The researchers could interpret the memories through electrodes inserted into the rats’ brains, including into special neurons in the hippocampus. These neurons are known as “place cells” because each is activated when the rat passes a specific location, as if they were part of a map in the brain. The activation is so reliable that one can tell where a rat is in its cage by seeing which of its place cells is firing.

Earlier this year Dr. Wilson reported that after running a maze, rats would replay their route during idle moments, as if to consolidate the memory, although the replay, surprisingly, was in reverse order of travel. These fast rewinds lasted a small fraction of the actual time spent on the journey.

In the findings reported today, the M.I.T. researchers say they detected the same replays occurring in the neocortex as well as in the hippocampus as the rats slept. The rewinds appeared as components of repeated cycles of neural activity, each of which lasted just under a second. Because the cycles in the hippocampus and neocortex were synchronized, they seemed to be part of a dialogue between the two regions.

The researchers recorded electrical activity only in the visual neocortex, the region that handles input from the eyes, but they assumed many other regions participated in the memory replay activity. One reason is that there is no direct connection between the visual neocortex and the hippocampus, suggesting that a third brain region coordinates a general dialogue between the hippocampus and all necessary components of the neocortex.

Larry Squire, a neuroscientist who studies memory at the University of California, San Diego, noted that the replay system in the neocortex had not been seen before. The fact that it occurred during sleep “would certainly provide one clue that part of the function of sleep is to let us process and stabilize the experiences we have during the day,” Dr. Squire said.