Memories can be decoded from brain waves during sleep

The University of Birmingham reports that in a study it carried out with colleagues from the University of York, published in Current Biology, researchers found new insight into the process of memory consolidation during sleep. Their findings could also suggest new ways to help people remember things better, according to the researchers.

Scientists have long known that sleep spindles, sudden bursts of oscillatory brain activity, play an important role in the formation and retention of new memories. Sleep spindles are half second to two second bursts of brain activity that occur during deep sleep, and can be visualised and measured on an electroencephalogram (EEG).

Earlier studies have shown that the number of spindles that occur during the night could predict a person’s memory the next day. But many questions about the link between sleep spindles and how a person’s recently acquired information is “reactivated” and strengthened during sleep remained.

Now the researchers have demonstrated that there is a particular pattern of brain activity that supports this reactivation process. This new study has also shown that the content of reactivated memories can be decoded for brain activation patterns at the time that spindles occur.

The team devised an experiment in which people learned to associate particular words with particular objects and scenes. Some study participants then took a ninety minute nap after their study session, whereas others stayed awake. While people napped, the researchers cued those associative memories and unfamiliar words. The team monitored the participant’s brain activity during sleep using an EEG machine.

The results showed them that sleep spindles occurred when memories were reactivated by presenting the associated words.

Importantly, the researchers were able to differentiate the brain signals associated with reactivated objects and scenes. This demonstrates that spindles produce a specific code for the content of reactivated memories, a process that may underpin people’s ability to remember more after sleep.

Dr Bernhard Staresina, of the University of Birmingham’s School of Psychology, said “While it has been shown previously that targeted memory reactivation can boost memory consolidation during sleep, we have now showed that sleep spindles might represent the key underlying mechanism. Thus, direct induction of sleep spindles – for example, by stimulating the brain with electrodes – perhaps combined with targeted memory reactivation, may enable us to further improve memory performance while we sleep. Our data suggest that spindles facilitate processing of relevant memory features during sleep and that this process boosts memory consolidation.”

Dr Scott Cairney, from the University of York’s Department of Psychology, said “We are quite certain that memories are reactivated in the brain during sleep, but we don’t know the neural processes that underpin this phenomenon. Sleep spindles have been linked to the benefits of sleep for memory in previous research, so we wanted to investigate whether these brain waves mediate reactivation. If they support memory reactivation, we further reasoned that it could be possible to decipher memory signals at the time that these spindles took place. We asked participants in our study to learn associations between words and pictures of objects or scenes before a nap. Half of the words were then replayed during the nap to trigger the reactivation of the newly learned picture memories. When the participants woke after a good period of sleep, we presented them again with the words and asked them to recall the object and scene pictures. We found that their memory was better for the pictures that were connected to the words that were presented in sleep, compared to those words that weren’t.”

This new understanding of the way the brain normally processes and strengthens memories during sleep may help to explain how that process may go wrong in people with learning difficulties, according to the researchers. It might also lead to the development of effective interventions designed to boost memory for important information.

Scott Cairney said “When you are awake you learn new things, but when you are asleep you refine them, making it easier to retrieve them and apply them correctly when you need them the most. This is important for how we learn but also for how we might help retain healthy brain functions.”

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