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Direct Evidence of the Role of Sleep in Memory Formation Uncovered

The mechanism that takes place during sleep that causes learning and memory formation has been pinpointed for the first time, an international research team announced today.

“It’s been known for more than a century that sleep somehow is important for learning and memory,” Rutgers University said in a statement about the research.

“Sigmund Freud further suspected that what we learned during the day was ‘rehearsed’ by the brain during dreaming, allowing memories to form.

“And while much recent research has focused on the correlative links between the hippocampus and memory consolidation, what had not been identified was the specific processes that cause long-term memories to form,” Rutgers said.

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NGS photo by B. Anthony Stewart

The researchers 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, the university added.

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llustration from “Grays Anatomy,” 1918

The study, posted online on September 11 by Nature Neuroscience,was done by György Buzsaki, professor at the Center for Molecular and Behavioral Neuroscience, Rutgers University, Newark, and his co-researchers, Gabrielle Girardeau, Karim Benchenane, Sidney I. Wiener and Michaël B. Zugaro of the Collége de France, Paris.

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, Rutgers said.

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NGS photo by Ted V. Tamburo

During stage four sleep, Buzsaki explained, “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.”

“Those ripples also explain why certain events may only take place once in the waking state and yet can be remembered for a lifetime.”

The intensity and multiple occurrence 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, Buzsaki said.

The researchers were able to pinpoint that sharp wave ripples are the cause behind 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,” Rutgers said. “Those 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.”

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NGS photo by James L. Stanfield 

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,” Buzsaki said.

The research also represents a move toward a new direction in neuroscience research, Rutgers added.

“While previous research largely has focused on correlating behavior with specific brain events through electroencephalogram, neuronal spiking and functional magnetic resonance imaging studies, increasingly researchers are challenging those correlations as they seek to identify the specific process or processes that cause certain events and behaviors to take place.”

The research was performed at the Collége de France, Paris where Buzsaki worked as a distinguished visiting professor in 2008.

Rutgers Board of Governors Professor of Neuroscience Gyorgy Buzsaki is recognized worldwide for his work in expanding the boundaries of scientific understanding about the brain’s ability to process and store information, according to the university. At the Center for Molecular and Behavioral Neuroscience at Rutgers University in Newark, he has focused on the hippocampus and the role of neurons located there in maintaining long-term memory, as well as related research into epilepsy, depression, schizophrenia and Parkinsons disease.

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