The brain develops in childhood and adolescence from back to front with the higher order brain centers developing last.
The brain's center of reasoning and problem solving is among the last to mature, a new study graphically reveals. The decade-long magnetic resonance imaging (MRI) study of normal brain development, from ages 4 to 21, by researchers at NIH's National Institute of Mental Health (NIMH) and University of California Los Angeles (UCLA) shows that such "higher-order" brain centers, such as the prefrontal cortex, don't fully develop until young adulthood.
A time-lapse 3-D movie that compresses 15 years of human brain maturation, ages 5 to 20, into seconds shows gray matter - the working tissue of the brain's cortex - diminishing in a back-to-front wave, likely reflecting the pruning of unused neuronal connections during the teen years. Cortex areas can be seen maturing at ages in which relevant cognitive and functional developmental milestones occur. The sequence of maturation also roughly parallels the evolution of the mammalian brain, suggest Drs. Nitin Gogtay, Judith Rapoport, NIMH, and Paul Thompson, Arthur Toga, UCLA, and colleagues, whose study is published online during the week of May 17, 2004 in The Proceedings of the National Academy of Sciences.
"To interpret brain changes we were seeing in neurodevelopmental disorders like schizophrenia, we needed a better picture of how the brain normally develops," explained Rapoport.
The researchers scanned the same 13 healthy children and teens every two years as they grew up, for 10 years. After co-registering the scans with each other, using an intricate set brain anatomical landmarks, they visualized the ebb and flow of gray matter - neurons and their branch-like extensions - in maps that, together, form the movie showing brain maturation from ages 5 to 20.
It was long believed that a spurt of overproduction of gray matter during the first 18 months of life was followed by a steady decline as unused circuitry is discarded. Then, in the late l990s, NIMH's Dr. Jay Giedd, a co-author of the current study, and colleagues, discovered a second wave of overproduction of gray matter just prior to puberty, followed by a second bout of "use-it-or-lose-it" pruning during the teen years.
The new study found that the first areas to mature (e.g., extreme front and back of the brain) are those with the most basic functions, such as processing the senses and movement. Areas involved in spatial orientation and language (parietal lobes) follow. Areas with more advanced functions -- integrating information from the senses, reasoning and other "executive" functions (prefrontal cortex) - mature last.
In a related study published a few years ago, Rapoport and colleagues discovered an exaggerated wave of gray matter loss in teens with early onset schizophrenia. These teens, who became psychotic prior to puberty, lost four times the normal amount of gray matter in their frontal lobes, suggesting that childhood onset schizophrenia "may be an exaggeration of a normal maturation process, perhaps related to excessive synaptic pruning," note the researchers. By contrast, children with autism show an abnormal back-to-front wave of gray matter increases, rather than decreases, suggesting "a specific faulty step in early development."
Also participating in the new study were: Leslie Lusk, Cathy Vaituzis, Tom Nugent, David Herman, Drs. Deanna Greenstein, Liv Clasen, NIMH; Kiralee Hayashi, UCLA.
This next article reviews a number of recent studies on brain development and includes quotes from scientists arguing over whether the brain scan studies and other advances in neurobiology are evidence for reducing the legal responsibility of adolescents for criminal actions.
The ambiguities of science don't mix with social and political causes, contends neuroscientist Bradley S. Peterson of the Columbia College of Physicians and Surgeons in New York City. For instance, it's impossible to say at what age teenagers become biologically mature because the brain continues to develop in crucial ways well into adulthood, he argues.
A team led by Sowell and Peterson used an MRI scanner to probe the volume of white and gray matter throughout the brains of 176 healthy volunteers, ages 7 to 87. The researchers reported in the March 2003 Nature Neuroscience that myelin formationómeasured by the total volume of white matter in the entire brainódoesn't reach its peak until around age 45.
Although gray matter volume generally declines beginning around age 7, it steadily increases until age 30 in a temporal-lobe region associated with language comprehension.
That previous article opens up with arguments by David Fassler and Ruben Gur against the application of the death penalty to adolescents. Fassler and Gur believe the brains of adolescents are not yet fully enough formed to allow them to understand the intentions of others or to control themselves. Surely there is lots of evidence for on-going changes in the brain in adolescence and beyond. Some of the results are a bit surprising. For instance, while language processing starts out on one side of the brain language processing becomes more evenly distributed across both sides of the brain in one's mid-20s.
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.
One particularly important (yet still preliminary) finding about adolescents by Deborah Yurgelun-Todd and colleagues at Harvard and McLean Hospital is that adolescents tend to misinterpret fearful expressions on faces as anger or other emotions.
What does your work tell you about young teenagers?
One of the implications of this work is that the brain is responding differently to the outside world in teenagers compared to adults. And in particular, with emotional information, the teenager's brain may be responding with more of a gut reaction than an executive or more thinking kind of response. And if that's the case, then one of the things that you expect is that you'll have more of an impulsive behavioral response, instead of a necessarily thoughtful or measured kind of response.
Does this research go part of the way to explaining the miscues between adult and teenagers?
Yes, I do think this research goes to helping understand differences between adults and teenagers in terms of communications. And I think that it does for two reasons. One, we saw that adults can actually look at fearful faces and perceive them as fearful faces, and they label them as such, whereas teenagers ... don't label them the same way. So it means that they're reading external visual cues [differently], or they're looking at affect differently.
The second aspect of the findings are that the frontal region, or this executive region, is activating differentially in the teenagers compared to adults. And I think that has important implications in terms of modulating their own responses, or trying to inhibit their own gut responses.
But even if all the evidence does point to problems with adolescent brains that is not necessarily an argument for less severe punishment for adolescents. One reason for that is the lower levels of punishment can be ruthlessly exploited by adolescents aware of the fact that they face less severe consequences for their actions. Also, if adolescents really are more prone to violent behavior then perhaps more severe punishment is needed to deter them than is needed deter adults.
My more fundamental objection to the argument for reduced legal liability is that it can not be enacted unilaterally without other compensatory changes. A free society is built on the assumption that its members are competent moral agents. While this is obviously not always true and scientific advances are likely going to chip away at that assumption even more in the future it really is a necessary assumption for a free society. People are given a great deal of latitude on what they can do and what legal protections they have because most are considered to be morally competent.
A reduction in assumed moral competence would need to be accompanied with a reduction in latitude of actions and autonomy which is allowed. For example, of kids can't be punished as severely as adults for, say, killing someone on the streets at night then it doesn't make sense to allow kids to be out on the streets at night in the first place. Or once it becomes possible to measure individual tendencies to violence perhaps a biotechnologically more advanced response might be to require teens to be tested for indicators of violent tendencies. Then those who are most at risk of lashing out could be given the choice of either drugs to reduce violent urges or exile to a remote special community designed to handle dangerous adolescents until their brains develop enough to allow their prefrontal cortexes to reign in their more primitive urges.
While some may quibble with these specific suggestions the larger point here is that regardless of the particular methods chosen to respond society needs to be protected from brains that are not operating as fully responsible moral agents.
Also see my previous post Adolescence Is Tough On The Brain.
|Share |||Randall Parker, 2004 May 17 09:48 PM Brain Development|