A specific firing pattern of brain cells, immediately after a daytime event, determines whether the memory of that experience is ‘replayed’ during sleep and is imprinted in the memory. Scientists already knew that this pattern occurs during sleep, while recording memories.
But the fact that it also occurs during the day to label an experience as valuable is completely new. If this signal did not occur shortly after an experience, that memory was not stored. The research, which was done in mice, was published in the scientific journal on Thursday Science.
The American researchers studied jointly firing brain cells in a brain region that is the hub of memory, the hippocampus. Large groups of brain cells often produce the same pattern of fast electrical signals at the same time, so-called sharp wave ripples – to the shape of the graph when this pattern is measured with electrodes. In this way, they pass information to other brain areas and encode it for storage. Until now, it was known that this process during sleep is crucial for memory.
Register day and night
In the new study, the scientists discovered that the striking pattern also occurs during the day. When there are five to twenty of those sharp wave ripples occurred after an experience, that experience was ‘replayed’ and stored during sleep.
For their study, they equipped mice with electrodes in their hippocampus, which allowed them to record the electrical signals of 500 brain cells simultaneously day and night. This involved brain cells that are specialized in determining location – each brain cell fires at a different location.
They had the mice walk around a figure-of-eight maze again and again. The researchers were able to link each location in that maze to a specific electrical activity of those brain cells. A unique neurological pattern was created for each round through the maze.
Sometimes the mice found a little water in the maze as a reward. Then they stood still for a moment. When at such a time five to twenty of those sharp wave ripples occurred, the previous round through the maze was replayed more often during sleep, and thus perpetuated as a permanent memory. If no or few ripples occurred, the neurological pattern of the previous experience was not reactivated or reactivated less often during sleep.
brain researcherLisa Genzel Sleep is not a passive waste of time
“A wonderful study,” says brain researcher Lisa Genzel of the Donders Center for Brain Science at Radboud University in Nijmegen. “Many memories are erased during sleep, which is the most important function of sleep. We cannot possibly remember everything we experience. Only the important events are reactivated during sleep, incorporated into other knowledge and stored. It has long been a big question how the brain knows which memory should be stored and which should not.”
“This study shows that this is determined by the fact that those most important memories are also reactivated during the day.” Genzel is impressed by how detailed the study is. “They were able to measure hundreds of brain cells, record the activity of each of those individual brain cells day and night, and they used sophisticated mathematical analyses.”
The reactivations are not dreams, Genzel emphasizes. “They do not occur during dream sleep or REM sleep. They are very short signals, lasting only milliseconds. That all happens unconsciously.” The study underlines how important it is not to skimp on your sleep, she says. “Sleep is not a passive waste of time, it is a very active and important state.”
To share
Email the editor
