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The mysterious workings of the adolescent brain

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    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,
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    neuroscientists have started to look inside the living
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    human brain of all ages, and to track changes
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    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
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    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
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    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.
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    (Laughter)
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    It can go on a long time. (Laughter)
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    One of the brain regions that changes most dramatically
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    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
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    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
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    or next week or next year, inhibiting
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    inappropriate behavior, so stopping yourself saying
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    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.
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    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
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    increases during childhood, which is what you can see
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    on this graph. It peaks in early adolescence.
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    The arrows indicate peak gray matter volume
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    in prefrontal cortex. You can see that that peak happens
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    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.
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    This is a really important process. It's partly dependent
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    on the environment that the animal or the human is in,
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    and the synapses that are being used are strengthened,
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    and synapses that aren't being used
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    in that particular environment are pruned away.
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    You can think of it a bit like pruning a rosebush.
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    You prune away the weaker branches so that
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    the remaining, important branches, can grow stronger,
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    and this process, which effectively fine-tunes brain tissue
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    according to the species-specific environment,
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    is happening in prefrontal cortex and in other brain regions
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    during the period of human adolescence.
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    So a second line of inquiry that we use to track changes
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    in the adolescent brain is using functional MRI
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    to look at changes in brain activity across age.
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    So I'll just give you an example from my lab.
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    So in my lab, we're interested in the social brain, that is
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    the network of brain regions that we use to understand
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    other people and to interact with other people.
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    So I like to show a photograph of a soccer game
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    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
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    on the ground, and the first aspect of the social brain
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    that this picture really nicely illustrates is how automatic
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    and instinctive social emotional responses are,
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    so within a split second of Michael Owen missing this goal,
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    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
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    with his arms, and presumably has a similar
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    facial expression, and the only people who don't
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    are the guys in yellow at the back — (Laughs) —
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    and I think they're on the wrong end of the stadium,
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    and they're doing another social emotional response
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    that we all instantly recognize, and that's the second aspect
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    of the social brain that this picture really nicely illustrates,
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    how good we are at reading other people's behavior,
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    their actions, their gestures, their facial expressions,
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    in terms of their underlying emotions and mental states.
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    So you don't have to ask any of these guys.
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    You have a pretty good idea of what they're feeling
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    and thinking at this precise moment in time.
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    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
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    to have a brain scan, we give them some kind of task
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    that involves thinking about other people, their minds,
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    their mental states, their emotions, and one of the findings
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    that we've found several times now, as have other labs
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    around the world, is part of the prefrontal cortex called
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    medial prefrontal cortex, which is shown in blue on the slide,
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    and it's right in the middle of prefrontal cortex
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    in the midline of your head.
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    This region is more active in adolescents when they make
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    these social decisions and think about other people
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    than it is in adults, and this is actually a meta-analysis
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    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
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    in this medial prefrontal cortex area decreases
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    during the period of adolescence.
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    And we think that might be because adolescents and adults
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    use a different mental approach, a different
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    cognitive strategy, to make social decisions,
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    and one way of looking at that is to do behavioral studies
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    whereby we bring people into the lab and we give them
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    some kind of behavioral task, and I'll just give you
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    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
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    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
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    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
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    gray piece of wood.
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    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.
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    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
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    condition whereby there's a kind of conflict
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    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
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    from your perspective, but then you have to remember,
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    "Oh, he can't see that truck, so he must mean
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    me to move the blue truck," which is the top truck
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    from his perspective. Now believe it or not,
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    normal, healthy, intelligent adults like you make errors
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    about 50 percent of the time on that kind of trial.
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    They move the white truck instead of the blue truck.
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    So we give this kind of task to adolescents and adults,
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    and we also have a control condition
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    where there's no director and instead we give people a rule.
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    We tell them, okay, we're going to do exactly the same thing
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    but this time there's no director. Instead you've got to
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    ignore objects with the dark gray background.
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    You'll see that this is exactly the same condition, only
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    in the no-director condition they just have to remember
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    to apply this somewhat arbitrary rule, whereas
