Neuroscientist Lisa Marshall of the University of L¿beck and her colleagues wired the scalps of 13 medical students with electrodes. They strengthened the slow oscillations of neurons during so-called non-rapid-eye-movement rest, applying 0.517 milliampere at the 0.75 hertz frequency, which most closely mimics the brain's signal during such deep sleep. Marshall's team applied this boost five times--with each jolt lasting five minutes--over the course of the night, although the students slept until morning undisturbed. Prior to sleep, they had memorized 46 word pairs and, on average, recalled 36.5 of them. After the stimulating sleep, however, they improved their recall to 41.2 words, compared with just 39.5 words for the control group.
"The slow oscillations probably drive a replay of recently encoded memories in the hippocampus, where the declarative memories are temporarily stored," Marshall says, referring to the type of memory used to recall facts and events consciously. Replay appears to be the brain's primary method of consolidating declarative memory, she notes: "This replay stimulates a transfer of the memories back to the neocortex where they are stored for the long term." But the boost does not work for a task that involves remembering a sequence of finger taps, nor does boosting other frequencies of brain waves improve memory, according to the paper presenting the results published online November 5 in Nature.
The finding strengthens the case that electrically stimulating the brain can exert a synchronizing effect on individual neurons and produce beneficial effects. "Not only are there applications for healthy people to better realize their potential, but EEG-neurofeedback work has been extended to pathology, as in the case of children with attentional disabilities and [transcranial magnetic stimulation] for depression," notes psychologist John Gruzelier of Goldsmiths College in London, who has been working on training musicians to control their own brain waves, thereby improving performance. Oscillations faster than 0.7-0.8 Hz , Gruzelier says, "also have implications for other features of memory such as memory retrieval and working memory."
"They show that one can exert influence on brain activity by means of a super-simple method, electrical stimulation through the scalp. This method does not require expensive equipment or sophistication. Second, behavior can be modified by such intervention," adds neuroscientist Gy¿rgy Buzs¿ki of Rutgers University. For reasons that remain unclear, humans begin to lose the ability to sleep deeply around 40 years of age, at about the same time that memory begins to decline, he notes. "Restoring this critical stage of sleep by means of electrical stimulation," Buzs¿ki says, "is a truly intriguing possibility for restoring the declining effect of sleep on memory." The German researchers are now investigating just how long the improvement might last, Marshall notes. A little shock during slumber just might mean more--and longer-lasting--memories.