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The search continues for other cognitive skills that might be linked to sleep. In January, Jan Born and his colleagues at the University of Lübeck in Germany published a clever study that shows why sleeping on a problem often brings such good results. They asked 106 test subjects to transform a string of numbers into a different string of numbers, using a simple but tedious mathematical equation. Unbeknownst to the study volunteers, there was a hidden trick to the calculations that could cut their response time dramatically. A good night's sleep between practice sessions more than doubled--from 23% to 59%--the probability that participants caught on to the trick. In other words, sleep isn't absolutely necessary to gain insight into a problem, but it can be a big help.
So can new technology, which is allowing researchers to study sleep at a microscopic level for the first time. Neuroscientists have long been able to record the firing of a single nerve cell, using a tiny electrode implanted in a laboratory animal's brain. But it's only recently that they have had electrodes small enough and computers powerful enough to record scores of individual neurons at once. The goal is to identify the changing patterns of neuronal firing during sleep. "There are days when we can record up to 500 neurons, but that's not typical," says Bruce McNaughton, a psychologist and physiologist at the University of Arizona in Tucson, who studies rats. More typically, he is able to tap between 50 and 100 neurons. That's not a lot when you consider that even a rodent's brain has 125 million neurons. But it was enough to get him started.
What McNaughton's recordings have shown is that many of the same neurons that fire during the daytime--say, when a rat is learning to navigate a maze--are reactivated during the REM stage of sleep. "Basically, the brain is reviewing its recently stored data," he says. Eventually the brain consolidates those patterns into permanent connections--or, as neuroscientists like to say, "neurons that fire together, wire together." Interestingly, says McNaughton, that process appears to happen not just during sleep but during restful states throughout the day as well.
SLOW-WAVE LEARNING
Better equipment HAS ALSO GIVEN researchers a new respect for what can be accomplished during slow-wave sleep. In a study published in July in Nature, Wisconsin's Tononi and others showed that a specific part of the brain that had been busy learning a new skill while awake needed much more slow-wave sleep in order to improve performance.
The scientists had 11 volunteers play a simple video game that required them to reach for objects on the screen with a mouse-controlled cursor. What the volunteers didn't realize was that the game sometimes introduced a slight bias to the cursor's motion, forcing them to adjust their movements. Half the group slept between sessions and the other half did not. Among the sleepers, the part of the brain that was learning to compensate for the bias while awake turned out to have the largest slow waves during sleep. "The bigger the slow waves were in that part of the brain, the better they performed the next day," Tononi says.