Let's face it -- the typical teen will look for any excuse to bail out on a chore, dodge an obligation, or skip out on a task that reeks of responsibility. But the teen set may actually have a neurological reason to justify their skittish behavior: Their brains are still growing.
Challenging the commonly held belief that our brains have fully matured by the age of 12, the latest research suggests that this particular organ continues to blossom well past puberty, possibly explaining why teenagers are so, well, adolescent. In this case, the culprit is the overproduction of gray matter, mostly in the frontal and striatal regions. These areas are known for regulating self-control, judgment, planning, and organization.
"One of the critical issues (in neurological studies) is what's different about the brain at what time and at what age? Until these studies of normal brain maturation are conducted, it puts us at a disadvantage because we don't know where we are in the developmental trajectory," said Dr. Elizabeth Sowell, a post-doctoral fellow at UCLA's Laboratory of Neuro Imaging.
Sowell and her team compared the brains of 12-to-16-year-olds to a group of 20-year-olds. High-resolution MRI brain images were acquired for each subject in the same magnet with a gradient-echo, T1-weighted series with TR = 24 ms, TE = 5 ms, NEX = 2, and a flip angle of 45 degrees. The field of view was 24 cm and section thickness was 1.2 mm, with no gaps. First-image volumes were re-sliced into a standard orientation to define cerebral and non-cerebral regions.
"We used a simple voxel-by-voxel contrast to compare the average gray-matter map for adolescents to that for young adults, focusing on the negative effects of age on gray matter, the authors wrote in an article that appeared in Nature Neuroscience (Oct. 1999; Vol. 2(10): pp.859-861). "In regions of frontal cortex, we observed reduction in gray matter between adolescence and adulthood, probably reflecting increased myelination in peripheral regions of the cortex that may improve cognitive processing in adulthood."
In other words, teens may be a little too "brainy" for their own good, with an extra layer of myelin allowing the nerves to transmit signals with greater speed and efficiency in the frontal lobe.
"Neuropsychological studies show that the frontal lobes are essential for such functions as response inhibition, emotional regulation, planning and organization. Many of these aptitudes continue to develop between adolescence and young adulthood," the authors said.
Another study, which appeared in the same issue of Nature Neuroscience, found that the corpus callosum, or the nerves that connect the two sides of the brain, continues to branch out well beyond the teen years.
Researchers at the National Institute of Mental Health in Bethesda, MD, imaged healthy subjects ages 4 to 21 with a 1.5-tesla scanner using three-dimensional, spoiled-gradient, recalled echo in the steady-state protocol (axial-slice thickness of 1.5-mm, a time-to-echo of 5 ms, a repetition time of 24 ms, flip angle of 45 degrees and a field of view of 24 cm).
"(C)hanges in cortical gray matter were regionally specific, with developmental curves for the frontal and parietal lobe peaking at about age 12 and for the temporal lobe at about age 16, whereas cortical gray matter continued to increase in the occipital lobe through age 20," wrote Dr. Jay Giedd and his co-authors (pp.861-863).
Sowell said she is not sure what this type of information will mean for adolescents with behavioral and neurological disorders, except to provide a starting point for further investigation.
"I think the best reason for currently doing this kind of research is as a sort of baseline," she said. She said her next area of study will be the developmental relationship between structural changes in the brain, function, and cognitive behavior.
Psychiatrists at the University of London have already begun mapping neurodevelopmental trajectories with functional MRI (fMRI). In a paper published in Neuroscience and Biobehavioral Reviews, the group looked at whether frontal cortex development was related to brain activation (January 2000, Vol.24, pp.13-19).
Seventeen healthy adult and adolescent volunteers were scanned while performing a motor response inhibition task and a motor timing task. Gradient-echo echo-planar MR images were acquired using a 1.5-tesla GE Signa system fitted with Advanced NMR hardware and software. One hundred T2-weighted MR images depicting blood oxygenation level dependent (BOLD) contrast also were acquired.
In the delay task, adults performed better than the adolescents, with an increase in power of activation in the fronto-striato-parietal network. In the stop task, both groups performed equally well, although the adults showed more response in the left hemispheric middle and inferior prefrontal brain regions.
By Shalmali Pal
AuntMinnie.com staff writer
March 20, 2000
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