Under the Hood

Memory Formation, Consolidation During Sleep: Scientists Say They Understand Underlying Neural Processes

Stay up to study for the test or prioritize a good night's rest?

Many of us have had to make this difficult choice during our time as students. Scientific research already made clear that disrupted sleep can negatively affect our memory consolidation. Losing a few hours of rest at night can cause a loss of connectivity between neurons in the hippocampus, the area of the brain linked with learning and memory.

Now, researchers from the U.K. who used memory reactivation in a study with sleeping participants found that recently acquired memories could be strengthened in this manner. Scott Cairney, from the University of York, co-led the study with Bernhard Staresina, from the University of Birmingham. 

The key lies in sleep spindles which refer to the sudden bursts of brain activity we experience during stage 2 of our sleep. During this stage, our body and mind prepare to move into a deeper state of sleep as eye movement stops and heart rate slows down. 

"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," Cairney says. "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."

46 participants were asked to learn associations between words and pictures of objects or scenes before a nap. During the nap, half the words were "cued" i.e. replayed to trigger the reactivation of the newly acquired memories of the pictures. 

"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," Cairney reveals.

Brain activity during sleep was monitored using an electroencephalogram (EEG) machine as researchers were able to witness how the sleep spindles were triggered during the presentation of the words. "Our data suggest that spindles facilitate processing of relevant memory features during sleep and that this process boosts memory consolidation," Staresina says.

The most significant outcome of the study was that researchers were able to differentiate the brain signals associated with reactivated objects and scenes. This could indicate that spindles produce a specific code for reactivated memory content. a process that may underpin our ability to remember more after sleep.

"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," Cairney elaborates.

The article also suggests that future studies could look into comparing the neural correlates of cue-evoked memory reactivation with spontaneous memory reactivation.