The New York Times-20080129-Study Gives Key Role To Sleep in Helping Brain Learn Anew

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Study Gives Key Role To Sleep in Helping Brain Learn Anew

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Researchers who study the brain know that it's far from an immutable object. It's much more plastic than most people think, said Giulio Tononi, a psychiatrist at the University of Wisconsin. It's changing all the time.

One area of change is the synapses, the connections between neurons, which are altered as the brain receives stimuli. What happens when you're awake is you produce an overall strengthening of synapses, Dr. Tononi said. That's good, because that's how you learn.

But that is unsustainable in the long run, because stronger synapses require more energy and material, and there's a limit to how much of both is available. It's as if in the morning you start with a V-6 engine, he said, and in the evening you find yourself idling, but you're running a V-8. Stronger synapses are also bigger, but the brain cannot grow bigger or denser. If you strengthen synapses because you learn, soon you'd reach a point where you can't learn further, he added.

So Dr. Tononi and a colleague, Chiara Cirelli, have hypothesized that during sleep, the synapses weaken. The downscaling is across the board, so that the synapses' relative strength is maintained. Those that have been used (those involved in learning) stay stronger than those that haven't.

Drs. Tononi and Cirelli say this weakening is a crucial role of sleep, because it restores the brain for the next period of learning. But their ideas run counter to another theory, that brain circuits that were activated during wakefulness become reactivated during sleep, consolidating learning by making these synapses stronger.

Now the Wisconsin researchers have produced experimental findings in rats that support their hypothesis. In a paper in Nature Neuroscience, they show that rats had stronger synapses after periods of wakefulness than after periods of sleep. They measured the number of a certain type of neuroreceptor in the synapses and found that there were 50 percent fewer in rats that had been asleep. They also measured the electrical response across neurons in brains of live rats and found that the response was weaker. The results, Dr. Tononi said, suggest that after sleep we get a leaner brain -- there's a gain in terms of energy, space and supplies, and you are ready to learn anew.

[Illustration]ILLUSTRATION (ILLUSTRATION BY CHRIS GASH)