The mysterious goings-on inside teen brains have befuddled countless parents over the years. Now some insights are being provided by recent neuroscience research.

Between ages 11 and 17, children's brain waves reduce significantly while they sleep, a new study found. Scientists think this change reflects a trimming-down process going on inside teenagers' brains during these years, where extraneous mental connections made during childhood are lost.

"When a child is born, their brain is not fully-formed, and over the first few years there's a great proliferation of connections between cells," said physiologist Ian Campbell of the University of California, Davis. "Over adolescence there is a pruning back of these connections. The brain decides which connections are important to keep, and which can be let go."

Scientists call this process synaptic pruning, and speculate that the brain decides which neural links to keep based on how frequently they are used. Connections that are rarely called upon are deemed superfluous and eliminated. Sometimes in adolescence, that pruning process goes awry and important connections are lost, which could lead to psychiatric disorders such as schizophrenia, the researchers think.

Brain pruning

Synaptic pruning is thought to help the brain transition from childhood, when it is able to learn and make new connections easily, to adulthood, when it is a bit more settled in its structure, but can focus on a single problem for longer and carry out more complex thought processes.

For example, if a child receives a brain injury before age 10, another area of the brain can often take over the functions of the damaged region. If the same injury occurs at age 20, however, the person may lose a vital ability, because the brain has lost the flexibility to transfer that function to another area.

"The fact that there are more connections [in a child's brain] allows things to be moved around," Campbell told LiveScience. "After adolescence, that alternate route is no longer available. You lose the ability to recover from a brain injury, or the ability to learn a language without an accent. But you gain adult cognitive powers."

Campbell and UC-Davis psychiatrist Irwin Feinberg recorded the sleep brain waves (called EEG) two times a year over five years in 59 children, beginning at either age 9 or age 12. They found that brain waves in the frequency range 1–4 Hz remained unchanged between ages 9 and 11 and then fell sharply, by about 66 percent, between ages 11 and 16.5. In the 4–8 Hz frequency range, which corresponds to a different part of the brain, brain waves started to decline earlier and fell by about 60 percent between ages 11 and 16.5 years.

Overall, these changes are consistent with synaptic pruning, because as neural connections are lost in those areas of the brain, brain waves in the corresponding frequencies decrease. Campbell and Feinberg report their findings in the March 23 issue of the journal Proceedings of the National Academy of Sciences.

Tumultuous years

Synaptic pruning is just one of many changes thought to be going on inside teenagers' brains. For example, a 2005 study found that teenagers can't multi-task as well as adults because their brains are still learning how to process multiple pieces of information at once they way adults can.

In addition to changes that affect how they think, teenagers' brains also undergo developments that affect how they feel. For example, during adolescence people begin to empathize more with others, and take into account how their actions will affect not just themselves, but people around them.

A 2006 study found that the teenage medial prefrontal cortex, the part of the brain associated with higher-level thinking, empathy, and guilt, is underused compared to adults. But as adolescents mature, they begin to use this region more when making decisions, indicating that they increasingly consider others when making choices.