Why do we remember certain moments while forgetting others? Every day, our minds are bombarded with thousands of pieces of information, yet our brains have a limited capacity to consolidate these experiences into lasting memories. This means we need a way to hold on to what is most important and disregard irrelevant details.

Now researchers at the University of York have uncovered the neural mechanism that instructs the brain which daily experiences to save as long-term memories during sleep.

In the study published in the journal PLOS Biology, researchers analyzed brainwave activity while participants were learning new associations between words and pictures. It is already known that after learning, sleep plays a central role in strengthening new learning into long-term memories.

Theta rhythms mark key memories
The researchers discovered that during learning, the brain "tags" certain memories for processing during later sleep. This tagging provides a way for the sleeping brain to strengthen only what is deemed important to remember.

This tagging was linked to a specific brainwave pattern known as a "theta rhythm" present during learning. The strength of these theta rhythms provided an instructional cue to help organize brain activity during sleep.

Those memories with a particularly strong theta rhythm during learning were preferentially strengthened during sleep, ultimately leading to better memory recall, the scientists said.

They say the findings show that the brain prioritizes and strengthens important information by using specific neural signals during learning that guide memory-strengthening processes during sleep. Determining what our brains decide to strengthen during sleep could help us better understand why some individuals are vulnerable to conditions such as depression.

When filtering becomes maladaptive
Lead author Dr. Dan Denis, from the Department of Psychology at the University of York, said, "It has long been known that a good night's sleep is important for processing memories.

"How the brain decides what is important to remember and what can be forgotten is still a mystery. These new findings help to answer that question by uncovering for the first time a signature of neural activity that instructs the brain which experiences to process during sleep and form into long-term memories."

"If we remembered everything that we experienced, our brains would quickly reach system overload. By selectively prioritizing important events in our lives, be they emotionally salient or important for the future, we are able to use our past experiences to help guide our interactions in the world."

The researchers note that while selective memory consolidation is vital for daily functioning, disruptions to this filtering process could play a role in psychiatric conditions.