The Power Nap
Memory Stimulation During Sleep Can Reinforce Skills
Decades ago, “sleep learning” lured us with promises of mastering a brand new skill—Greek or chess, say—in our sleep. Perhaps, it was thought, we could bypass the daytime drudgery of memorization by listening to instruction in our sleep.
Scientific evidence quickly disproved the efficacy of that kind of learning. But a recent study at Northwestern shows that sleep may indeed be a time for strengthening skills that we have already learned in our waking moments.
“Our protocols are different today,” says neuroscientist and psychology professor Ken Paller, co-author of the study. “We are not trying to give people an easy way out. You have to devote effort to learn while you are awake. Then it continues while you are asleep. In this way, it’s possible to reinforce things you’ve already learned.”
And that holds promise, Paller says, for everyone from stroke victims to Alzheimer’s patients to ball players hoping to improve their pitch.
“Sleep does many things,” Paller says, “but one is to sort out the things we learn each day and integrate them and process them further. During sleep, you are probably going to process memories that you’ve acquired recently. You might rehearse commercials you saw on TV, for example, for products you don’t really want. But we would like to know how much choice there can be about what gets rehearsed. So our research group asked whether we could bias sleep processing to specific, recently learned information to give you that advantage.”
Name That Tune
Previous studies by Paller and others have shown that sound and odor cues can help people remember where objects are located. Their most recent project expands upon those findings by showing that learned skills can also improve during sleep.
Here’s how it worked: Volunteers, many of them students, came to Paller’s lab one by one and were taught to play two simple melodies on a computer keyboard. Then, while undergoing EEG monitoring, they tried to fall asleep (not difficult for most sleep-deprived students).
While the subjects slept, the notes of one melody or the other were played back to them. Upon waking, they were asked to play the two songs, and their performance of each was measured for mistakes.
Consistently, volunteers made fewer mistakes with the song that was played during their naps.
The results of the study have been published in Nature Neuroscience and heralded in Science News and other scientific publications. The project was led by neuroscience graduate student James Antony ’14, working with another psychology professor, Paul Reber, and two other students, Eric Gobel ’08 and Justin O’Hare ’12.
“Our angle was to go against the common assumption that you are basically closed off to the world when you are asleep,” Paller explains. “By presenting subtle sounds, we found you could influence what the brain is doing during sleep. So our approach was to manipulate the memory processing during sleep using specific sounds that connected with what people learned in the lab prior to going to sleep.”
The EEG equipment used to measure sleep has been around for decades, so the melody experiment technically could have been done years ago. But Paller says that scientists were looking for answers in the wrong part of the sleep cycle: rapid eye movement (REM) sleep instead of slow-wave sleep. Recent experiments have shown that slow-wave sleep is a key time for the processing of recently acquired knowledge. This processing influences what one is going to remember.
A Clue to Memory Loss?
Everyone goes through sleep cycles that are roughly 90 minutes long, moving from light sleep to deeper sleep. Slow-wave sleep is characterized by electrical signals in the EEG, including slow waves of activity that are quite prominent and distinctive. This stage transitions into REM sleep, with brain activity that looks a lot like waking, with the eyes rapidly moving. Then the pattern repeats itself and the sleeper goes back to stage one. One might have four or five such cycles during eight hours of sleep.
Researchers are trying to determine just how these intricate brain mechanisms relate to memory. “Why is there a 90-minute cycle?” Paller asks. “Why is it good to go through all of these stages? Why is it detrimental if you don’t? What are the various factors that influence slow-wave sleep?”
One of the factors, Paller observes, is age, so that as we age, we get less slow-wave sleep. “This might contribute to the less-than-perfect memory that most people experience as we get older,” he says.
Paller’s recent work has been funded by the National Science Foundation, but much of his time these days is spent writing proposals for new grants. He is excited about the possibilities of further sleep-based research, especially collaborative work with Phyllis Zee, a neurology professor at the Feinberg School of Medicine, who conducts groundbreaking investigations on sleep and its disorders.
The melody-performance experiment tested one type of skill, but Paller and his group want to think about other skills, such as those involved in stroke rehabilitation.
“If you have a stroke, you go through therapy as a learning process,” says Paller. “Your arm is just fine, but you’ve lost some brainpower and you have to learn a new way of performing that function. To get those brain networks to do what they need to do, you have to practice. So our thought is that if you can practice during your sleep as well as during your rehab sessions, you may get a boost in learning what you need to learn to regain function.”
Paller has to wait nine months to find out if one particularly promising grant application is successful, but he’s already thinking about the implications for issues such as post-partum depression, anxiety, PTSD, and smoking.
“I have colleagues in the medical school who are ready to collaborate with me and there are lots of possibilities, “ he says. “I say, ‘Let’s go try them.’”