Fwd: When Rats Dream, It Seems, It's After a Day at the Mazes

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    When Rats Dream, It Seems, It's After a Day at the Mazes



    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
    of Technology.

    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
    recent events.

    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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