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The Remains of the Day
Sleep and Memory: At night, does the brain replay the day's events in order to form long-term memories?
 
Anyone who has ever watched a dog twitching and growling in its sleep knows that even our pets have vivid dreams. But what is the brain really doing when it dreams?

Some neuroscientists now suspect that the vivid mental videos that play in our heads when we sleep may reflect more than just the brain's after-hours random noodling. Dreaming-and sleep in general-may be part of the process that transforms important snippets of the day's experiences into enduring long-term memories.

Sleep and memory

One of the connections of sleep and memory is fairly obvious: the lack of sleep causes fatigue, which impairs memory and other mental faculties. But even in slumber, the brain appears to be extraordinarily busy. The phase of sleep in which we appear to dream the most is called REM, or "rapid eye movement" sleep. Experiments show that depriving an animal or person of the normal amount of REM sleep hurts performance on a variety of learning and memory tasks.

Photonica/Tina Merandon


Why should this be? One intriguing explanation centers on the concept of "offline memory reprocessing." According to this theory, when we sleep the brain accomplishes an important task called memory consolidation. During our waking hours, we see, hear, touch, taste, and observe the world. A large amount of information streams into consciousness. But only a small amount of that information is stored as long-term memories. The sleep-deprivation studies suggest that some aspects of memory consolidation occur during REM sleep, although it's clear that not ALL memories are formed during sleep.

But one of the fundamental problems with sleep-deprivation research is that it necessarily involves jarring lab animals and human volunteers from their slumber. It's hard to say for sure whether memory consolidation is hobbled by lack of REM sleep or just the mental and physical stress of breaking the normal sleep cycle.

The dreams of mice and men

Matthew Wilson, Ph.D., an associate professor of biology at the Massachusetts Institute of Technology, has found a way to circumvent this problem. He and his colleagues at MIT's Center for Learning and Memory developed a way to record the electrical activity of small groups, or "ensembles," of brain cells in sleeping rats. This enables them to eavesdrop on the rats' dreams, opening a window into the role of sleep in creating new memories.

"With animals, because you can't wake them up and ask them what they're dreaming about, you have to go directly to the source," Wilson explains. "By recording neural activity and trying to interpret those patterns, you are able to study sleep in its unperturbed form. You can see what's going on during sleep itself, which is something you can't do in humans."

The "bug" they use to eavesdrop in rat dreams is a special electrode, implanted in the rat's head, that picks up the electrical messages exchanged between brain cells. They've used it to listen in on various parts of the rats' brains, including the hippocampus-a region long known to play a key role in transforming recent experiences into long-term memories.

Instant replay

Wilson and one of his graduate students, Kenway Louie, trained rats to navigate a special circular maze to obtain food. The researchers simultaneously recorded activity in the hippocampus. Then, as the rats took a break from their training and slept, another set of recordings were taken. A computer program allowed the researchers to compare the recordings later and identify any similarities.

In about half of the sleep recordings, stretches of brain activity lasting up to several minutes matched segments recorded while the rats ran the maze. The match was so close that the Wilson and Louie could track the movements of the sleeping rats through their dream maze. They were, in a very real sense, watching the dreams of rats.

The interpretation of dreams

What can rat dreams tell us about how memory works? Wilson and some other neuroscientists have proposed that when we dream, we "replay" recent experiences as part of a more widespread process in the brain that allows us to learn and remember.

In Wilson's rats, the hippocampus appeared to store a kind of tape recording of the animals' movements through the maze. While asleep, the animals replayed that record. Wilson speculates that re-living the maze may help the rats sort out the important parts of their recent experiences that should be transferred to long-term memory.

"At the time we're experiencing things, we're not able to make that assessment," he says. "But by reevaluating it offline, while we're asleep, we're able to carry out some kind of selective filtering to determine what was important, what was useful." Memory consolidation, he says, may be this process of selecting and reinforcing "just those bits that were important." Depending on which nerve cell connections get strengthened, the memory takes shape.

Memory is closely related to learning, so replay has implications for the lessons we take away from our experiences. "Dreams may provide the opportunity to bring together experiences that were related, but didn't occur at the same time, in order to learn from them," Wilson explains. For example, replay could allow us to identify what a series of pleasant or unpleasant experiences had in common and use these lessons to guide future behavior.

Alternative views

Wilson's account of memory replay is appealing and is certainly consistent with what we know about how the brain works. However, there are other possibilities. For example, he says, "one possibility is that nothing goes on in sleep at all, that the replay itself is just a consequence of random activity. There's no function to it at all."

Another strong contender: Sleep simply gives the brain time to solidify connections between brain cells-representing memories-that were forged during waking hours. "It could be a time when chemical processes occur. You've already established these connections; they just need to be locked in."

Or there is the sleep-as-housekeeping theory: The brain makes memories continuously, and "you need the system to be running well in order to perform," Wilson says. Memory gets no preference, here. Sleep is just as important to keeping your liver running as your brain. This would explain why depriving a person of REM sleep would weaken learning and memory. It stresses out the brain, disrupting the process that locks in previously formed memories.

A good start

No one can say which theory is right, but Wilson's rat recordings represent a step forward. Scientists now have a technology that allows them to observe at least a portion of undisturbed sleep. "This can and will be resolved," Wilson says. "The important thing is that there is a methodological handle on this."

Wilson is certain, however, of one thing: His experiments show that the rat hippocampus creates a record of its experience. "The question now is whether it is used for anything."


Further Reading:

Deep Sleep 101, by Gregg Jacobs. (Blue Marble, 2001, 52 pp plus music CD, $23.95.). Author Gregg Jacobs, offers a concise guidebook of ways to improve sleep.

"Temporally structured replay of awake hippocampal ensemble activity during rapid eye movement sleep," by Kenway Louie and Matthew A. Wilson. (Neuron, January 2001), Volume 29, pp. 145-156.)|

·"Sleep, learning, and dreams: off-line memory reprocessing," by Robert Stickgold, et al. (Science, November 2, 2001, Volume 294, pp. 1052-105).

·www.SleepNet.com: This educational, non-commercial website is devoted to improving sleep health and bills itself as a source of "everything you wanted to know about sleep but were too tired to ask."

National Center on Sleep Disorders Research: This web site posted by the National Institutes of Health includes patient information on sleep and sleep disorders. Visit the website: www.nhlbi.nih.gov/about/ncsdr

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