The critical role of sleep in the formation of memories

Imagine you’re a student facing finals week and preparing for a crucial exam: do you stay up all night studying or ensure you get some rest?

Many exhausted students staring blankly at their test papers know that sleep deprivation makes it incredibly hard to retain information.

Two new studies from the University of Michigan shed light on why this happens, revealing what occurs in the brain during sleep and sleep deprivation that affects memory formation.

Specific neurons can be tuned to specific stimuli. For instance, when rats navigate a maze, certain neurons, known as place neurons, become active at particular spots. These neurons also exist in humans and aid in environmental navigation.

But what occurs during sleep?

“If a neuron responds during sleep, what can you infer from that?” asked 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., explored neurons in the hippocampus, a brain structure crucial for memory formation. They discovered a way to visualize neuronal patterns associated with locations while an animal slept.

During restful states and sleep, a type of electrical activity called sharp-wave ripples emanates from the hippocampus every few seconds over several hours. Researchers are fascinated by the synchronization of these ripples and their extensive travel, seemingly transmitting information across the brain.

These firings are believed to help neurons form and update memories, including spatial memories.

For the study, the team measured a rat’s brain activity during sleep after it had completed a new maze. Using Bayesian learning, they tracked which neurons responded to specific places in the maze.

“Let’s say a neuron prefers a certain corner of the maze. We might see that neuron activate with others showing similar preferences during sleep. But sometimes neurons linked to different areas might co-activate with that cell. We then observed that when the rat returned to the maze, the neurons’ location preferences changed based on which cells fired together during sleep,” explained Diba.

This method allows visualization of neuronal plasticity or representational drift in real time and supports the theory that reactivation of neurons during sleep is crucial for memory.

Recognizing sleep’s importance, Diba’s team also investigated the brain’s behavior during sleep deprivation.

In the second study, also published in Nature, Diba and former graduate student Bapun Giri, Ph.D., compared neuron reactivation – where place neurons that fired during maze exploration spontaneously fire again at rest – and the sequence of their reactivation (quantified as replay) during sleep versus sleep deprivation.

They found that neuron firing patterns involved in reactivating and replaying the maze experience were more frequent during sleep than during sleep deprivation. Sleep deprivation was associated with similar or higher rates of sharp-wave ripples but with 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,” noted Diba.

When sleep-deprived rats were allowed to catch up on sleep, the reactivation slightly rebounded but never matched that of rats with normal sleep. Additionally, replay was similarly impaired and did not recover with regained sleep.

Since reactivation and replay are vital for memory, these findings highlight the harmful effects of sleep deprivation on memory.

Source: Science Daily