When lab rats sleep, their brains revisit the maze they navigated during the day, according to a new study released yesterday, offering some of the strongest evidence yet that animals do indeed dream.

Experiments with sleeping rats found that cells in the animals' brains fire in a distinctive pattern identical to the pattern that occurs when they are awake and trying to learn their way around a maze, the researchers found.

Based on the results, the researchers concluded the rats were dreaming about the maze, in essence reviewing what they had learned while awake to consolidate the memories.

"What we've shown is we can identify the content of brain activity during sleep and trace it back to an experience an animal had -- what it's thinking about and dreaming about during sleep, which is the first time that's been done," said Matthew A. Wilson of the Massachusetts Institute of Technology in Cambridge, Mass., who conducted the work.

Why humans sleep and dream has long been one of science's great mysteries. Researchers have long known that animals go through the same types of sleep phases that people do, including rapid eye movement sleep or REM sleep, which is when people dream. But aside from the occasional twitching, growling or barking that any dog owner has witnessed in a sleeping pet, there's been little direct evidence that animals really do dream.

"We know that they have dream-like states. The question is whether they actually dream. For the content of dream-like states in humans you can wake them up and ask them what they are dreaming about. You can't do that with animals," Wilson said.

If animals dream, it suggests they may have more complex mental functions than had been appreciated, Wilson said.

"We have as humans felt that this property of memory -- our ability to recall sequences of experiences -- was something that was uniquely human," Wilson said. "The fact that we see this in rodents does suggest they can evaluate their experience in a significant way. The ability to recall, reflect and evaluate prior experience is something that goes on in animals at many levels. They may be thinking about more than we had previously considered."

The findings also provide new support for a leading theory for why humans sleep -- to solidify new learning.

"People are now really nailing down the fact that the brain during sleep is reviewing its activity at least for the time immediately before sleep and almost undoubtedly using that review to consolidate or integrate those memories into more usable forms," said Robert Stickgold, an assistant professor of psychiatry at Harvard Medical School.

Carlyle Smith of Trent University in Peterborough, Ontario, called the research "really exciting" because it provided new evidence that "something important happens during REM sleep that probably has to do with memory.

"It's the first time this particular kind of activity has been shown in animals."

In the new experiment, Wilson and graduate student Kenway Louis implanted tiny electrodes in the brains of rats while the rodents learned to maneuver through a circular laboratory maze. In particular, the researchers measured the activity in a part of the brain called the hippocampus, which in humans is located deep in the brain behind the temples. The hippocampus has long been believed to be involved in forming and storing memories.

The rats showed a distinctive pattern. "That pattern is only generated when the animal runs on that maze. It's a unique signature for that experience. It's like having a movie of the experience we can record. Then the trick is to look for instances of that pattern while the animal is asleep," Wilson said.

The same pattern often appeared when the animals entered REM sleep, Wilson and his colleagues reported in today's issue of the journal Neuron. In fact, the pattern was so precise the researchers could actually chart where the rats were in the maze in their dreams and whether they were running or standing still.

"That tells us that in fact during those REM periods the memory of those events of the animal running on that maze is being replayed. The significance is that it says during these dream states there is in fact dream content. This is the animal correlate of dreaming," he said.

A sleeping rat is superimposed against brain firing patterns that may be dream content.MIT graduate student Kenway Louie, left, and Professor Matthew A. Wilson work with a bank of computers where they collect data on rats and their dreaming brain activity.