DIMENSIONS Spring 2011

Sleep and Memory

By By Michael Rafii, MD
Associate Medical Core Director, Alzheimerís Disease Cooperative Study Alzheimerís Disease Information Network Monthly E-Newsletter, February 2011

photo of sleeping father and daughter

The best way to not forget a newly learned memory may be to take a quick nap after initially learning the new material. In experiments, researchers in Germany showed that the brain is better during sleep than during wakefulness at resisting attempts to scramble or corrupt a recent memory.

Their study, published in Nature Neuroscience, provides new insights into the hugely complex process by which we store and retrieve deliberately acquired information - learning, in short. Earlier research showed that fresh memories, stored temporarily in a region of the brain called the hippocampus, do not gel immediately. It was also known that reactivation of those memories soon after learning plays a crucial role in their transfer to more permanent storage in the brain's "hard drive," the neocortex. During wakefulness, however, this period of reactivation renders the memories more fragile. Learning a second item at this juncture, for example, 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.

Twenty-four volunteers were asked to memorize 15 pairs of cards showing pictures of animals and everyday objects. While performing the exercise, they were exposed to a slightly unpleasant odor. Forty minutes later, half the subjects who had stayed awake were asked to learn a second, slightly different pattern of cards. Just before starting, they were again made to smell the same odor, designed to trigger their memory of the first exercise. The 12 other subjects, meanwhile, 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. Interestingly, the sleep group performed significantly better, retaining on average 85 percent of the patterns, compared to 60 percent for those who had remained awake. After only 40 minutes of sleep, 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.

It is thought that sleep problems negatively affect the brainís ability to consolidate short-term memories, and may exaggerate the short term forgetfulness seen as part of age-associated memory decline.

This piece is reprinted from the Alzheimer's Disease Information Network Monthly E-Newsletter, February 2011. The article is based on the findings from, "Labile or stable: opposing consequences for memory when reactivated during waking and sleep." Diekelmann S, BŁchel C, Born J, Rasch B., Nature Neuroscience, 2011 Jan 23.

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