June 7, 2004

Patch of brain put to sleep

Local snoozing makes for better learning.

by Tanguy Chouard
Nature News

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A good night's rest is hard work for parts of your brain, say US neuroscientists. Regions related to learning show increased activity in sleepers who spent their evening mastering a new skill, they say.

The discovery shows that sleep is valuable for consolidating new information and is not a simple 'standby' mode. Local brain processing during the night led to new skills being more firmly cemented, the research indicates.

Giulio Tononi of the University of Wisconsin-Madison and his colleagues measured electrical brain signals in subjects who learned a simple computer game before going to sleep.

The kind of activity that occurs during sleep was increased in a penny-sized region in the brains of slumbering subjects who had learned the game. Just playing the game did not have this effect. The researchers conclude that sleep falls on brain circuits that have been changed, not just used, during the day.

And someone with more of such activity in this area, which is in the top right hemisphere, tends to perform better in the morning, they report in a paper published online by Nature1.

This is the first time that waking behaviour has been shown to affect a specific part of the human brain during slumber. "It's a very elegant study," says Robert Knight, a neuroscientist at the University of California, Berkeley.

Night shift

When the brain goes to sleep, its nerve cells synchronize their firing to generate a pattern called slow-wave activity (SWA). SWA characterizes the long periods of deep sleep, which are interrupted by short bouts of rapid eye movement when dreaming occurs.

Sleep specialists know that SWA somehow reflects a need to rest. Someone who has been awake for a long time will display more pronounced SWA at the beginning of the night.

"But the real questions are: what is it in you that really needs to rest, and what for?" says Tononi. "Is it your whole body or just the brain cells that had something special to do during your day?"

If SWA is needed by local brain regions, he reasoned, one should be able to increase SWA locally as a result of a specific task. He points out that some animals, such as dolphins, can send the two halves of their brain to sleep independently, and carry on regardless.

Playing catch-up

The researchers asked a dozen people to play a simple computer game involving moving a cursor to a prescribed position. During some sessions, without telling the players, they skewed the cursor's trajectory a tiny bit. So the players had to subconsciously adapt to the handicap, which created an implicit learning task.

After a straight game, all 256 electrodes taped to a sleeper's scalp showed normal SWA. But if the subject had to battle with a skewed game, he or she showed boosted SWA activity on six electrodes that all indicated one small brain region: the posterior parietal cortex of the right hemisphere.

Crucially, this region is already known to control the learning of eye-hand coordination during waking hours. "Had they hit another region, nobody would have believed this story," comments Knight.

"The amazing thing is when you compare the different people," adds Tononi. The ones who had the most initial trouble controlling the cursor, and who therefore worked the hardest at learning the task, showed the greatest local SWA increases.

This shows that the brain circuits are reshuffling themselves to aid learning, rather than recovering from simple strain, says Tononi. "It's learning, not usage, that elicits sleep," he explains. And these people also improved overnight more than others.

So why don’t more animals follow dolphins' example, remaining alert while part of their brain catches 40 winks? "Why an animal should go to sleep all at once is still a mystery," says Knight. "It's extremely dangerous, yet most species keep doing it."

Tanguy Chouard is a senior editor at Nature

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