Teenage Brain - The Prefrontal Cortex

Excerpted from
The Primal Teen: What the New Discoveries About the Teenage Brain Tell Us About Our Kids
By Barbara Strauch

When scientists talk of the frontal lobes of the brain, they're most often referring to the prefrontal cortex, the section of the brain located directly behind the forehead. Human brains are little wads of evolutionary history. Sonic parts, such as the brain stem and parts of the inner limbic system-areas that regulate emotions and gut reactions, the "I'd like to cat you now" instincts-we share with your basic crocodile.

But the prefrontal cortex is something else again. Neuroscience textbooks are often illustrated with stark diagrams that show how the prefrontal cortex has grown disproportionately huge in human beings. First, there's usually a drawing of a rat's brain with its tiny smooth prefrontal cortex, followed by a cat's brain with a bigger prefrontal cortex, then the monkey with its bigger-still brain and prefrontal cortex. And then, by way of finale, the human brain, outsized, folded in on itself with its puffed-out prefrontal cortex. By some estimates, the human prefrontal cortex, over the course of evolutionary history, has increased a whopping 29 percent, while the same region in our closest relative, the chimpanzee, has grown perhaps 17 percent and that of a cat only 3 percent.

Development of a baby's brain in the womb roughly follows a step-by-step path. The brain stem and parts of the limbic system develop early, only later followed by the wrinkled prefrontal cortex and other more sophisticated sections, in rudimentary forms. (Interestingly, parts of the late-developing frontal cortex are also often the first to disintegrate in degenerative diseases such as Alzheimer's. Scientists speculate that that might be a price the area has to pay for its sophistication. It may simply get worn out faster than other parts of the brain due to its life of plasticity, its unparalleled knack for interacting with and adapting to the environment.)

The frontal lobes, in other words-the same parts that scientists have now found to be undergoing so much change at adolescence-are a big deal in the human brain. They also have, to my mind, some of the most intriguing stories. Neurologist Oliver Sacks, author of The Man Who Mistook His Wife for a Hat and other books of neurological oddities, once told the story of a man with damage to his frontal lobes who became, through a distinct lack of impulse control, a compulsive "toaster." "He would rise to his feet when dining in restaurants," Sacks wrote in the journal Neurology, "clear his throat loudly to command attention and propose a toast to Her Majesty the Queen. The diners around him would be surprised, but rise to their feet and obediently lift their glasses. A minute or two later, the performance would be repeated, this toast, perhaps to the Lord Mayor of London. Other toasts would follow at frequent intervals, until his embarrassed family had to take him from the restaurant. The patient... seemed to enjoy his toasting but (while of considerable intelligence) showed no insight into it."

On Chuck Nelson's wall at his office at the University of Minnesota is a picture of a white skull with a red pole sticking out of the forehead, the head of Phineas Gage, the subject of one of the better-known stories in brain science lore.

Phineas had been an extremely capable and likable fellow until, while working as foreman on a railroad in Vermont in 1848, an explosion sent a thirteen-pound iron tamping rod through the front of his skull. After the accident, Phineas could function fine physically and converse. But he was a changed man. He lied, he stole, he cursed. He became impulsive and could not, for the life of him, plan ahead.

It was Phineas who started scientists speculating in the nineteenth century that the front part of the brain was somehow quite important in a whole range of humanlike behavior, particularly

the ability to inhibit impulses and plan. More than a hundred years later, Antonio Daniasio and his wife. Hanna, two of the country's leading neuroscientists at the University of Iowa, examined Phineas s skull, preserved at Harvard, and, using modern imaging techniques, pinpointed the site of the damage. The rod had, indeed, gone through a section of the prefrontal cortex.

In recent years, neuroscience has been engaged in an all-out effort to zero in on exactly what this important brain part does and how it does it. In the 1970s and 1980s, neuroscientists Patricia Goldman-Rakic at Yale and Adele Diamond, now at the Eunice Kennedy Shriver Center in Massachusetts, did a series of elegant experiments that showed when and how the prefrontal cortex kicks into action. The tests they did-Goldman-Rakic on rhesus monkeys and Diamond on human infants-are called delayed response tests, or A not 13 tests, designed to find out how long a subject can retain information that is no longer in view. To do this, a monkey or child must hold a representation of the picture in mind and inhibit the impulse to let other irrelevant information take its place.

The A not B task has been linked to a function of the prefrontal cortex, more precisely the dorsolateral prefrontal cortex, that neuroscientists often refer to as working memory-the ability to keep a seven-digit phone number in your mind just long enough to dial the number, for instance. It is believed that working memory, often referred to as the brains blackboard or Post-it note, is linked to impulse control. As Goldman-Rakic puts it: "If you are not able to direct responses by mental representations, you also are not able to withhold reflexive responses to irrelevant or salient stimuli." In other words, if you can't inhibit your brain from responding to every urgent e-mail from your friends, you'll forget your homework again.

In the Goldman-Rakic and Diamond tests, monkeys and humans did versions of similar tests. First they watched as a bit of food or toy was hidden in one of two or three receptacles, and then these receptacles were blocked from their view. After a few seconds, the receptacles were brought back and the animals and children had to remember which one contained the hidden food or toy.

Young monkeys and young infants largely flunked. But the researchers found that as they got a bit older, they could keep information about the items location in their heads for longer and longer periods. Monkeys got good at the task at around four months and human infants started to improve at an equivalent age of about seven months. By a year, the human babies could remember where the food was hidden for ten seconds.

As it turned out, the monkeys and babies had acquired a hugely important basic skill as, according to timetables established in earlier studies, synapses in their brains' prefrontal cortex began to flourish. And later studies showed that the precision of this skill improves up and through late adolescence along a path that mirrors the delayed refinement of the prefrontal cortex.

From this experiment and others that track development of functions-for example, vision-many scientists have concluded that a flourishing or exuberance of synapses in the prefrontal cortex-an overproduction similar to what Giedd found in adolescent brains-may be linked to establishing and refining crucial brain functions.

As Goldman-Rakic explains it. the brain must be at a stage of readiness, in terms of densities of synapses, before it acquires certain basic skills. "The brain seems to have to get to a certain level of circuitry, to be fully wired, before certain behaviors emerge."