Knocking the brain out of rhythm during sleep can interrupt memory formation and lead to learning disruptions, says a new animal study.
Researchers have known that disrupting sleep can impede memory creation and storage, yet the physiological cause remained a mystery. Now, researchers from the University of Michigan believe they’ve zeroed in on the cause of memory mishaps.
It starts in the hippocampus, the brain’s central storage unit for memories, and involves the patterns and “oscillations” of neurons firing away. In mice studies, the researchers discovered that disrupting the normal neural rhythm during sleep in a subsection of the hippocampus known as CA1 wiped out any memories that normally would have been gained.
During the study, the researchers used a common apparatus to test the mice’s behavior. They inserted the mice into a new environment and let them run around freely. Then they applied a mild shock to the feet of the mice and returned them to their normal cages to let them sleep.
“If you return the mouse to that same structure a day or even a couple months later, they will have this very stereotyped fear response, which is that they freeze,” said senior author Sara Aton, an assistant professor in the Department of Molecular, Cellular and Developmental Biology. “But if you sleep-deprive an animal for a few hours after that context-shock pairing, the mouse won’t remember it the next day.”
When mice slept normally after a test like the one described above, the “sleep-associated oscillations” were significantly higher than normal, which suggests that the memories were being ingrained in their minds.
However, when the researchers gave some mice a drug to suppress neural activity in the CA1 section during sleep, those mice appeared to lose any memory of the shock. The researchers surmise that preventing the neurons from “firing together regularly and rhythmically” forestalled the development of critical, new memories.
“There’s an old theorem called Hebb’s Law, which is, ‘Fire together, wire together,'” Aton said. “If you can get two neurons to fire with great regularity in close proximity to each other, it’s very likely you’re going to affect the strength of connections between them.”
The interplay of distinct neurons seems to create a complex tapestry through which memories can form, suggests the study.
“It seems like this population of neurons that is generating rhythms in the brain during sleep is providing some informational content for reinforcing memories,” Aton said. “The rhythm itself seems to be the most critical part, and possibly why you need to have sleep in order to form these memories.”
That marks a big step forward in the scientific community’s understanding of how the hippocampus operates and the precise role of individual cells in memory formation and learning ability.
“The dominant oscillatory activity, which is so critical for learning, is controlled by a very small number of the total cell population in the hippocampus,” said Nicolette Ognjanovski, a graduate student and first author of the study.
“This changes the narrative of what we understand about how networks work,” added Ognjanovski.