News release

From: PLOS

Brain activity patterns may “tag” experiences for later memory making during sleep

Theta oscillations during learning predicted which experiences were remembered after sleep

The brain may “tag” experiences during learning to better consolidate the memory of them during sleep, according to a study published August 27^th in the open access journal PLOS Biology by Dan Denis from the University of York, United Kingdom, and colleagues.

Most people do not remember every experience they’ve ever had. Instead, the brain consolidates some experiences—but not others—into long-term memory during sleep, suggesting that some experiences are “tagged” during learning for later encoding into memory.

To better understand how the brain selected experiences for later memory consolidation during sleep, the authors of this study collected data from 31 participants using electroencephalography (EEG) to measure patterns of brain activity. The participants learned sets of object-word pairs and were tested on them immediately and after a two-hour break. During one visit, participants were allowed to sleep, and in the other, they remained awake. As they learned and rested, the scientists analyzed their EEG readings, matching the repetitive oscillations of brain activity during learning to those that were remembered after the participants’ sleep period.

The authors found that word-object pairs associated with theta oscillations between 3-8Hz during learning were more likely to be remembered after a period of sleep, but not after staying awake. The theta activity during learning predicted more coupling between slow neural oscillations during sleep and sleep spindles—bursts of brain activity. The slow oscillations and the sleep spindles were in turn associated with better memory performance after sleep. While the study only tested memory after two hours, and further studies would be needed to determine if memories persisted, the authors suggest that theta oscillations may indicate a mechanism by which the brain identifies experiences to consolidate into memories during sleep.

The authors add, “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 which 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 prioritising 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.

“Although it is typically adaptive to prioritise certain kinds of information, there are cases where this becomes maladaptive. For example, in depression, individuals tend to over allocate attentional resources to negative information, whilst disregarding or downplaying more positive experiences. Our new findings shed new light on the brain processes that dictate what is ultimately remembered, and may open new avenues for understanding and treating common mental health problems such as depression.”