Just as a conductor coordinates different instruments in an orchestra to produce a symphony, breathing coordinates hippocampal brain waves to strengthen memory while we sleep, reports a new Northwestern Medicine study.
This is the first time breathing rhythms during sleep have been linked to these hippocampal brain waves -- called slow waves, spindles and ripples -- in humans. Scientists knew these waves were linked to memory but their underlying driver was unknown. The study is published in the Proceedings of the National Academy of Sciences.
"To strengthen memories, three special neural oscillations emerge and synchronize in the hippocampus during sleep, but they were thought to come and go at random times," said senior study author Christina Zelano, professor of neurology at Northwestern University Feinberg School of Medicine. "We discovered that they are coordinated by breathing rhythms."
Northwestern scientists discovered that hippocampal oscillations occur at particular points in the breathing cycle, suggesting that breathing is a critical rhythm for proper memory consolidation during sleep.
"Memory consolidation relies on orchestration of brain waves during sleep, and we show that this process is closely timed by breathing," said corresponding author Andrew Sheriff, a postdoctoral student in Zelano's lab.
The findings have important implications for disordered breathing during sleep -- such as sleep apnea -- which is linked with poor memory consolidation.
We've all had the experience of better memories after a night of sleep. This was noted as far back as ancient Rome, when the scholar Quintillion wrote of the "curious fact" that "the interval of a single night will greatly increase the strength of the memory," the study authors said. He was describing what we now call memory consolidation, which is accomplished by the exquisitely tuned coordination of different brain waves in the hippocampus.
"When you're sleeping, your brain is actively replaying experiences you had during the day," Sheriff said.
Sheriff had just returned from a conference in Reykjavik, Iceland, where he had to learn his way around a new city.
"The hippocampus plays a major role in forming a map of a new area," Sheriff said. "I would wake up and feel I had a better representation of the city around me. That was facilitated by the oscillations that occurred during my sleep, which we found are coordinated by breathing."
The study indicates people with disrupted breathing during sleep should seek treatment for it, Sheriff said.
"When you don't get sleep your brain suffers, your cognition suffers, you get foggy," Sheriff said. "We also know that sleep-disordered breathing is connected with stroke, dementia and neurodegenerative disorders like Alzheimer's disease.
"If you listen to someone breathing, you might be able to tell when they are asleep, because breathing is paced differently when you're sleeping. One reason for that may be that breathing is performing a careful task: coordinating brain waves that are related to memory."
Other Northwestern authors include Guangyu Zhou, Justin Morgenthaler, Christopher Cyr, Katherina K. Hauner, Mahmoud Omidbeigi, Joshua Rosenow, Stephan Schuele and Gregory Lane.