Sleep or Cram? Sleep is Key for Memory Retention

Summary: Sleep is crucial for memory formation, while sleep deprivation severely disrupts this process. Neurons in the hippocampus reactivating during sleep help to consolidate memories, but this reactivation is impaired without proper rest. These findings highlight the importance of sleep for effective learning and memory retention.

Key Facts:

  1. Sleep reactivates neurons in the hippocampus, aiding memory formation.
  2. Sleep deprivation suppresses neuron reactivation, harming memory retention.
  3. Catching up on sleep doesn’t fully recover the lost memory consolidation.

Source: University of Michigan

Imagine you’re a student, it’s finals week, and you’re preparing for a big exam: do you pull an all-nighter or do you get some rest? 

As many a groggy-eyed person who’s stared blankly at a test knows, a lack of sleep can make it extraordinarily difficult to retain information.

Two new studies from University of Michigan uncover why this is and what is happening inside the brain during sleep and sleep deprivation to help or harm the formation of memories.

This shows a bed and alarm clock.
Given sleep’s importance, Diba’s team wanted to look at what happens in the brain in the context of sleep deprivation. Credit: Neuroscience News

Specific neurons can be tuned to specific stimuli. 

For example, rats in a maze will have neurons that light up once the animal reaches specific spots in the maze. These neurons, called place neurons, are also active in people and help people navigate their environment. 

But what happens during sleep?

“If that neuron is responding during sleep, what can you infer from that?” said Kamran Diba, Ph.D., associate professor of Anesthesiology at U-M Medical School. 

A study, summarized in the journal Nature and led by Diba and former graduate student Kourosh Maboudi, Ph.D., looks at 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 location while an animal was asleep.

A type of electrical activity called sharp-wave ripples emanate from the hippocampus every couple of seconds, over a period of many hours, during restful states and sleep. 

Researchers have been intrigued by how synchronous the ripples are and how far they travel, seemingly to spread information from one part of the brain to another. 

These firings are thought to allow neurons to form and update memories, including of place.

For the study, the team measured a rat’s brain activity during sleep, after the rat 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 places in the maze.

“Let’s say a neuron prefers a certain corner of the maze. We might see that neuron activate with others that show a similar preference during sleep. But sometimes neurons associated with other areas might co-activate with that cell.

“We then saw that when we put it back on the maze, the location preferences of neurons changed depending on which cells they fired with during sleep,” said Diba.

The method allows them to visualize the plasticity or representational drift of the neurons in real time. 

It also gives more support to the long-standing theory that reactivation of neurons during sleep is part of why sleep is important for memories.

Given sleep’s importance, Diba’s team wanted to look at what happens in the brain in the context of sleep 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—wherein the place neurons that fired during maze exploration spontaneously fire again at rest—and the sequence of their reactivation (quantified as replay), during sleep vs. during sleep loss. 

They discovered that the firing patterns of neurons involved in reactivating and replaying the maze experience were higher in sleep compared to during sleep deprivation.

Sleep deprivation corresponded with a similar or higher rate of sharp-wave ripples, but lower amplitude waves and lower power ripples. 

“In almost half the cases, however, reactivation of the maze experience during sharp-wave ripples was completely suppressed during sleep deprivation,” said Diba. 

When sleep deprived rats were able to catch up on sleep, he added, while the reactivation rebounded slightly, it never matched that of rats who slept normally. Furthermore, replay was similarly impaired but was not recovered when lost sleep was regained. 

Since reactivation and replay are important for memory, the findings demonstrate the detrimental effects of sleep deprivation on memory.

Diba’s team hopes to continue looking at the nature of memory processing during sleep and why they need to be reactivated and the effects of sleep pressure on memory.

Additional authors include Hiroyuki Miyawaki, Caleb Kemere, Nathaniel Kinshy, Utku Kaya and Ted Abel.

About this sleep and memory research news

Author: Kelly Malcom
Source: University of Michigan
Contact: Kelly Malcom – University of Michigan
Image: The image is credited to Neuroscience News

Original Research: Closed access.
Sleep loss diminishes hippocampal reactivation and replay” by Kamran Diba et al. Nature


Sleep loss diminishes hippocampal reactivation and replay

Memories benefit from sleep, and the reactivation and replay of waking experiences during hippocampal sharp-wave ripples (SWRs) are considered to be crucial for this process. However, little is known about how these patterns are impacted by sleep loss.

Here we recorded CA1 neuronal activity over 12 h in rats across maze exploration, sleep and sleep deprivation, followed by recovery sleep. We found that SWRs showed sustained or higher rates during sleep deprivation but with lower power and higher frequency ripples.

Pyramidal cells exhibited sustained firing during sleep deprivation and reduced firing during sleep, yet their firing rates were comparable during SWRs regardless of sleep state.

Despite the robust firing and abundance of SWRs during sleep deprivation, we found that the reactivation and replay of neuronal firing patterns was diminished during these periods and, in some cases, completely abolished compared to ad libitum sleep. Reactivation partially rebounded after recovery sleep but failed to reach the levels found in natural sleep.

These results delineate the adverse consequences of sleep loss on hippocampal function at the network level and reveal a dissociation between the many SWRs elicited during sleep deprivation and the few reactivations and replays that occur during these events.

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