Brain learns while you snooze

Sleeping mind can make associations between smells and sounds

Even while in a deep slumber, people can still learn brand new information. Sleepers soak in new associations between smells and sounds, knowledge that lingers into the next waking day, researchers report online August 26 in Nature Neuroscience.

The new study is the first to show that entirely new information can creep into the sleeping mind, says study coauthor Anat Arzi of the Weizmann Institute of Science in Israel. Sleep used to be considered a kind of reversible death, she says. “But the brain is not passive while you sleep. It’s quite active. You can do quite a lot of things while you are asleep.”

But before overtaxed students rejoice, the results don’t mean that Spanish vocabulary tapes now have a place on the nightstand. Researchers have tried and largely failed to find evidence that trickier information, such as new pairs of words, can make its way into the brain during sleep.

Instead of trying to teach people something complicated like a new language, Arzi and her colleagues relied on subjects’ sense of smell. As anyone who has walked by a dumpster in July knows, smells can elicit a nose-jerk reaction. Catching a whiff of hot trash automatically makes people inhale less, curbing the size of the inhale. But a scent of fresh bread spurs a long, deep inhale. Arzi and her team took advantage of this sniff reflex for their experiment.

As people slept in the laboratory, the researchers delivered pleasant scent, such as shampoo. As this nice smell crept into the sleepers’ noses, the researchers played a particular tone.  Later, a disgusting smell, such as rotten fish or carrion, was paired with a different tone. Neither the smell nor the sound woke people up.

After just four exposures to the smell-tone pair during a single night, the sleepers started to automatically respond to the tones without the accompanying smells, taking in bigger breaths when the shampoo-associated tone played and smaller breaths when played the sound linked to the rotten fish smell.

This new learned association lingered into the next waking day, too. Even though the sleepers had no idea they had been exposed to smells or sounds, their behavior proved otherwise. As before, the shampoo sound elicited a long, deep inhale, while the rotten fish tone caused more shallow breaths. “They learned what the tone signified,” Arzi says.

One of the reasons these researchers found evidence for sleep learning when others had not is because they relied on people’s sense of smell, says neuroscientist James Antony of Northwestern University. “It’s a clever way of using this system that has a strong response.”

Scientists don’t yet know the limits of sleep learning, says Antony, who recently found that musical skills could be strengthened if a person listened to a recording of a song during sleep. Complex subjects like calculus, European history and Arabic might not work, but perhaps acquiring more subtle information — shifting preferences or habits, for example — might be possible, he says.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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