A team of researchers in Germany has found that a certain type of memory improves when a person’s brain is stimulated with a mild electric current during a particular phase of the sleep cycle.
Neuroscientist Jan Born, of the University of Lubeck, has been studying the role of sleep in human memory for the past decade. In recent years, there’s been evidence to suggest that REM sleep and non-REM sleep serve to strengthen neuron connections for different kinds of memories. While the scientific community is split on just how these phases influence memory, Born and his colleagues have recently focused on non-REM sleep, specifically the initial, drowsy, slow-wave phase. They’re interested in its role in strengthening declarative memories, otherwise known as fact-based memories, as opposed to other types of memory such as motor-skill, or procedural learning.
“You remember the things consolidated during sleep better than not during sleep,” says Born. “Our research is finding out which stages are more important for memories.”
In previous studies, scientists have found that different phases of sleep are characterized by different patterns of brain activity. The most well-known is that of REM sleep, a period of heightened activity within the cortex when dreaming usually occurs. During a full night’s rest, REM sleep occupies 20 percent of a person’s sleep.
The other 80 percent is devoted to non-REM sleep: a period of relatively little action, mostly consisting of slow, synchronized waves that travel across the surface of the brain. Less is known about this relatively dormant phase, and Born theorizes that, in fact, it’s not a dormant phase at all. Rather, this slow-wave sleep may serve as a sort of “covert replay” of what was learned before an individual went to sleep. That is, this synchronized activity may somehow reinforce neuron connections involved in establishing long-term memories, particularly for facts.
To test this hypothesis, Born and his colleagues applied electrodes to the scalps of 13 medical-student volunteers. The whole group was subjected to successive nights of testing through various kinds of stimulation. Before going to sleep, they were asked to learn a list of word pairs (a declarative-learning task). They also learned a finger-tapping exercise normally associated with motor-based learning. As they slept, researchers electrically stimulated their brains with a mild, .75 hertz current at 5-minute intervals during an early, 30-minute period of non-REM sleep. The current applied matched the naturally occurring slow-wave frequency of non-REM sleep, and it was low enough not to disturb volunteers as they slept. On other nights, researchers altered the current, and they also applied currents during both non-REM and REM phases of sleep. Sometimes, as a control, they applied no current at all.
After each night’s sleep, volunteers, once fully alert, were asked to recall the tasks they had learned the day before. Of all the trials, they improved most significantly after having been stimulated at .75 hertz during non-REM sleep. What’s more, the improvement, although small (8 percent), was only seen on the word-pair task, suggesting a link between non-REM sleep and fact-based, or declarative, memory. “I think it’s a very good indicator that makes us think we induced a really physiological pattern in these brain networks,” says Born. “This [activity] in turn stimulates the brain to replay these memories.”
He says that while the improvement measured was small, the fact that the experiment was tested on “high performers”–medical students who are used to memorizing large amounts of information in a single sitting–suggests that any improvement is significant.
Terry Sejnowski, professor and head of the Computational Neurobiology Laboratory at the Salk Institute for Biological Studies, has also studied sleep’s role in memory. Sejnowski sees Born’s findings as an important advance in the study of memory consolidation.
“Previous studies only showed that memory improvements were correlated with brain oscillations, and only for nonfactual forms of memory, such as motor learning,” says Sejnowski. “This study provides evidence that the link between sleep and memory is causal and may lead to a practical way to improve memory.”
Born agrees with this last point, and he believes that someday electrical stimulation during sleep may be a possible therapy for those with memory-loss problems.
However, others like Jerome Siegel are wary of linking sleep with memory. Siegel, a psychiatrist at the University of California at Los Angeles and head of its Center for Sleep Research, says that while sleep may have some role in forming memories, it is not an essential role. He warns that there is a danger in misinterpreting data when trying to establish a causal role.
“You have to wonder to what extent you are getting chance results just because you measure a lot of variables,” says Siegel. “Also, there’s the performance issue, where if sleep is deeper, performance is better, and it’s different from memory.”
In future studies, Born plans to tease out the many variables from this initial experiment by looking at the effects of specific currents and whether applying them for varying amounts of time will have significant effects.
“We also have a plan to see if you can use stimulation not only to intensify an ongoing sleep stage like non-REM sleep,” says Born, “but if you can, for example, change the brain state from the waking state into the sleeping state, which is so far what we see as a dream–the researcher’s dream.”