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.
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
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
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
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."
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).
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:
Alzheimer's Disease Education and Referral Center (ADEAR)
P.O. Boz 8250
Silver Spring MD 20907-8250