January 24, 2011

Naps Help Memories Transfer To Brain’s Hard Drive

Scientists, surprised by their own findings, report that the best way to not forget a newly learned poem, card trick or algebra equation may be to take a quick nap.

Researchers in Germany showed in experiments that the brain is better during sleep than during wakefulness at resisting attempts to scramble or corrupt a recent memory.

Their study provides new insights into the hugely complex process by which people store and retrieve deliberately acquired information.

Earlier research showed that fresh memories, which are stored temporarily in a region of the brain known as hippocampus, do not gel immediately.

Reactivating these memories soon after learning plays a crucial role in their transfer to more permanent storage in the brain's "hard drive," also known as the neocortex.

However, during wakefulness this period of reactivation renders the memories more fragile.

For example, learning a second poem at this juncture will likely make it harder to commit the first one to deep memory.

Bjorn Rasch of the University of Lubeck in Germany and three colleagues assumed that the same thing happens when we sleep, and designed an experiment to find out if they were right.

Volunteers were asked to memorize 15 pairs of cards showing pictures of animals and everyday objects.  They were exposed to a slightly unpleasant odor.

Half the 24 volunteers who stayed awake 40 minutes later were asked to learn a second, slightly different pattern of cards.

They were again made to smell the same odor just before starting, which was designed to trigger their memory of the first exercise.

The 12 other subjects did the second exercise after a brief snooze, during which they were exposed to the odor while in a state called slow-wave sleep.

Both groups were then tested on the original task.

The sleep group performed significantly better, retaining on average 85 percent of the patterns, compared to 60 percent of those who had remained awake.

"Reactivation of memories had completely different effects on the state of wakefulness and sleep," lead author Susanne Diekelmann, also from the University of Lubeck, told AFP.

"Based on brain imaging data, we suggest the reason for this unexpected result is that already during the first few minutes of sleep, the transfer from hippocampus to neocortex has been initiated," she said in an email exchange.

She said that after 40 minutes of shuteye, significant chunks of memory were already "downloaded" and stored where they "could no longer be disrupted by new information that is encoded in the hippocampus."

Diekelmann said the positive impact of short periods of sleep on memory consolidation could have implications for memory-intensive activities like language training.

She said that the findings also point to a strategy for helping victims of post-traumatic stress syndrome, a debilitating condition caused by extreme experiences.

The reactivation techniques "might prove useful in re-processing and un-learning unwanted memories," she said. "And reactivation of newly learned memories during ensuing sleep could then help consolidate the desired therapeutic effects for the long-term."

Diekelmann cautioned that computers are an imperfect metaphor for the way memories are stored in the brain.

"Human memory is absolutely dynamic. Memories are not statically 'archived' in the neocortex but are subject to constant changes by various influences," she said.

She said that the act of remembering does not simply entail "reading" the stored data. 

"Recall is a reconstructive process in which memories can be changed and distorted."

The study results were published this week in Nature Neuroscience.


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