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New recordings of electrical activity in the brain may explain a major part of its function, including how it consolidates daily memories, why it needs to dream and how it constructs models of the world to guide behavior.

The recordings capture dialogue between the hippocampus, where initial memories of the day’s events are formed, and the neocortex, the sheet of neurons on the outer surface of the brain that mediates conscious thought and contains long-term memories.

Such a dialogue had been thought to exist, but no one had been able to eavesdrop on it successfully. The new insight has emerged from recordings of rat brains but is likely to occur in much the same way in the human brain, which has analogous structures and the same basic principles of operation.

The finding, reported on the Web site of the journal Nature Neuroscience by Daoyun Ji and Matthew A. Wilson, researchers at the Massachusetts Institute of Technology, showed that during nondreaming sleep, the neurons of both the hippocampus and the neocortex replayed memories — in repeated simultaneous bursts of electrical activity — of a task the rat learned the previous day.

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.

Because the fast rewinds in the neocortex tended to occur fractionally sooner than their counterparts in the hippocampus, the dialogue is probably being initiated by the neocortex, and reflects a querying of the hippocampus’s raw memory data, Dr. Wilson said.

Brain researchers have long assumed that immediate memories are laid down in the hippocampus and later transferred to the neocortex for long-term storage. Dr. Wilson said the process was not just a transfer of memory, however, but more probably a sophisticated processing of data in which the neocortex learned selective information from the hippocampus.

“The neocortex is essentially asking the hippocampus to replay events that contain a certain image, place or sound,” he said. “The neocortex is trying to make sense of what is going on in the hippocampus and to build models of the world, to understand how and why things happen.”

These models are presumably used to direct behavior, Dr. Wilson said. They are able to generate expectations about the world and plausibly fill in blanks in memory.

Though the neocortex learns from the hippocampus, the raw memory traces, from childhood onward, are not transferred and are probably retained in the hippocampus, Dr. Wilson said. If so, the forgetfulness of age would arise because of problems in accessing the hippocampus, not because the data has vanished.

The subject matter of the neocortex-hippocampus dialogue in rats seems mostly to concern recent events. This is consistent with what people report when awoken from nondreaming sleep — usually small snatches of information about recent events. Dr. Wilson also said that the new findings, by showing activity in the visual neocortex, confirmed that rats had humanlike dreams with visual imagery, a possibility some researchers had doubted.