The study shows that brain activity specifically during REM sleep - the stage when dreams occur - is required for normal memory consolidation

For decades, scientists have known rapid eye movement (REM) sleep - the phase of the sleep cycle when dreams occur - is linked to spatial and emotional memory consolidation but have run into problems trying to prove it.

Now, a mouse study from McGill University in Montreal, Canada, and the University of Bern in Switzerland - published in the journal Science - provides evidence that REM sleep does play this role.

Sylvain Williams, a professor of psychiatry at McGill and joint senior author of the study, explains that while they already knew the brain stores new information in different types of memory - spatial or emotional - before consolidating it, it was not clear, until this study, how it went about it. He adds:

"We were able to prove for the first time that REM sleep is indeed critical for normal spatial memory formation in mice."

In their study, he and his colleagues used a different approach to the ones attempted by hundreds of previous studies that have tried to isolate brain activity during REM sleep.

REM sleep brain cell activity key to memory consolidation

Prof. Williams says they decided to target the brain cells that control the activity of the hippocampus - a part of the brain that is important for forming memories while we are awake. It is also known as the brain's "inner GPS" because it helps us map and navigate our environment.

For their study, the team ran experiments where mice completed spatial memory tasks that trained them to recognize new objects placed in a controlled environment already containing other objects of similar shape and size.

However, when they used light pulses to switch off a certain group of memory-associated brain cells while the mice were in REM sleep, the researchers found the mice could not successfully complete the memory tasks they had learned the previous day.

The brain cells the researchers targeted are called "medial septum γ-aminobutyric acid-releasing" neurons. They used an approach called optogenetics to switch them off. This method uses mice that have been genetically engineered to have brain cells that can be switched off using light pulses.

Compared with a control group of mice that did not have the light-pulse treatment, the memories of the treated mice appeared to have been erased, or at least disrupted.

The researchers ran the tests several times, experimenting with switching off other groups of brain cells, or the same group of brain cells during other, non-REM phases of sleep, or while the mice were awake. However, none of them had the same effect.

"Silencing the same neurons for similar durations outside REM episodes had no effect on memory. This indicates that neuronal activity specifically during REM sleep is required for normal memory consolidation."

Lead author Richard Boyce

REM is known to be an important component of sleep for humans as well as mice and other mammals, and poor quality of sleep is linked to several brain disorders, including Alzheimer's disease and Parkinson's disease.

Researchers have found that REM sleep in particular is disrupted in Alzheimer's disease. The team in this study say its findings suggest disruption of REM sleep could be factor in causing the memory problems seen in Alzheimer's disease.