May 11, 2004: Scripps Howard News Service
New research shows stark differences in teen brains
By Lee Bowman
Recent popular films depicting teenagers suddenly housed in adult bodies have more than a little truth in them.
The latest brain research has found strong evidence that when it comes to maturity, organization and control, key parts of the brain related to emotions, judgment and "thinking ahead" are the last to arrive.
"It seems that regulation of impulse control is the last on board and often the first to leave in the brain as we age," said Dr. Ruben Gur, a professor of psychology and director of the Brain Behavior Laboratory at the University of Pennsylvania who has been researching brain development in young adults.
Until recently, most brain experts thought the human command center stopped growing at around 18 months, and that neurons were pretty much set for life by age 3.
In fact, the brain's gray matter has a final growth spurt around the ages of 11 to 13 in the frontal lobes of the brain, the regions that guide human intellect and planning.
But it seems to take most of the teen years for youngsters to link these new cells to the rest of their brains and solidify the millions of connections that allow them to think and behave like adults.
At the same time, the release of a cascade of adolescent hormones during and after puberty causes other areas of the brain, particularly the amygdala, which governs basic emotional response, to fire up or expand.
The result is that teens look at things differently than adults. This has tremendous implications for education, mental health, drug abuse and moral and legal responsibility of adolescents.
Deborah Yurgelun-Todd of Harvard Medical School and McClean Hospital in Boston has studied how teenagers and adults respond differently to the same images. Shown a set of photos of people's faces contorted in fear, adults named the right emotion, but teens seldom did, often saying the person was angry.
When Yurgelun-Todd and her team did the same test while doing functional magnetic resonance imaging of the subject's brains, they found a stark difference in the parts being used. Adults used both the advanced prefrontal cortex and the more basic amygdala to evaluate what they had seen; younger teens relied entirely on the amygdala, while older teens (top age in the group was 17) showed a progressive shift toward using the frontal area of the brain.
"Just because teens are physically mature, they may not appreciate the consequences or weigh information the same way as adults do," Yurgelun-Todd said. "Good judgment is learned, but you can't learn it if you don't have the necessary hardware."
There is more evidence of the differences:
A recent imaging study by researchers at the National Institute on Alcohol Abuse and Alcoholism found that teens taking an experimental gambling test are less likely to activate a region in the base of the brain that motivates behavior to work to obtain rewards than a control group of young adults, ages 22-28, playing the same games.
Numerous studies show alcohol and perhaps other drugs hit teen brains harder than they do adult brains. The frontal lobes and the hippocampus, which is involved in memory formation, are particularly vulnerable.
It has been known for some time that children have sharp growth spurts in brain connections among regions specialized for language and spatial relationships between ages 6 and 12. That language capacity tends to reside mostly in a person's nondominant side - the left hemisphere of the brain in right-handers, for instance. But a recent imaging study by researchers at the University of Cincinnati Medical Center found that this distinction ends in the mid-20s when the brain shifts to use both sides in language processing.
The story of teen brain development lies in a process called myelinization, in which a layer of fat coats wire-like fibers connecting regions of the brain, back-to-front, side-to-side, and everywhere in between. Over time, this makes the operation of the brain more precise and efficient, affecting not just thinking and problem-solving, but also coordination and mastery of skills ranging from throwing a baseball to playing the trombone.
But there's a price for this greater efficiency -brain cells that aren't hooked up to other parts tend to get killed off.
"If they're not on the network, they die and their place is taken up with cerebral fluid. This goes on well beyond age 18," said Dr. David Fassler, a psychiatrist at the University of Vermont.
Even in adulthood, the wiring job is not completely done. Imaging done on the brains of people in their 40s and 50s show there's another surge of connections being made, perhaps in response to menopause or to prepare the brain to better compensate for the loss of brain cells as we age.
Still, it's a slow, arduous road to maturity and insight for teens.
"We have some new insight into the 16 year-old that doesn't think twice about getting in a car with a friend who's been drinking, but they're still not going to appreciate adults arguments for why they shouldn't," said Fassler.
At the National Institute of Mental Health, Dr. Jay Giedd, who helps run the ongoing imaging studies that first detected the middle school growth spurt, said the new understanding of teen brains "argues for doing a lot of things as a teenager. You are hard-wiring you brain in adolescence. Do you want to hard-wire it for sports and playing music and doing mathematics, or for lying on the couch in front of the television?"
The new understanding of adolescent brains leads to questions of ethics and legalities.
The Supreme Court already has decided that people should not be executed for crimes committed when they were age 15 or younger, and in the fall is scheduled to consider whether the restriction should be extended to everyone under 18.
Two years ago, the court banned execution of mentally retarded people because of deficiencies that "diminish their personal culpability."
"With the new biological explanation that adolescent brains are different, we think there's scientific evidence that they, too, are less culpable," said Stephen Harper, an adjunct professor of juvenile justice at the University of Miami School of Law who specializes in capital cases.
Gur said some scientists would put off the age of legal majority to 22 or 23, and said there will likely be considerable debate over how to tell when a person's brain physically looks like an adult's as imaging research continues and efforts to set standards and norms develop.
Fassler predicts that within a decade, brain images will be sophisticated enough to "help us determine the age for appropriate treatment of addictions and therapy models for adults and adolescents with disorders."
Other researchers say that while it's possible to gain general understanding about brain development and function from the images, the notion that medicine, law enforcement or anyone else should work from some ideal, normal brain model is troubling.
"Each individual is not an exact map, and the difficulties in determining what the range of variations are is really dangerous. The data is incredibly easy to be over-interpreted," said Sonia Miller, a New York attorney who specializes in cases dealing with new technologies.
Some courts are already accepting brain scans as evidence of a person's mental capacity in criminal cases, she said, and "as the neuroscience of intentional behavior develops, the way we assign responsibility and blame will be challenged. This raises a lot of questions about how much neural privacy can we expect, how much the authorities can get into your brain."
Dr. Peter Bandettini, a brain-imaging researcher at the National Institutes of Health, said the science of understanding what small structures and chemicals are doing within the brain is far from a gold standard for mental function or age.
"Right now, I personally think you'd get more information about a person's mental age by going to a set of behavioral tests. But I'd agree that as these technologies become more powerful, there's going to be a greater need for checks and balances to determine how the imaging information should be used."
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