are revealing how the cortex develops -- the executive part of the brain that handles things like planning, self-awareness, analysis of consequences and behavioral choices. It turns out that these regions develop more slowly during adolescence, and in fascinating ways that relate to risk-taking, peer pressure and learning.

Which leads to a bigger question: How can we better target education to speak to teenagers' growing, changing brains?

More profile about the speaker
Sarah-Jayne Blakemore | Speaker | TED.com