Everybody needs to sleep, even if researchers aren’t entirely sure why.
One of the leading theories suggests sleep plays a critical role in the consolidation of memory, but characterizing this process has been difficult. The basic idea is that a sleeping brain reactivates the same neural pathways that were activated when the memory was first formed. This reactivation strengthens synaptic connections between neurons, which results in stronger memories. Indeed, a growing body of research has shown that people are able to remember information better after ‘sleeping on it.’
“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,” Scott Cairney, a professor of psychology at the University of York, said in a statement.
New research published today by Cairney and his colleagues in Current Biology has made significant headway on this issue by linking sleep spindles—spontaneous bursts of brain activity—to memory processing in a sleeping brain. Incredibly, the researchers claim it is possible to determine the content of the memory being processed by analyzing this brain activity.
“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 said. “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.”
To test this theory, Cairney and his colleagues had research subjects study associations between words and pictures of objects on flashcards before taking a nap. For example, a picture of an apple might be matched with the word ‘lively,’ or a picture of a mountain range matched with the word ‘open.’ While the subjects were sleeping, the researchers would play audio of half of those words back to the subjects to trigger the reactivation of picture memories. At the same time, they monitored the electrical activity in the subjects’ brain with an electroencephalogram (EEG) and found that sleep spindles were triggered when memories were reactivated by the words.
“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,” Cairney said. “We found their memory was better for the pictures that were connected to the words that were presented in their sleep compared to those words that weren’t.”
This suggests that spindles play a significant role in memory processing during sleep, but the truly cool part about the study is that the researchers were able to tell the spindles apart based on the words that were presented to the sleeping subjects. In other words, the spindles were able to encode the content of specific memories.
Although this isn’t at the point yet where researchers can ‘read’ your brain activity just based on the content of an EEG spindle (they still need to know which words the subject was exposed to in advanced), research is headed in that direction. Last year, a German researcher developed an algorithm that was able to tease out the content of consolidated memories in sleeping subjects based on nearly imperceptible changes in brain activity.
In a similar vein, the researchers involved with this new paper think it may be possible to purposefully induce brain spindles in sleeping subjects using electrodes to target specific memories for consolidation in the future.