What an age we live in. Science is progressing in ever greater leaps and bounds. The way things are going, we might one day even understand that most enigmatic and mysterious of natural phenomena, the teenager.
“If youth knew; if age could.” Such was the lament of the French publisher Henri Estienne in the 16th century. But youth doesn’t know and age can’t: Such is the tragedy, as Estienne saw it, of human life. Young people don’t know — but good luck to them. It is the prerogative of the young to be different and inscrutable to their elders.
And now we know why. A 10-year study of people ages 4 to 21 has revealed that the higher centers of the brain — those involved in reasoning and problem solving — are among the last to mature. The prefrontal cortex, the site of the higher centers, is the last to come “on line.” It might explain why some teenagers lack the ability to be reasonable (though it doesn’t explain why the same is true of many adults).
As well as an insight into teenage behavior, the work will help us understand brain disorders such as schizophrenia and autism.
Researchers from the National Institute of Mental Health (NIMH) in Bethseda, Md., and the University of California, Los Angeles produced a dynamic sequence of brain scans that showed that the brain’s gray matter diminishes during the teen years. Gray matter is the tissue that does the work in the cortex. And it disappears as parts that are unused are “pruned back.”
Neural connections that get exercised are retained, while those that don’t are lost. So when it comes to gray matter, the brain seems to follow a principle of “use it or lose it.” Previous research has shown that gray matter is overproduced at the fetal and early childhood stage and is pruned back at around 18 months. But the new research, published in the Proceedings of the National Academy of Science last week, confirms other studies showing that a second major bout of pruning occurs during the teenage years.
Could this be the beginning of an evolutionary explanation of teen moodiness? The “use it or lose it” principle suggests that humans’ uniquely long childhood and adolescence functions to make the most of the brain’s potential. It protects and shapes the brain.
This could be why teenagers require more sleep than the rest of us, as a study found in 2002. Teenagers are not (just) lazy and moody; they need to sleep longer. And we know that the brain is affected by experience. Musicians trained from an early age, for example, have a different distribution of gray matter than nonmusicians. The differences may be a response to differential use at a critical period of brain maturation.
In the 13 healthy children used in the current study, the gray matter diminished over time in a wave from the back to the front of the brain. The first areas to be pruned — to mature — are those concerned with the most basic functions, like moving and sensing. Areas involved in spatial orientation and language (the parietal lobes) follow. The last to mature is the prefrontal cortex, the area with advanced functions concerned with integrating information from the senses, reasoning and other “executive” functions.
The prefrontal cortex distinguishes primates from other mammals (other mammals don’t have one), and it marks humans out from other primates (ours is much bigger than theirs). So, in a sense, the brain’s maturation sequence mirrors that of its own evolution. Only primates (higher-order mammals) have a prefrontal cortex, and it matures last in the brain.
The brain is a staggeringly complex entity. It comprises a hundred billion nerve cells with several trillion connections between them. It is no wonder that sometimes things go wrong during its development, and this was the reason for the current study: to understand how the brain develops with the aim of understanding disorders.
“To interpret brain changes we were seeing in neurodevelopmental disorders like schizophrenia, we needed a better picture of how the brain normally develops,” said Judith Rapoport, an author of the paper and a member of NIMH.
Rapoport and colleagues used magnetic resonance imaging to scan the brains of the same 13 healthy children and teens every two years as they grew up, for 10 years. When they’d accumulated a set of brain maps — snapshots of the brain’s architecture at different developmental stages — they laid the scans atop one another using brain anatomical landmarks. The result was a movie showing the movement of gray matter over 10 years.
Rapoport and colleagues’ earlier MRI studies had found that there is an exaggerated wave of gray matter loss in teens with early onset schizophrenia. These teens become psychotic prior to puberty, losing four times the normal amount of gray matter in their frontal lobes, which suggested that childhood-onset schizophrenia might be a result of the normal pruning process going out of control. In contrast, children with autism show an abnormal back-to-front wave of gray matter increases, rather than decreases.
Shakespeare said, “Time doth transfix the flourish set on youth/And delves the parallels in beauty’s brow”; neuroscientists might add: “And MRI will show us how”.