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To Sleep, Perchance to Forget

29 July 2008 (All day)
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Michael Firbank

Sleep on it. Older brains may be forgetful because a key memory process that occurs during sleep in the hippocampus (where the lines intersect) is impaired.

While we snooze, scientists think, our brains are busy forming new memories by replaying the events of the day. But aging may rob us of this process and set us up for having "senior moments." A new study has found that older rats seem to replay previous events less and, as a result, have more trouble remembering than younger animals.

How our brains form memories is not entirely understood, but sleep may be vital. The hippocampus region of our brain seems to rerun experiences we had while awake, a process that scientists believe helps cement memories. A team led by neuroscientist Carol Barnes at the University of Arizona, Tucson, noticed that older rats--just like older people--sometimes have trouble remembering. Could those memory problems be due to a decline in the brain's replay during sleep?

In the study, 11 young and 11 older rats learned the locations of food rewards in several mazes. Meanwhile, researchers recorded the electrical activity of their hippocampi with probes inserted into their brains. That night, while the rats slept, the researchers monitored the hippocampus activity again. The young rats showed roughly the same kind of hippocampus activity in their sleep as they did when they were navigating the mazes; apparently, they were replaying the events. But the older rats did not, indicating that the replay process was impaired, the researchers report in the 30 July issue of The Journal of Neuroscience. Further experiments showed that the whippersnappers had a sharper memory; they were faster and more accurate than the older animals in remembering where a hidden platform was located while swimming in a tank of water.

The findings suggest that at least some of the short-term memory loss experienced by elderly people could be due to a decline in automatic replay during sleep, says Michael Hasselmo, a neuroscientist at Boston University. The results could pave the way for treatments to improve memory, Hasselmo says, by targeting brain chemicals that play a role in replay.

The study also raises an intriguing question, says neuroscientist Lisa Marshall of the University of Lübeck in Germany: Is the increased daytime napping that's common among seniors the body's attempt to compensate for the impaired replay process?

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