A rat pauses at the end of a track. He dips his nose into a cup containing food and munches contentedly, nose twitching. Meanwhile, brain cells in the hippocampus of his brain replay his recent route to this repast, according to the results of a new study.

Previous research had shown that rats replayed specific brain firing sequences while sleeping. David Foster and Matthew Wilson at the Massachusetts Institute of Technology wanted to find out what happened when they were awake. After implanting measuring devices to catch neurons firing in the hippocampuses of four rats, they let the animals run up and down a track with food at each end. Upon reaching the end of the track, the rats paused to eat, groom or just be still.

But their hippocampus cells were in a frenzy of activity. These cells are known to play a role in the formation of memories in rats and primates, including humans. By measuring the amount and location of the hippocampus cell firing, the researchers were able to determine that the neurons fired in the exact reverse order of the firing that occurred when the rat scurried from one end of the track to the other. In essence, the rats' brains replayed the recent route, possibly committing it to memory.

Such activity did not occur when the animals simply rested outside the track or when they were in a more familiar environment. "Reverse replay in the hippocampus might have a critical role in support of learning," the researchers conclude in their paper detailingthe findings, published online yesterday by Nature.

"When awake, reverse replay occurs in situ, allowing immediately preceding events to be evaluated in precise temporal relation to a current anchoring event, and so may be an integral mechanism for learning about recent events," they continue. "Understanding this replay is likely to be critical to understanding how animals learn from experience."