Each night as you sleep, your brain buzzes with electrical activity. Neuroscientists suspect that that this activity helps solidify memories formed during the day. Now, they've bolstered their case: for the first time, researchers have shown that electrically stimulating the brain during sleep can enhance memory performance the following day.
Many previous studies have linked sleep and memory. Waking people up during the slowly oscillating pattern of brain activity known as slow wave sleep, for example, makes them perform poorly on tasks that require remembering recently learned facts. If these slow waves really do have a role in making memories stick, enhancing the waves might enhance memory, reasoned neuroscientists Lisa Marshall and Jan Born of the University of Lübeck, Germany.
To test the idea, Marshall, Born, and colleagues recruited 13 medical students for a sleepover in the lab. Before settling down for the night, the students studied pairs of words and pairs of figures, learned to tap a sequence of keys on a keyboard, and practiced drawing figures while looking at a mirror. Then each student was fitted with electrodes that allowed the researchers to record their brain waves and stimulate their brains through their scalps. When the electrodes revealed that a student had drifted into slow wave sleep, the researchers turned on the stimulating electrodes for about half an hour. The idea was to provide low frequency stimulation that enhanced the brain's natural slow waves, Born says. The same 13 students also spent another night in the lab in which they went through the same routine--minus the stimulation.
Although the students weren't able to tell whether they'd received the stimulation, memory tests given the following day revealed a difference. After stimulation, students showed a modest but consistent improvement on the word pair and figure pair tests compared to their scores after a nonstimulated night's sleep, the researchers report online 5 November in Nature. Their performance on the typing and drawing tasks wasn't affected by stimulation, however.
The research adds to growing evidence that different types of memory gel during different stages of sleep at different times of night, says Robert Stickgold, a neurobiologist at Harvard University. But Stickgold doesn't foresee a future in which people keep brain stimulators by their bedside tables. While he concedes that the idea is "way cool," Stickgold suspects that evolution has already done a pretty good job of optimizing brain activity during sleep. "Tweaking it further isn't going to do all that much good for you," he says.