Published online 15 November 2007 | Nature | doi:10.1038/news.2007.253

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Sleeping brain plays back events in fast-forward

Rat study shows long-term memory formation at work.

How are long term memories laid down while we sleep?Punchstock

Research on slumbering rats has shed light on how the brain processes its recent experiences into long-term memories. The experiment suggests that the brain creates such memories by 'playing back' the day's events several times faster than they actually happened.

The study, carried out by neuroscientists at the University of Arizona in Tucson, boosts the theory that the brain region responsible for organizing long-term recall, known as the medial prefrontal cortex (mPFC), consolidates memory by playing back events during sleep. It also shows that the brain is quicker at re-running these events than it is at actually performing them.

The researchers, led by Bruce McNaughton, trained two rats to run to a series of different locations within an area during 50-minute activity sessions. After the task, they allowed the rats to sleep for up to an hour.

During the experiment, the researchers monitored the activity of selected brain cells in the rats' mPFC. While performing the task, the cells showed a characteristic pattern of activity. During the subsequent sleep, the same cells showed the same patterns, but at a higher speed.

During sleep, the rats' brain replayed this characteristic activity at roughly six or seven times the original speed, McNaughton and his colleagues report in this week's Science1.

Fast-forward

The observed playback is consistent with the role of the mPFC in long-term memory storage. Very recent experiences are recalled with the help of a brain structure called the hippocampus, and neuroscientists think that, during sleep, memories are 'transferred' from short-term hippocampal storage to the longer-term repository in the mPFC.

Accelerated playback has been seen in other brain regions linked to memory, including the hippocampus. But this is the first time it has been seen in the prefrontal cortex, says Matthew Wilson, a neurobiologist at Massachusetts Institute of Technology in Cambridge. "It puts the prefrontal cortex firmly into the picture of coordinated memory processing during sleep," he says.

It's not clear why the brain replays events faster than they actually happened, McNaughton and his colleauges admit. They suggest that the pathways made up by connections between brain cells may have an intrinsic speed that results from the movement of electrical signals through them — and that the brain has to 'slow down' when working under the constraints of having to perform actual behaviours.

"I think that's a very compelling idea," agrees Wilson. The observed bursts of replay in the rat brains lasted just a few hundred milliseconds each, suggesting that the rodents' brains were playing back tiny chunks of their experience and processing them into small packages of memory.

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While you were sleeping

Similar processes almost certainly occur in the human brain during sleep, says Wilson, and may even be linked to dreaming. "I believe these phenomena are directly correlated with dream-like states that humans report," he adds.

It is not usually possible to investigate the sleeping human brain in as much detail as in rats, because of ethical considerations. But Wilson expects that, as both rat brain studies and human sleep research improve, the gap between the two will be closed.

"In human sleep studies it's difficult to see what is going on, but it's possible to see the consequences of sleep-related activities," Wilson says. But he thinks that "there is an overlap - I see there being convergence, and I see there being consistency" between what goes on inside human and rat brains as they slumber. 

  • References

    1. Euston, D. R., Tatsuno, M. & McNaughton, B. L. Science 318, 1147-1150 (2007). | Article |
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