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    in the director condition, they have to remember
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    to take into account the director's perspective
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    in order to guide their ongoing behavior.
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    Okay, so if I just show you the percentage errors
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    in a large developmental study we did,
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    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
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    in the adult group in both conditions,
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    so the gray is the director condition, and you see
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    that our intelligent adults are making errors about 50 percent
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    of the time, whereas they make far fewer errors
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    when there's no director present, when they just have
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    to remember that rule of ignoring the gray background.
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    Developmentally, these two conditions develop
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    in exactly the same way. Between late childhood
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    and mid-adolescence, there's an improvement,
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    in other words a reduction of errors, in both of these trials,
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    in both of these conditions.
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    But it's when you compare the last two groups,
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    the mid-adolescent group and the adult group
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    where things get really interesting, because there, there is
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    no continued improvement in the no-director condition.
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    In other words, everything you need to do in order to
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    remember the rule and apply it seems to be fully developed
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    by mid-adolescence, whereas in contrast,
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    if you look at the last two gray bars, there's still
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    a significant improvement in the director condition
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    between mid-adolescence and adulthood, and what
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    this means is that the ability to take into account someone
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    else's perspective in order to guide ongoing behavior,
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    which is something, by the way, that we do in everyday life all
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    the time, is still developing in mid-to-late adolescence.
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    So if you have a teenage son or a daughter and you
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    sometimes think they have problems taking other people's
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    perspectives, you're right. They do. And this is why.
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    So we sometimes laugh about teenagers.
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    They're parodied, sometimes even demonized in the media
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    for their kind of typical teenage behavior. They take risks,
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    they're sometimes moody, they're very self-conscious.
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    I have a really nice anecdote from a friend of mine
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    who said that the thing he noticed most
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    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,
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    stop messing around and I'll sing your favorite song,'
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    and instantly they'd stop messing around and he'd sing
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    their favorite song. After puberty, that became the threat.
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    (Laughter)
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    The very notion of their dad singing in public
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    was enough to make them behave.
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    So people often ask,
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    "Well, is adolescence a kind of recent phenomenon?
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    Is it something we've invented recently in the West?"
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    And actually, the answer is probably not. There are lots
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    of descriptions of adolescence in history that sound
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    very similar to the descriptions we use today.
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    So there's a famous quote by Shakespeare from "The Winter's Tale"
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    where he describes adolescence as follows:
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    "I would there were no age between ten and
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    three-and-twenty, or that youth would sleep out the rest;
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    for there is nothing in the between but getting wenches
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    with child, wronging the ancientry, stealing, fighting." (Laughter)
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    He then goes on to say, "Having said that, would any
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    but these boiled brains of nineteen and two-and-twenty
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    hunt in this weather?" (Laughter)
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    So almost 400 years ago, Shakespeare was portraying
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    adolescents in a very similar light to the light that we
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    portray them in today, but today we try to understand
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    their behavior in terms of the underlying changes
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    that are going on in their brain.
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    So for example, take risk-taking. We know that adolescents
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    have a tendency to take risks. They do.
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    They take more risks than children or adults,
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    and they are particularly prone to taking risks
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    when they're with their friends. There's an important drive
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    to become independent from one's parents
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    and to impress one's friends in adolescence.
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    But now we try to understand that in terms of
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    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.
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    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
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    out of doing fun things, including taking risks.
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    It gives you the kick out of taking risks.
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    And this region, the regions within the limbic system,
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    have been found to be hypersensitive to the rewarding
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    feeling of risk-taking in adolescents compared with adults,
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    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.
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    So brain research has shown that the adolescent brain
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    undergoes really quite profound development,
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    and this has implications for education, for rehabilitation,
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    and intervention. The environment, including teaching,
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    can and does shape the developing adolescent brain,
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    and yet it's only relatively recently that we have been
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    routinely educating teenagers in the West.
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    All four of my grandparents, for example, left school
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    in their early adolescence. They had no choice.
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    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.
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    And yet, this is a period of life where the brain is
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    particularly adaptable and malleable.
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    It's a fantastic opportunity for learning and creativity.
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    So what's sometimes seen as the problem
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    with adolescents — heightened risk-taking, poor impulse
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    control, self-consciousness — shouldn't be stigmatized.
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    It actually reflects changes in the brain that provide
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    an excellent opportunity for education
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    and social development. Thank you. (Applause)
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    (Applause)
Title:
The mysterious workings of the adolescent brain
Speaker:
Sarah-Jayne Blakemore
Description:

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.

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Video Language:
English
Team:
closed TED
Project:
TEDTalks
Duration:
14:26

English subtitles

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