the fundamental role of sleep in the formation of memories

Two new studies from the University of Michigan have discovered why that happens and what happens inside the brain during it sleep and sleep deprivation to help or harm the formation of memories.

The importance of sleep for preserving memories

Specific neurons can be tuned to specific stimuli. For example, rats in a maze will have neurons that fire when the animal reaches specific points in the maze. These neurons, called place neurons, are also active in people and help people orient themselves in their environment.

What happens during sleep?

“If that neuron responds during night rest, what can you infer from that?” said Kamran Diba, Ph.D., associate professor of Anesthesiology at UM Medical School.

A study, summarized in the journal Nature and led by Diba and former graduate student Kourosh Maboudi, Ph.D., examines neurons in the hippocampus, a seahorse-shaped structure deep in the brain involved in memory formation, and discovered a way to visualize the tuning of neuronal patterns associated with a position while an animal slept.

A type of electrical activity called sharp wave ripples emanates from the hippocampus every couple of seconds, over a period of many hours, during resting states.

The researchers were intrigued by how synchronous the ripples are and how far they travel, apparently to spread information from one part of the brain to another. These activations are thought to allow neurons to form and update memories, including memories of place.

For the study, the team measured a rat’s brain activity during a night’s rest, after the rat had completed a new maze. Using a type of statistical inference called Bayesian learning, they were for the first time able to track which neurons would respond to which points in the maze.

“Let’s say a neuron prefers a certain corner of the maze. We might see that neuron firing with others showing a similar preference during nighttime rest. But sometimes neurons associated with other areas might coactivate with that cell.

We then saw that when we put it back in the maze, the position preferences of the neurons changed depending on which cells they activated during night rest,” Diba said.

The method allows them to visualize the plasticity or representational drift of neurons in real time. It also provides further support for the long-held theory that reactivation of neurons during nighttime rest is part of the reason why sleep is important for memories. Given the importance of night’s rest, Diba’s team wanted to observe what happens in the brain in the context of night’s rest deprivation.

In the second study , also published in Nature , the team, led by Diba and former graduate student Bapun Giri, Ph.D., compared the amount of neuron reactivation, in which neurons that fired during maze exploration they activate again spontaneously at rest. and the sequence of their reactivation (quantified as replay), during sleep versus during loss of nocturnal rest.

They found that the firing patterns of neurons involved in reactivating and reproducing the maze experience were higher during sleep compared to nighttime rest deprivation.

Nocturnal rest deprivation corresponded to a similar or higher rate of sharp wave ripples, but lower amplitude waves and lower power ripples.

“In almost half of the cases, however, the reactivation of the labyrinth experience during sharp wave ripples was completely suppressed during night rest deprivation,” Diba said.

When the sleep-deprived rats were able to catch up on sleep, he added, although reactivation increased slightly, it never matched that of the normally sleeping rats. Furthermore, reproduction was similarly impaired but was not recovered when lost night’s rest was regained.

Because reactivation and replay are important for memory, the findings demonstrate the detrimental effects of nighttime rest deprivation on memory. Diba’s team hopes to continue studying the nature of memory processing during nighttime rest, why it needs to be reactivated, and the effects of sleep pressure on memory.

Sleep restores brain connections

During sleep, the brain weakens new connections between neurons that were forged while awake, but only during the first half of the night’s sleep, according to a new study in fish by UCL scientists.

The researchers say their findings, published in Nature, provide insight into the role of sleep, but still leave an open question about what function the second half of nighttime sleep serves.

The researchers say the study supports the synaptic homeostasis hypothesis, a key theory about the purpose of sleep that proposes that sleep acts as a reset for the brain.

Lead author Professor Jason Rihel (UCL Cell & Developmental Biology) said: “When we are awake, connections between brain cells become stronger and more complex. If this activity were to continue unabated, it would be energetically unsustainable. Too many active connections between brain cells could prevent new connections from being created the next day.

“Although the function of sleep remains mysterious, it may serve as an ‘off-line’ period in which such connections can be weakened in the brain, in preparation for learning new things the next day.”

For the study, the scientists used optically translucent zebrafish, with genes that allow them to easily visualize synapses (structures that communicate between brain cells). The research team monitored the fish during several sleep-wake cycles.

Researchers have found that brain cells gain more connections during waking hours and then lose them during sleep. They found that this depended on how much sleep pressure (sleep need) the animal had built up before it could rest; if the scientists deprived the fish of sleep for a few more hours, the connections continued to increase until the animal could sleep.

Professor Rihel added: “If the patterns we observed hold true in humans, our findings suggest that this remodeling of synapses may be less effective during a midday nap, when sleep pressure is still low, rather than during night, when we really need to sleep.” the sleep.”

The researchers also found that these rearrangements of connections between neurons occurred mainly in the first half of the animal’s nocturnal sleep. This reflects the pattern of slow-wave activity, which is part of the sleep cycle and is strongest at the beginning of the night.

First author Dr Anya Suppermpool (UCL Cell & Developmental Biology and UCL Ear Institute) said: “Our findings add weight to the theory that sleep serves to dampen connections within the brain, preparing for further learning and new connections the next day. But our study tells us nothing about what happens in the second half of the night.

“There are other theories that sleep is a time for eliminating waste in the brain or repairing damaged cells. Perhaps other functions come into play in the second half of the night.”

Sleep improves the ability to remember complex events

Researchers have long known that sleep consolidates our memory of episodic facts and events. However, so far research has focused mainly on simple associations, that is, on connections between elements, such as those we make when learning new vocabulary.

In real life, events generally consist of numerous components, for example a location, people and objects, which are connected to each other in the brain,” explains Dr. Nicolas Lutz from the Institute of Medical Psychology at LMU.

These associations may vary in strength and some elements may be linked to each other only indirectly. “Thanks to the neural connections that underlie these associations, a single keyword is often enough for someone to remember not just single aspects of an event but multiple aspects at once,” says Lutz.

This process, known as schema completion, is a key feature of episodic memory. Lutz is the lead author of a study recently published in Proceedings of the National Academy of Sciences, which investigated the effect of sleep on memory for such complex events.

After study participants had learned about events with complex associations, in one condition they spent the night in a sleep laboratory, where they could sleep as usual, while in another condition they had to stay awake all night.

In both conditions, participants were allowed to spend the next night at home to recover. Then they were tested how well they could remember different associations between elements of the learned events.

“We were able to demonstrate that sleep specifically consolidates weak associations and strengthens new associations between elements that were not directly connected to each other during learning. Furthermore, the ability to remember multiple elements of an event together, after only one of them was presented after sleep, improved compared to the condition in which the participants remained awake,” summarizes Nicolas Lutz. This demonstrates the importance of sleep for completing partial information and processing complex events in the brain.

By monitoring the study participants’ brain activity during sleep, the study authors were also able to demonstrate that improved memory performance is linked to so-called sleep spindles, bursts of neural oscillatory activity during sleep, which are associated with consolidation active memory contents. This occurs through the reactivation of underlying neural structures during sleep.

“This finding suggests that sleep spindles play an important role in the consolidation of complex associations, which underlie the completion of memories of entire events,” says Professor Luciana Besedovsky, lead researcher on the study.

According to Lutz and Besedovsky, the identified effects of sleep on memory can be seen as an important adaptation of the human brain, because they help people paint a more coherent picture of their environment, which in turn allows them to make more complete predictions about the future. . events.

“And so our findings reveal a new function by which sleep may offer an evolutionary advantage,” says Besedovsky. “Furthermore, they open up new perspectives on how we store and access information about complex multi-element events.”