Memory boosting happens early in sleep, during the period of slow-wave sleep that comes before dreaming and rapid-eye-movement (REM) sleep. During this time, the electric currents in the brain slowly oscillate up and down.
Some scientists say these oscillations are good vibrations that bring brain networks carrying new information into harmony with the rest of the brain. Other scientists say these vibrations are just a side effect of the brain trying to synchronize itself.
Lisa Marshall, PhD, Jan Born, PhD, and colleagues at the University of Lubeck thought of a way to test these ideas. They got 13 brave volunteers -- mostly medical students -- to let them place electrodes on their heads. While the students slept, the researchers turned on a mild electric current. The pulsing current tuned the student's brains to a slow frequency.
For an hour and a half before going to sleep, the students studied a list of paired words. They were tested just before sleeping and just after waking up the next morning. As expected, the students did better the next morning. But they did even better when their brains were electronically tuned.
"This improvement in retention following stimulation is striking considering that most subjects were medical students, who were highly trained in memorizing facts and already performed well in the sham condition," Marshall and colleagues note.
The brain tuning improved memory only when given during early, slow-wave sleep and not during other phases of the sleep cycle.
The researchers conclude that the slow vibrations of early sleep are not the mere humming of the brain while it works on more important things. Instead, they may be the music of which memories are made.
Marshall and colleagues report the findings in the early online edition of the journal Nature.