The mechanism that causes learning and memory formation during sleep has been uncovered for the first ever time by a team of scientists.
Researchers led by Gyorgy Buzsaki, professor at the Center for Molecular and Behavioral Neuroscience at Rutgers University, Newark, have determined that short transient brain events, called "sharp wave ripples," are responsible for consolidating memory and transferring the learned information from the hippocampus to the neocortex, where long-term memories are stored.
Sharp wave ripples are intense, compressed oscillations that occur in the hippocampus when the hippocampus is working "off-line," most often during stage four sleep, which, along with stage three, is the deepest level of sleep.
"(During stage four sleep) it's as if many instruments and members of the orchestra come together to generate a loud sound, a sound so loud that it is heard by wide areas of the neocortex. These sharp, 'loud' transient events occur hundreds to thousands of times during sleep and 'teach' the neocortex to form a long-term form of the memory, a process referred to as memory consolidation," Nature quoted Buzsaki as saying.
The intensity and multiple occurrences of those ripples also explain why certain events may only take place once in the waking state and yet can be remembered for a lifetime, added Buzsaki.
The scientists could pinpoint that sharp wave ripples caused memory formation by eliminating those ripple events in rats during sleep.
The rats were trained in a spatial navigation task and then allowed to sleep after each session.
It was found that rats that selectively had all ripple events eliminated by electrical stimulation were impeded in their ability to learn from the training, as compressed information was unable to leave the hippocampus and transfer to the neocortex.
Identification of a specific brain pattern responsible for strengthening learned information could facilitate applied research for more effective treatment of memory disorders.
"This is the first example that if a well-defined pattern of activity in the brain is reliably and selectively eliminated, it results in memory deficit; a demonstration that this specific brain pattern is the cause behind long-term memory formation," said Buzsaki.
The study has been published in Nature Neuroscience.