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 limited.''
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.
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