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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)