It is late in the evening rush hour, typical stop-and-go traffic. Finally, there is a break; the tightly packed group around you is soon cruising together at 55 mph. Suddenly, you see brake lights flare up ahead. As you prepare to brake, you glance in the rearview mirror and see an alarming sight--a car closing way too fast on your rear fender. The teenage driver looks panicked, one hand clutching the steering wheel, the other hand clenching a cell phone. You brace for the terrible impact...
We are quick to blame adolescents for getting themselves into predicaments that adults believe could be easily avoided. But recent research indicates that simple irresponsibility may not be the full explanation. When teenagers perform certain tasks, their prefrontal cortex, which handles decision making, is working much harder than the same region in adults facing the same circumstances. The teen brain also makes less use of other regions that could help out. Under challenging conditions, adolescents may assess and react less efficiently than adults.
Understanding the capabilities and limitations of the brain at different developmental stages is crucial for education and psychological assessment. Ironically, although the teenage years are widely recognized as a period of tremendous growth and change, the mental capabilities of teens have been less studied than those of children or adults. As more work is completed, it is becoming apparent that society should not be fooled into thinking that a teen has the mental prowess of an adult just because he or she looks and, most of the time, behaves like one. Brain processes that support cognitive control of behavior are not yet mature. Add stressors to the mix--like a sudden highway jam--and a teen can be an accident waiting to happen.
As recent studies underscore, differences in the prefrontal cortex--responsible for the so-called executive function that underlies planning and voluntary behavior--may be one of the most important distinctions between adolescents and adults. Beatriz Luna, director of the Laboratory of Neurocognitive Development at the University of Pittsburgh, has pinpointed differences by scanning the brains of teens and adults with functional magnetic resonance imaging (fMRI) during demanding tests of the visual-motor system.
In one setup, subjects faced a computer that flashed lights randomly. They were told either to rapidly focus on the lights or to try to avoid looking at them. Luna found that, when trying to block a strong reflexive tendency and make a considered response, "teens used more of their prefrontal cortex resources than adults did." Indeed, the amount of prefrontal cortex employed was similar to what adult brains use when performing a much more complex task. This excessive reliance, Luna says, "can lead to error, especially when difficulty increases."
Psychologists distinguish between two types of behavior control: exogenous and endogenous. Exogenous control is reflexive, generated in response to external stimuli--for example, focusing on lights as they appear on the screen. Endogenous control is voluntary and generated by an internal plan--trying not to look at the lights. A mature prefrontal cortex makes it easier for endogenous behavior to override exogenous behavior. In the traffic scenario, the exogenous response of the teen who suddenly realizes he is going to hit your rear bumper would be to freeze and scream, whereas the endogenous response would be to brake hard and steer out of the way. But for teen brains, deliberately overriding the exogenous reaction is more difficult than it is for adult brains.
Experts such as Luna maintain that although adolescents can at times demonstrate adult-level cognitive control of decision making, this endogenous power is only beginning to mature. In the visual-motor tests, she explains, subjects must use the prefrontal cortex to tell the rest of the brain how to behave. "Adolescents show similar capabilities of inhibition compared with adults, but the fMRIs show that they are using up prefrontal cortex like crazy," Luna notes. Adults call on other parts of the brain "to collaborate and better distribute the workload," she adds.
The implication is that if something unexpected occurs in an already stressful situation, an adolescent may exhaust his or her prefrontal cortex resources. Adults can better handle the stress by tapping other brain regions. And in everyday life, general overtaxing of the prefrontal cortex may undermine executive function, impairing planned behaviors and choices. That may explain why adolescents exhibit impulsive or thoughtless behavior. For example, Luna says, it may be easier for adults to suppress bad responses to peer pressure. They may be better able to keep themselves in line, rather than succumbing to temptation.
Overloading the Cortex
Full maturation of executive function occurs only as a completely integrated, collaborative brain system emerges, in the late teens and even in the early 20s, according to psychologists. But in adolescents, a key contributor that helps to guide voluntary behavior--working memory--is also still developing. Lunas fMRI images support the conclusion that adolescents are not as efficient in recruiting areas that support working memory.
Weak integration has also been found by Susan F. Tapert, associate professor of psychiatry at the University of California, San Diego, who investigated spatial working memory in earlier and later adolescence. Tapert tested 25 young teens (ages 12 to 14) and 24 older teens (ages 15 to 17) using fMRI. Older adolescents, she says, "showed more intense and widespread dorsolateral prefrontal activation as they performed a working-memory task, and used more right inferior parietal cortex but less superior parietal cortex than younger adolescents."
Tapert infers that older adolescents recruit different neural networks and employ different strategies to perform the same job. Older teens used regions that suggested they solved the task through a verbal strategy rather than through simple (yet taxing) rote spatial rehearsal, which appears to be how the younger teens performed the task. Over the course of adolescence, the brain involves more areas in general and distributes certain tasks to specialized regions, thereby reducing the neuronal effort necessary to achieve the same level of performance. "I was surprised with the magnitude of change we observed across this relatively narrow age range," Tapert says.
Early adolescents can perform well on spatial working-memory tests, but it appears they need to engage in more neural activity to do so. They also become much less efficient if they are stressed when asked to perform an additional task. Only at the end of adolescence, Tapert says, is spatial working memory efficiently distributed across brain regions.
Recent structural MRI images of adolescent brains lend credence to the notion that regions of the teen brain involved in decision making and behavior control undergo significant physical changes. Jay N. Giedd, a psychiatrist and investigator in the Child Psychiatry Branch at the National Institute of Mental Health, has shown that the dorsolateral prefrontal cortex, important in controlling impulses, undergoes synaptic pruning--the elimination of unnecessary connections between neurons. This results in more efficient transmission of nerve impulses.
