How Does Sleep Affect Memory

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When we sleep, one type of slow brain wave helps strengthen memories, but the competing wave weakens them.

How Does Sleep Affect Memory

How does the brain balance between remembering and forgetting information during sleep? This can be explained by the dynamics of two identical brain waves – one that strengthens memories and one that weakens them.

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The brain accumulates more memories than it can store. We absorb new information during the day, but retain only some from overnight. Sleep appears to be critical to this balance between learning and forgetting, with the brain’s electrical signal patterns consolidating some memories and erasing others, but the mechanisms at work are unclear. However, research reported earlier this month dispelled the mystery by distinguishing between two types of brainwaves’ opposing actions: one that strengthens memories and one that weakens them.

By distinguishing these brain waves, the researchers began to formulate an explanation that reconciled competing theories about how the brain processes memories, retaining some and losing others. “There was a gap in our understanding of how sleep is important for remembering and forgetting,” said Karunesh Ganguly, associate professor of neurology at the University of California, San Francisco and senior author of the study.

Theories of memory consolidation generally fall into one of two camps, with some evidence supporting each. Long-term studies have been attributed to patterns of brain activity that are replicated during sleep. These synchronizations of neural firing mimic the signals involved in actual learning, and that repetition strengthens the synaptic connections between neurons that are anchored in the memory. Without reactivation, other connections are not cemented in theory, and these memories fade away.

As an alternative, many researchers share the idea of ​​”synaptic downscaling,” in which the brain more actively purges itself of less useful memories. Because learning involves neural activity that strengthens brain connections, it becomes an energy drain. During sleep, less energy goes into connections, allowing those of less long-term importance to weaken. Removing this background noise from unwanted memories clarifies the brain’s signals and keeps it more efficient.

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New research bridges these theories by examining the role of different sleep-related brain wave patterns in memory retention.

Decades of research into the consolidation and loss of memories has focused on two brain wave patterns: slow oscillations and “sleep spindles.” Slow oscillations with high peaks and low frequencies span large parts of the brain. Along with sleep spindles, the high-frequency bursts of activity that occur every second during non-REM sleep (a generally dreamless state without rapid eye movement), they may be critical to memory consolidation. In contrast, delta waves are slightly shorter than slow oscillations and occur localized in the brain. Slow oscillations and delta waves are often difficult to tell apart because they usually occur together during sleep and are often classified as slow waves.

But distinguishing between slow oscillations and delta waves was key to the findings in the newly published research.

Jaekyung Kim, a postdoctoral fellow in neurology at UCSF and lead author of the study, worked with Tanuj Gulati, assistant professor of biomedical sciences and neurology at Cedars-Sinai Medical Center. With rewards, they trained the rats in a new skill that involved a specific neural activity in the motor cortex. At night, when the mice sleep, they suppress the selected wave patterns in the animals’ brains. The next day, they tested whether the mice’s memory for the new skill improved or worsened, as an indication of different brain wave activity.

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Drawing on differences first observed in a study of cats more than 20 years ago, Kim used precise criteria to distinguish slow oscillations from delta waves in rat brain recordings. When he and his colleagues knocked out the slow oscillations, the mice’s learning worsened—as expected, given the known importance of slow oscillations in consolidating memories.

But to the researchers’ surprise, when they interrupted the delta waves, they saw the opposite effect: the mice’s memory improved.

“I had no idea that dampening slow oscillations would have a different effect than dampening delta oscillations,” said Gina Poe, professor of integrative biology and physiology at the University of California, Los Angeles. She didn’t expect the differences between these waves to be so significant, but she said the study’s findings are consistent with several other findings. “It’s kind of like a missing puzzle piece,” she added.

These competing functions suggest mechanisms for both central theories of memory consolidation. Ganguly’s team had previously linked spindles with slow oscillations to reactivate synchronicities of neural activity and strengthen memory. At the same time, delta waves appear to degrade memories, perhaps by weakening brain connections in some form of synaptic downscaling.

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Ganguly said that delta waves have not been studied in detail in this context. Considering their results, the balance between these two types of waves was revealed – between learning and forgetting. “It’s a push-pull system,” he said. “There is a dynamic in the brain that can turn things on and off based on other information.”

In fact, slow oscillations and delta waves compete for synchronization with the spindles. The speed at which spindles synchronize to slow oscillations correlates with how well rats remember their new skills, but taking into account the synchronization speed of slow oscillations and delta waves provides a much better prediction. Slow oscillations are not the only ones that affect learning; Delta waves also seem to have an influence. “Both are really critical to not just remembering, but remembering the right things at the right time,” said Bryce Munter, assistant professor of psychiatry and human behavior at the University of California, Irvine.

But this balance is delicate and can be disrupted by anything from a stroke to lack of sleep. Because changes in brain waves often follow brain damage, Ganguly said, the proportion of delta waves and slow oscillations may contribute to long-term memory loss. The relationship between these waves may play a role in a very common phenomenon – the cognitive decline that accompanies aging.

Ganguly and his team observed in their research that the sleep spindle behaves differently with different waves. With slow oscillations, the spindles are synchronized to one phase of the wave, but they lock to another phase of delta waves. Spindles show a similar change in the aging brain, Mander said, as they start nesting at a different time during slow waves. If there are more delta waves in the aging brain, this may help explain this change.

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Delta waves are also directly linked to dementia. Delta waves propagate in the brain and show the accumulation of amyloid plaques that are characteristic of Alzheimer’s disease. In advanced Alzheimer’s, they are present not only during sleep, but also during wakefulness. “You see delta waves popping up everywhere,” Mander said. Their presence indicates how memory impairment can develop in the brain.

Extending the study of these competing brain waves to new areas, such as aging, will depend on whether the mechanism is applicable to other types of studies. For example, motor skill learning relies on brain areas that are not involved in other types of learning, so it is not yet clear whether this competing brain wave mechanism may be present in all types of memory, Poe said.

The recognition that delta waves may have a distinct function from slow oscillations suggests a promising approach to many problems in sleep and memory research. Some medicines, e.g. improves sleep, still cannot improve learning and memory; Future studies of the balance of slow waves produced under their influence may clarify why. Data tracking of slow waves during sleep may reveal additional new insights into the specific effects of slow oscillations and delta waves. It may ultimately prove to improve memory by disrupting delta waves rather than promoting slower oscillations. Studies of brain activity in people with abnormal memories can provide ideas for interventions to benefit learning. “This paper opens up a new area of ​​study,” Mander said.

One question that remains after Kim’s work is why do we and other animals sleep the way we do? Evidence suggests that the integration of slow waves with sleep spindles can result from hours to seconds of sleep; Only a few instances of this integration are necessary to produce long-term changes in memory durability. “That’s enough for our circuit to form,” Poe said. When you learn something new, the best way to remember it is to sleep on it. This is because sleep helps reinforce the memories you formed during the day. That helps too

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