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German researchers found they could use odors to re-activate new memories in the brains of people while they slept -- and the volunteers remembered better later.

Writing in the journal Science, they said their study showed that memories are indeed consolidated during sleep, and show that smells and perhaps other stimuli can reinforce brain learning pathways.

Jan Born of the University of Lubeck in Germany and colleagues had 74 volunteers learn to play games similar to the game of "Concentration" in which they must find matched pairs of objects or cards by turning only one over at a time.

While doing this task, some of the volunteers inhaled the scent of roses. The volunteers then agreed to sleep inside an MRI tube. Functional magnetic resonance imaging was used to "watch" their brains while they slept.

At various stages during sleep, Born's team wafted in the same scent of roses.

The volunteers were tested again the next day on what they had learned. "After the odor night, participants remembered 97.2 percent of the card pairs they had learned before sleep," the researchers wrote.

But they only remembered 86 percent of the pairs if they did not get the rose smell while sleeping.

And the stage of sleep was important too, the researchers said in a finding that will add to the debate over whether people "learn" in their sleep the way some animals have been shown to.

Research has shown, for example, that rats learning a new maze will rehearse their movements during sleep, and that songbirds rehearse their songs.

Born's team said the scent improved learning when it was administered during slow-wave sleep, but had no effect during rapid eye movement or REM sleep.

The MRI showed that the hippocampus, the part of the brain associated with learning new things, was activated when the odor was wafted over the volunteers during slow wave sleep.