Most researchers agree that pruning is a fundamental mechanism for brain maturation. So is adding more myelin--insulation around the axons that send signals from neuron to neuron. Both changes translate into improved brain function. Synaptic pruning increases efficiency of local computations, whereas myelination speeds up neuronal transmissions. As a result, Luna notes, the prefrontal cortex is more able to impose voluntary and planned behaviors.
Giedd evaluates data from ongoing MRI studies conducted at the Child Psychiatry Branch. A recent study draws from a pool consisting of 307 children and adolescents who underwent MRI scans and neuropsychological testing. Many have been retested every two years. Giedd says the initial surprise is "that the brain doesnt change that much in size from age six on." The skull thickens, but the brain is at 90 percent adult size. Its overall breadth is stable during the teen years, "but the components change in size and shape," he adds.
The MRI images show alterations in the wiring among neurons involved in decision making, judgment and impulse control, as well as in the wiring the prefrontal cortex uses to tie brain regions together. Along with other studies, the images show that the prefrontal cortex seems to continue maturing well into the 20s. "It is striking how dynamically the brain changes during the teen years and how long it changes into young adulthood," Giedd says. "Frankly, it surprised us that [ongoing change] lasted so long." Whereas much change occurs during the teen years, adaptation in the prefrontal cortex continues for a number of years afterward.
Not all neuroscientists or psychologists are ready to accept that the teen brains innate biology explains reckless behavior, however. Robert Epstein, a psychologist, visiting scholar at the University of California, San Diego, and founder of the Cambridge Center for Behavioral Studies, says he is "infuriated" by the very concept that there is a teen brain that is so different from an adult brain. "There is no such thing. Its a hoax, pushed to some extent by drug companies who are funding research," he asserts.
To bust the myth that routine brain development underlies teenage behavioral problems, Epstein cites the influential book Blaming the Brain, by Elliot S. Valenstein (Free Press, 1998), now psychology professor emeritus at the University of Michigan at Ann Arbor. It implies that some neuroscientists come under the influence of drug companies that want to develop the idea that the brain is at fault, easing the way for doctors to prescribe psychoactive drugs. (Note that none of the studies discussed in this article were funded by drug companies.)
Perhaps more persuasive is Epsteins observation that studies that implicate a teen brain tend to look only at American adolescents. He says research shows that "teens in other countries and developing nations dont behave or feel like American teens. If you look at multicultural and causation issues, there is no teen brain" that is universally different from adult brains.
American culture has come to define teenage years as tumultuous. "But most teens around the world dont experience any such turmoil," Epstein notes, citing a massive study by anthropologist Alice Schlegel of the University of Arizona and psychologist Herbert Barry III of the University of Pittsburgh. Their book Adolescence: An Anthropological Inquiry (Free Press, 1991) examined teens in 186 preindustrial cultures. Schlegel and Barry found that 60 percent of the cultures do not even have a word for adolescence and that most teens spend much of their time with adults, not segregated with only their peers. Antisocial behavior was absent in over half the cultures; where it was found, it was mild.
This is "mind-boggling," Epstein declares, because in America "we define the teen years as storm and stress. To point to the brain as the cause of everything bad is wrong, because environment changes the brain. We live in a society where kids are isolated from adults, so they learn from each other." And that, he says, can be a recipe for trouble. Epstein contends that when a society raises adolescents to experience a smooth, swift transition to adulthood, much of the angst assumed to be a given with teens is absent.
Ready or Not
Luna calls Epsteins view "interesting," although she does not agree. Either way, she says, her experiments control for cultural differences because they look at brain function based on emotionally neutral stimuli, not socially relevant behavioral decisions.
As for environmental influence, Luna says the fMRI images suggest that the brain is a vulnerable system and that in an environment with many stresses it is more difficult for adolescents to show self-control as compared with adults. She points out that the structure of the teen brain is "not ready" and that this is a good thing, because it allows the brain to develop more consistently with the particular environment in which it matures. "We're trying to understand the brain-behavior relationship," she adds. "Its not like the teen brain is different from other brains. There is a continuum."
The visual-motor test, she observes, is very difficult, "because the whole brain is wired to look at a visual stimulus." Asking subjects to not look at the light requires frontal regions to communicate with subcortical regions to enforce a planned, endogenous response ("I will not look at the light") that overrules the reflexive, exogenous response ("Look at the light"). "We're asking a teen to do something" that, at most, is only remotely related to risk-taking behavior, she says. "It is a way to look at the basic ability to inhibit a response." Because adolescents have a much harder time performing tasks that require voluntary control, they could be more prone to bad decision making.
Yet when adolescents are in situations with few competing demands, they do indeed behave like adults, Luna says. In preindustrial cultures that is the more likely environment, "so, of course, those teens might not exhibit risk-taking behavior. That doesnt mean their brain is not pruning," she explains. "Or that there isnt something uniquely special about adolescence."
LESLIE SABBAGH is a science journalist who specializes in medicine and aerospace. She has flown on combat medevac missions in Iraq and NASA science and microgravity flights.
- Adolescent Brain Development: Vulnerabilities and Opportunities. Edited by Ronald E. Dahl and Linda Patia Spear. Annals of the New York Academy of Sciences, Vol. 1021; June 2004.
- fMRI Reveals Alteration of Spatial Working Memory Networks across Adolescence. A. D. Schweinsburg, B. J. Nagel and S. F. Tapert in Journal of the International Neuropsychological Society, Vol. 11, pages 631644; 2005.
- Intellectual Ability and Cortical Development in Children and Adolescents. P. Shaw et al. in Nature, Vol. 440, pages 676679; March 30, 2006.