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When Rats Dream, It Seems, It's After a Day at the Mazes
By ERICA GOODE
Elephants dream of munching sweet grass under a starry savannah sky.
Dogs, paws aquiver, tails thumping faintly in slumber, chase squirrels in
the park. And cats, of course, dream of mice.
Or so humans, prone to anthropomorphic conjecture about the four- legged
world, have long suspected.
Yet what animals dream about ‹ or indeed, whether they dream at all ‹ has
remained resistant to scientific scrutiny, if only because animals cannot
describe their closed-eye experiences in words.
Now, however, two researchers studying memory have offered compelling
evidence that the brains of sleeping animals are at work in a way
irresistably suggestive of dreaming. And the animals in question ‹ four
pink-eared, black-and-white laboratory rats ‹ appeared to be dreaming
about something very specific: the maze they were learning to run.
The researchers, who reported their findings in today's issue of the
journal Neuron, found that patterns of brain activity identified when the
rats ran a circular maze ‹ receiving a reward of chocolate-flavored
sprinkles ‹ were exactly duplicated when the rats were sleeping.
In particular, the patterns, detected in the firing of clusters of cells
in the hippocampus, an area involved with memory formation and storage,
were reproduced during phases of sleep that in humans are strongly linked
to dreaming. And they were so precise the scientists could tell where in
the maze the rat would be if it were awake, and whether it would be
moving or standing still.
"The animal is certainly recalling memories of those events as they
occurred during the awake state, and it is doing so during dream sleep,"
said Dr. Matthew Wilson, the senior author of the report and an associate
professor of brain and cognitive sciences at the Massachusetts Institute
Dr. Wilson added that the research was not proof, in the purest sense,
that animals dream, because the dreaming experience is subjective and,
"our ability to ask the animal to report the content of these states is
But the findings, he said, "are the strongest evidence we have to date
that animals have something close to human dreaming," adding, "Call it
whatever you want."
Though only four rats were studied, Dr. Wilson and other scientists said
the number was sufficient to attain statistical significance. Also, they
said, the elaborate nature of the controls used in the study made it
unlikely that the results were spurious, though more research is needed
to replicate and extend the findings.
"The likelihood that this would occur by chance is exceedingly small,"
Dr. Wilson said.
Scientists familiar with the work said the research was important not
only for the glimpse it offered of the sleeping animal brain, but also
because it lent support to the idea that sleep played a critical role in
the encoding and storage of memories. The study demonstrates, for the
first time, that complex, episodic memories are replayed or "rehearsed"
in the hippocampus during sleep, perhaps representing a process by which
memory is gradually consolidated and passed to other parts of the brain,
a model championed by several researchers.
"I am delighted," said Dr. John Allan Hobson, a professor of psychiatry
at Harvard and the director of the Laboratory of Neurophysiology at the
Massachusetts Mental Health Center in Boston, "because it suggests that,
as we have long suspected, sleep has a lot of functional significance for
learning and memory."
The relationship between sleep and memory is still debated within the
field, but studies by Dr. Robert Stickgold, of Dr. Hobson's laboratory,
and others indicate that when people learn new skills, their performance
is dependent on how much they get of two types of sleep: Nondreaming or
slow-wave sleep early in the night, and so-called rapid eye movement
sleep, or R.E.M., later in the night. In humans, R.E.M. sleep is when
most dreaming occurs.
But Dr. Howard Eichenbaum, university professor of psychology of Boston
University, said that even with the new study, the case for sleep as
memory consolidator was far from proved.
"We're not quite ready yet to say that getting a good night's sleep is
specifically related to memory," Dr. Eichenbaum said.
Still, Dr. Eichenbaum and other scientists said, the work is exciting
because the sophisticated technology the researchers used opens new
possibilities for understanding the biology of sleep. Such studies, which
involve implanting electrodes in animals' brains, cannot be done in
humans for ethical reasons.
Like humans, slumbering animals pass through different stages of sleep,
and most mammals exhibit periods of R.E.M. sleep, characterized by
intense activity in the brain similar to that during waking, and rapid
movements of the eye. The rat, Dr. Wilson said, which has a 12-hour sleep
cycle, generally passes through R.E.M. about every 20 minutes, with each
R.E.M. episode lasting an average of 2 minutes.
In the study, Dr. Wilson and Kenway Louie, a biology graduate student,
first trained the rats to run through the maze, receiving rewards when
they reached a point three- fourths of the way around it.
Electrophysiological activity from clusters of neurons in the hippocampus
was then recorded using multiple electrodes, made from fine wire,
implanted in the rats' brains. Recordings were taken while the rats ran
through the maze, and during periods of sleep before and afterward.
In previous work, Dr. Wilson and other researchers had found that while
rats ran a maze, hippocampal neurons fired in specific patterns,
producing, as they wrote, a "unique signature of the behavioral
experience." The pattern was distinct from that produced when the rats
ran a different maze, ran the same maze under different conditions, or
engaged in random activity.
"Due to the repetitive nature of the task," the researchers wrote, "such
patterns of activity were consistently repeated throughout a given
session. The repeated activation of these robust patterns led us to
hypothesize that such patterns may be good candidates for subsequent
reproduction during sleep."
In fact, of the 45 R.E.M. episodes ‹ each lasting 60 seconds to 250
seconds ‹ recorded while the rats slept, 20 contained a replication of
the signature maze-running pattern. Nineteen of those occurred in R.E.M.
periods recorded before the rats' daily session in the maze.
Dr. Wilson speculated that the more frequent appearance of the firing
sequences in R.E.M. episodes before the rats ran the maze might mean that
R.E.M. is "more precisely concerned with the remote past," involving a
re-evaluation of past experience, rather than a simple translation of
The pattern could also be seen during periods of slow-wave sleep, the
researchers found, perhaps reflecting the processing of more recent
memory, he said.
The study builds on work a decade ago by Dr. Jonathan Winson and his
colleagues, which found that single neurons in a rat's hippocampus were
reactivated during sleep as a result of experiences during waking.
Dr. Wilson and his colleagues hope to extend their work to other parts of
the brain, for example, examining patterns of activity in areas
responsible for sensory experience, like sight and smell.
The result, he said, might be "a kind of animal correlate of Freudian
psychoanalysis," a way to explore how waking life influences the
complexity and content of dreams, and how dreaming affects memory and
performance when awake.
And though the apparent dreams of the laboratory rats turned out to be
somewhat prosaic, Dr. Wilson said, this could be simply because they tend
to lead boring lives.
"It's not necessarily that rodents have simpler dreams," he said, "but we
limit them by restricting the experiences they have. It might be that a
wild subway rat's dreams are as exciting as our epic adventures in sleep."
Copyright 2001 The New York Times Company
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