Adolescents are brain-dense — and that’s good

Density of gray matter may compensate for young brains’ low volume

Researchers have overturned a longstanding belief about the adolescent brain — that gray matter is decreasing. Instead, it just gets packed into a smaller space, increasing density.

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The brain consists of two types of tissues: gray matter and white matter. The white matter provides connections to brain’s processing regions. The gray matter coordinates that processing. The nerve cells of gray matter help control emotions, understanding and movement. They also respond to the senses, such as touch, smell and sound. Now, scientists have found that gray matter in adolescents is denser than that in children. This adds vital information to existing beliefs about the developing brain.

Neuroscientists study the brain and nervous system. Many years ago, they showed that kids lose gray matter as they enter adolescence. Yet, their brain function is not affected. Scientists have been trying ever since to explain how young adults cope with the loss of gray matter.

It turns out that scientists — just like everyone else — sometimes confuse two types of physical traits, such as density and volume. Volume is the three-dimensional space something takes up. Density is how much mass has been crammed into that space. Neuroscientists typically figure out the volume and density of gray matter by analyzing brain scans, explains Ruben Gur, who led the new research. He’s a neuroscientist at the University of Pennsylvania in Philadelphia.

“When people studied aging, they looked at density and volume [of gray matter],” he notes. “And they found that both go down.” Gur believes this led scientists to assume that both of these traits are so related that they essentially provide the same information. That led some neuroscientists who were studying the transition into adulthood to measure the volume only. They ignored gray matter’s density within that volume.

The new data now show that was a mistake.

What brain scans revealed

Indeed, Gur’s Pennsylvania colleague, Efstathios Gennatas, was skeptical about relying only on volume. Also a neuroscientist, he points out that an old brain is not the same as a brain transitioning from childhood to adulthood. Brain cells begin to die off in old age. That leaves the elderly with less gray matter. Not surprisingly, brain function tends to worsen in old people.

But there is no natural death of this tissue in teens. Scientists know that brain cells trim off their non-essential parts during adolescence. This is a normal part of the learning process. But this isn’t the same as brain cell death seen in old age. And Gennatas wondered if this loss of unused gray matter was also accompanied by other changes.

“When you hear the story, which has been reported many times, that adolescents lose gray matter, you think of it as something negative. But there has to be something that is improving because clearly they are improving functionally in almost every way,” Gennatas says.

To dig deeper, Gur’s team scanned the brains of 1,189 youths. All were between 8 and 23 years old. They used a method called magnetic resonance imaging. With MRI, a person is placed inside a machine that applies a strong magnetic field to image parts of the body. In this case, the MRI imaged the brain. From the images, the researchers were able to determine brain volume and density.

Those images showed that gray matter volume of adolescents tended to be lower than that of kids. It decreased on average by about 7 percent in males and 10 percent in females. This was similar to what’s already known. But the research team also showed for the first time that the original amount of gray matter was just packed more tightly into a smaller space in adolescents. This means that gray matter is not lost from childhood to adolescence. Rather, it’s just reorganized.

Armin Raznahan, who was not a part of this study, finds these data important. A child psychiatrist, he’s a doctor who studies mental health. He works at the National Institute of Mental Health in Bethesda, Md. Raznahan hopes the new findings will encourage other researchers to consider gray matter density as well as volume when they do MRI brain studies.

“It is important to bear in mind that these are very powerful measures of the brain,” Raznahan says. Yet, he points out, “they are still relatively crude.” He notes that a typical brain scan may focus on an area of about 1 cubic millimeter (0.00006 cubic inch). But it will not reveal much details of the million or so cells within that space.

Shruti Muralidhar suggests an idea, in addition to MRI scans, to study brain cells in detail. A neuroscientist at the Massachusetts Institute of Technology in Cambridge, Muralidhar was not involved in this research. She suggests coloring different parts of brain slices (from dead people) using specific dyes and viewing them under a microscope. This way scientists can measure changes in the brain cells more accurately, she says.

This study, reported in the May 17 Journal of Neuroscience, adds a new piece of information in adolescent brain research. But the Pennsylvania team seeks further information to understand the complete picture. They want to find out how density and volume of gray matter, individually or together, affect brain function.

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