Major changes in an adolescent's brainwaves occur soon after puberty, a finding that may point to a massive pruning of brain connections that occurs around this time, a new study has found.
Dr Ian Campbell, Professor Irwin Feinberg and colleagues at the University of California at Davis in the USA studied the brains of 67 children as they passed through adolescence.
Their results appear in the latest edition of Proceedings of the National Academy of Sciences.
The researchers took recordings of the sleeping adolescents' brainwaves twice a year using an electroencephalogram (EEG).
They also measured the stage of puberty for each child by assessing pubic hair growth and the development of sexual features, within a month of each EEG recording.
The data revealed a dramatic and permanent 60 per cent drop in the activity of brainwaves known as delta waves as the children passed through adolescence. The researchers published this discovery in 2009.
Delta waves occur in the deepest part of sleep known as non-rapid eye movement (non-REM) sleep. They involve "pools of neurons oscillating in unison", says Feinberg.
"We think that non-REM sleep is the recuperative phase of sleep where the parts [of the brain] that are more active during waking, recuperate in some way from their labours during the day," he says.
Pruning the neurons
For most children the plummet in brain wave activity occurred between the ages of 12 and 16.5 years.
The scientists suspect that the decline in delta activity reflects the huge transformation in the brain known as 'synaptic pruning' that occurs in adolescence.
Many neurons and the connections between them are removed, leading to improvements in brain efficiency.
In their latest paper the researchers go further, showing that there is strong correlation between physical signs of puberty and the delta wave decline.
"The kids who went through pubertal maturation earlier were the kids who showed the earliest declines in delta activity", says Campbell.
The children experienced their most rapid physical puberty changes about a year before the changes in brainwaves peaked.
Campbell and Feinberg say one interpretation of their results is that puberty hormones are driving the brain maturation, but that their work does not prove this.
To investigate the link between puberty and brain maturation further would require measurements of puberty hormones, they suggest.
Professor Mick Hunter of the University of Newcastle thinks it is most likely that puberty and the delta wave decline are causally linked. "The likelihood is that they are linked in some way. The question is, what is the nature of the link?"
"What they need next is far more precision in the measure of pubertal development", he says.
"People don't realise what massive changes occur in adolescence in a very short period of time. There are so many changes - physiologically, mentally and socially. The idea of them being independent is highly unlikely."
Insights for schizophrenia?
Feinberg sees the work as having potential for investigating the process of synaptic pruning in diseases such as schizophrenia.
"Sleep EEG is the aspect of brain activity that can be easily and non-invasively measured", he says.
"I'm a psychiatrist interested in schizophrenia", he adds. "Its onset is frequently at the end of adolescence. With such a massive reorganisation of the brain going on at that time there may be errors in the reorganisation that give rise to mental illness."