Hormones are going wild in a teenage body, but is sexual maturation at pace with mental maturation?

A team at University of California, Davis found that brain wave changes in adolescence are related to age, not sexual maturation, being linked to the brain's major reorganization projects: synaptic pruning.

The child brain has many synapses (neurons connections): more synapses means more intense brain activity. During adolescence, the brain reorganizes and eliminates many synaptic connections, a process known as synaptic pruning.

This pruning reorganizes and eliminates many useless connections, increasing brain's information processing ability while decreasing energy consumption. During pruning, there is a sharp decline in sleep slow wave activity, the delta wave. The authors hypothesized that the pruning is age-programmed.

Two subject groups were investigated over the course of two years: 31 children were 9 years old at the beginning of the study and 38 were 12. An in-home electroencephalograph (EEG) monitored the children's brain activity during sleep.
Measurements were taken at six-month intervals and computer analyzed. Brain activity changes were correlated to sexual maturity and physical growth (height, weight and body mass index) at each interval.

Delta wave intensity in the first group (9-11 age) group was unchanged and equal between girls and boys. In the second group (aged 12-14) declined by 25%, but was related to age, not to physical growth and sexual maturation. A lower intensity was measured in girls than in boys, as girls begin the brain reorganization sooner.

Later bed times and reduction in total sleep time that occurs during adolescence did not affect the process. "Previous studies had shown a delta wave activity decline of 50% between ages 10 and 20, but it was unclear when the change began and whether there were gender differences," Irwin Feinberg, one of the researchers, said. This research showed that decrease in delta wave activity during sleep begins at about 11 years of age and reaches 25% by the age of 14. Girls, on average, begin adolescent brain maturation at least one year earlier than boys, and this may explain the earlier mental maturation in girls compared to boys.

However, once this maturational process starts, it proceeds at the same rate in both sexes. This study strongly points to the fact that change in delta wave activity is due to age, not sexual maturity. "It may seem surprising that age is the (predominant) factor in the delta power density decline," the authors wrote. "However, many maturational events in the development of the nervous system proceed on a programmed schedule."

Still unknown brain processes that start sexual maturation in adolescence also may provoke the brain changes, but the processes then proceed independently. "Longitudinal sleep EEG measurement could also provide a new arena for clinical studies of subjects at high risk of schizophrenia and other neurodevelopmental disorders," the authors noted.

"The emergence of schizophrenia during adolescence and the dramatic change in delta wave activity during that time might both be related to synaptic pruning," Feinberg said. "It is possible that sleep EEG changes will prove a relatively direct indicator of synaptic pruning."