Sweet Rat Dreams.

Ever dream you're lost in a maze of hallways at school? If so, you're a lot like a rat. While rats sleep, new research shows, they replay events from their lives much like people do. And these dreams may help them learn from the past.

Since animals can't describe their dreams, researchers have never known exactly what dances through their slumbering heads. Now scientists at the Massachusetts Institute of Technology (MIT) have recorded and interpreted the brain activity of snoozing lab rats. And, says MIT biologist Matthew Wilson, "We found that rats seem to dream of running through a maze in a way that suggests they're practicing how to do it."

How do they know what a rat dreams about? First, Wilson needed to see what happens in a rat brain during its waking hours. Using chocolate sprinkles as rewards, he trained lab rats to find their way in and out of mazes. He monitored the rat's brain activity by implanting electrodes (wires that carry electrical activity) into its hippocampus a brain region that processes memory. Whenever the rat scurried to a different part of the maze, the electrode recorded electric impulses fired by different brain cells.

The next stage was to record the rat's brain activity while it slept. Wilson noticed that when rats fall into a stage of sleep called REM (rapid eye movement)--where dreaming most commonly occurs--the same cells that fired during the maze run burst into activity. "It's as though the animal was literally replaying its running in the maze," Wilson says.

The patterns were so specific, in fact, that Wilson could pinpoint where in the maze a sleeping rat would be running if it were awake. Special memory cells in the hippocampus called place cells may play a key role in a rat's ability to navigate through complicated mazes--even during sleep. Place cells may also determine why you remember your way to school and back home again.