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RENEE MONTAGNE, host:

It's MORNING EDITION from NPR News. I'm Renee Montagne.

STEVE INSKEEP, host:

And I'm Steve Inskeep. Good morning.

Today in Your Health, we will report on the mysteries of the middle-aged brain. Sure, maybe you're misplacing your keys a little more but in a minute, we'll tell you there's good news, too.

MONTAGNE: But first, we're going to see about what's going on in the teenage brain. When adolescence hits, you may find yourself the parent of a newly frustrating creature - sullen, self-centered and reckless. Or you might've been one at one time. Beneath that brand-new mohawk, there is a brain. It just works differently. One scientist mom decided to find out why. Here's NPR's Richard Knox.

RICHARD KNOX: When her two sons hit adolescence, Frances Jensen found herself saying the same thing over and over again.

Dr. FRANCES JENSEN (Pediatric Neurologist, Children's Hospital, Boston): What were you thinking? You know, this is that resounding mantra of parents and teachers.

KNOX: Jensen couldn't fathom her teenagers' behavior: the surly moods, the exasperating changes of plans, the assumption that somebody else will pick up their dirty clothes.

Now, Jensen's a pediatric neurologist at Children's Hospital in Boston. But her own teens suddenly seemed like aliens.

Dr. JENSEN: I thought, you know, I'm going to try to understand what's going on with this species, you know, these teenagers that we're living with.

KNOX: What really got her into gear was her older son, Andrew. It was when he turned 16.

Dr. JENSEN: He's a very compliant kid, was a great kid. And then his grades started to slip, and this hadn't been an issue before.

Mr. ANDREW JENSEN: I did dumb things. Yeah. I got in trouble with the school sometimes for just not really caring, doing really poorly in classes.

KNOX: He dyed his hair black with red streaks, started going off to school in studded leather and platform shoes.

Dr. JENSEN: I watched my child morph into another being, yet I knew deep down inside, it was the same Andrew.

KNOX: As Jensen dug up studies on teenage brains, she discovered it's not so much what they're thinking, it's how. Jensen says scientists used to think that the teen brain is just an adult brain with fewer miles on it. It's not. To begin with, a crucial part of the brain, the frontal lobe, is not fully connected.

Dr. JENSEN: It's the part of the brain that says: Is this a good idea? What is the consequence of this action?

(Soundbite of laughter)

KNOX: It turns out that the nerve cells that connect teenagers' frontal lobes with the rest of their brain don't work very well. They're sluggish. This is a little complicated, but it's really interesting. Teenagers don't have as much of the fatty coating, called myelin, that adult brains have in this area. Think of it as insulation on an electrical wire. Nerves need myelin for nerve signals to flow freely.

Dr. JENSEN: It's not that they don't have a frontal lobe. And they can use it. They're going to access it more slowly.

KNOX: This might explain what was going on in the brain of Jensen's younger son, Will, when he was 16. He's a straight-arrow student, good grades, high SAT scores. But one morning on the way to school, he turned left into oncoming traffic. He and the other driver were OK, but there was serious damage to the car.

Mr. WILL JENSEN: It was totaled. It was down and out. So it had to get towed away.

Dr. JENSEN: And lo and behold, who was the other driver? It was a 21-year-old -also probably not with a completely connected frontal lobe.

KNOX: Jensen says this also helps explain why teenagers often seem so maddeningly self-centered.

Dr. JENSEN: You think of them as these surly, rude, selfish people. Well, actually, that's the developmental stage they're at. They aren't yet at that place where they're thinking about - or capable, necessarily, of thinking about the effects of their behavior on others. That requires insight.

KNOX: And insight requires - that's right - a fully connected frontal lobe.

But that's not the only big difference in teenagers' brains. In children and adolescents, nature made the brain to be excitable, responsive to everything in the environment. That's what makes kids learn so easily. But this can work in ways that are not so good. Take alcohol, for example.

Dr. JENSEN: Addiction has been shown to be, essentially, a form of learning. So they're tapping into a much more robust habit-forming ability that adolescents have, compared to adults.

KNOX: This came in handy in Jensen's own household.

Mr. WILL JENSEN: Most parents, they'll say, don't drink. Don't do drugs. And I'm the type of kid who would say, why?

KNOX: That's Will, son number two. When he asked why, his mom could give him chapter and verse about how drug effects linger for days. So if her boys were tempted, they knew what to say to themselves.

Dr. JENSEN: It's a fact that, you know, if I smoke pot tonight and I have an exam in two days' time, I'm going to do worse. It's a fact.

KNOX: Will says having a neuroscientist for a mom was helpful with his homework, too, like when he was tempted to pull an all-nighter.

Mr. WILL JENSEN: She would say, read it all tonight and then go to sleep. And what she explained to me is that it will take from your short-term memory what you've been reading and what you know now, and while you sleep, it will consolidate it. And actually, you will know it better in the morning than right before, when you went to sleep.

KNOX: He says it worked every time. His older brother Andrew, the former Goth, agrees. He's now a senior in physics at Wesleyan University.

Mr. ANDREW JENSEN: I think she's great. I would not be where I am without her in my life.

KNOX: As a parent myself, who's survived two teenagers, I can tell you there are no sweeter words.

Richard Knox, NPR News, Boston.

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