Scientists find way to strengthen memories during sleep

If only we could make more constructive use of all the time that we spend asleep. People have tried playing various tapes to themselves while they're dozing, from foreign vocab lists to stop-smoking mantras, but they're all the wrong side of useless. What we do know for sure is that sleep is important for memory consolidation, if only we could tap into this somehow. Now, finally, John Rudoy and colleagues have provided some elusive evidence for how learning during sleep can be enhanced.

Twelve participants looked on as fifty objects appeared one at a time in various locations on a computer screen. Importantly, as each object appeared it was accompanied by a characteristic noise - for example a cat appeared with a meow and a kettle with a whistle. Several rounds of learning took place until the participants had estimated the approximate location of each object at least once. A final pre-nap test was then performed so that the researchers knew how well participants knew each object location before they went to sleep.

That the participants had nodded off was confirmed with brain wave recordings via scalp electrodes. But here's the clever bit. As the participants dozed off into non-REM slow-wave sleep, the researchers played the sounds associated with 25 of the objects. The objects that were cued in this way were carefully chosen such that pre-nap memory performance had been equal for cued and un-cued objects.

The participants woke up after about an hour and the exciting finding is that although their overall memory accuracy was lower compared with before the nap, their performance for the objects cued whilst they slept was superior to un-cued objects, even though pre-nap performance for the two object groups had been equal.

The researchers also looked back at the brain wave signals recorded during sleep, comparing the brain's response to sounds associated with objects that were better remembered on waking relative to objects for which memory had deteriorated. They found the brain had responded more to sounds belonging to better remembered objects. "We propose that sound cues presented during sleep prompted preferential processing of corresponding object-location associations," the researchers said.

For sceptics who think the results may have nothing to do with sleep, the researchers repeated the noise cueing exercise with twelve participants who remained awake. In their case, sounds presented after learning made no difference to subsequent memory performance.
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Rudoy, J., Voss, J., Westerberg, C., & Paller, K. (2009). Strengthening Individual Memories by Reactivating Them During Sleep. Science, 326 (5956), 1079-1079 DOI: 10.1126/science.1179013

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