Is 'instant replay' a learning tool?

A new discovery by Massachusetts Institute of Technology researchers suggests that the brain may be able to work much like the ''instant replay" on sports shows.

But -- and here is the especially surprising part -- it runs backward and super-fast: instant replay on ''rewind."

Scientists at MIT's Picower Institute for Learning and Memory have managed to record such reverse replay signals from rats that ran laps on a five-foot track, according to an online report in the journal Nature last week.

And that ''reverse replay" appears to occur only during pauses for reflection, helping the rodents learn from what they have just experienced, the researchers say -- as if it were the rapid review at the end of a college course.

''What this suggests is that, while there certainly is some record of your experience as it's occurring, that the actual learning -- when you try to figure out what was important, what should I keep and throw away -- that could happen after the fact, during periods of quiet, wakeful introspection," said Matthew A. Wilson, the paper's senior author. It is not clear whether the replay is conscious or unconscious, he said.

As the four rats ran, dozens of electrodes in their brains picked up patterns of electrical activity corresponding to different points on the track as the rodents moved from start to middle to end and a food reward.

Then, after the rats ate, they tended to rest a bit, and perhaps groom themselves. During that rest period, their brain patterns ran rapid-fire ''end-middle-start, end-middle-start, end-middle-start" patterns.

Wilson's lab had already found that when rats sleep, their brains often repeat -- in forward chronological order -- the patterns corresponding to the routes they have recently traveled, as if cementing the geography they had learned.

But if replay was an important learning mechanism, it never made sense that it would have to wait hours until an animal slept, said David Foster, first author on the Nature paper.

''Now, we actually see it happening immediately," he said. ''It's possible that replay could be more fundamentally involved" in learning.

In fact, reverse replay could help answer a central question about learning that applies to both animals and machines, said P. Read Montague, a Baylor College of Medicine neuroscience professor spending this year at Princeton University, who was not involved in the Nature paper.

Say that an animal is poking around, turns over a rock, runs around a bush, and then suddenly finds a delicious grubworm, Montague said. The animal needs to learn what it did right, he said, but so many things have happened that it does not know whether the most important step was the last thing it did or some action taken 50 events ago.

To learn efficiently, he said, an animal -- or a machine -- needs to ''rehearse its experience to be able to assign credit at the right point in time." And why begin at the beginning when there may have been 10 or 20 false starts that led to nothing? Rather, he said, better to begin with ''Bang! That really worked out! Now let's look back in time and see if we can assign proper credit to what really got me here."

The MIT researchers say there is every reason to believe that humans have the same kind of replay process as rats. The part of the brain they recorded from, a seahorse-shaped structure called the hippocampus that is known to be key to memory, is very similar between the species.

The technique used by Wilson and Foster involved placing tiny electrodes inside the rats' brains, and is too invasive to be performed in humans. But Montague said that a cleverly designed experiment could still detect reverse replay in humans using brain imaging technology.

It could be, the MIT researchers say, that reverse replay may even play a role in certain human disorders. For example, Foster said, it could be that hyperactivity in children is associated with learning impairments because their disorder doesn't allow them the rest periods needed for the reverse replay.

The replay findings fit well with educational research that has found that allowing students breaks between stints of learning can lead to better results, Wilson said. The rat research ''adds some additional weight or credibility to the idea that there is in fact some biological basis for the kinds of improved learning that can occur under those conditions," he said.

A key to the discovery, Wilson said, was Foster's idea to allow the rats to behave naturally and loaf in between laps, rather than push them, as researchers usually do, to constantly run, run, run. It was only when they were allowed that period as ''slacker rats," he said, that the replay patterns were detected.

So is there an immediate lesson for humans?

''We have this tendency to kind of force ourselves, push ourselves," Wilson said. ''To constantly go, and that gets reflected in the way we do our science. We want animals to perform, we want to perform. Just sitting back and allowing them to do nothing, it's hard to do that because you tend to think you're wasting time and energy." But what made this experiment work, he said, was ''letting the animals do what they naturally do."

Carey Goldberg is reachable at goldberg@globe.com.