A new study provides important physical evidence to support the idea that sleep helps cement and strengthen new memories. Published in the journal Science, the study shows sleep after learning causes very specific structural changes in the brain - namely growth of connections between brain cells that help them pass information to each other.
Senior investigator Wen-Biao Gan, professor of neuroscience and physiology at NYU Langone Medical Center in New York, NY, says while we have known for some time that sleep is important for learning and memory, the underlying mechanism has not been clear.
"Here we've shown how sleep helps neurons form very specific connections on dendritic branches that may facilitate long-term memory," he explains, "We also show how different types of learning form synapses on different branches of the same neurons, suggesting that learning causes very specific structural changes in the brain."
In experiments with mice, he and his team show for the first time that learning and sleep cause physical changes in the motor cortex, a brain region involved with voluntary movements.
While we may appear restful as we slumber, our cells are not. The brain cells that were active taking on new information during waking hours, reactivate during deep sleep or slow-wave sleep - a phase when brain waves slow right down, and rapid eye movement, and dreaming, come to a halt.
For some time now, scientists have believed slow-wave sleep is when we form and recall new memories. But exactly how this happens physically is what this study shows for the first time - using mice genetically modified so a particular protein in their brain cells fluoresces when seen with a laser-scanning microscope.
Using this approach, the team could track the growth of new spines along individual branches of dendrites. A brain cell typically has many thousands of dendrites. These connect to other neurons via synapses and carry information in the form of electrical impulses.
Mouse brain sprouted new dendritic spines within 6 hours of learning new task
The researchers got the mice to learn to balance on a spin rod. Eventually, the mice learned to balance on the rod as it spun faster and faster.
They noted that the mice sprouted new dendritic spines within 6 hours of training on the rod.
They then investigated the effect of sleep. They trained two groups of mice: one group trained on the spinning rod for an hour and then slept for 7 hours, while the other trained for the same time but were kept awake for 7 hours.
The mice that did not sleep after training showed significantly less new dendritic spine growth than the mice that slept after learning.
The researchers also noticed that different types of growth occurred for different types of learning.
For example, running forward on the spinning rod was followed by dendritic spine growth on one set of branches, while running backwards was followed by growth on another set. The researchers suggest this means learning specific tasks is linked to specific structural changes in the brain.
"Imagine a tree that grows leaves (spines) on one branch but not another branch," says Prof. Gan, "When we learn something new, it's like we're sprouting leaves on a specific branch."
Disrupting sleep prevents new dendritic spine growth
In a final set of experiments, he and his colleagues show that motor cortex brain cells that are active during wakeful learning reactivate during slow-wave sleep. And disrupting this prevents new dendritic spine growth.
They conclude this finding sheds light on "neuronal replay," where during sleep, the brain "practices" what has been learned during the day and consolidates it by growing specific connections within the motor cortex.
The National Institutes of Health and the Whitehall Foundation funded the study.
In April 2014, Medical News Today reported how researchers discovered another form of brain activity that may mark memory formation. After studying memory formation in rats, the team said humans and animals know where they are because "place cells" in the hippocampus that behave like "neural flags" pinpoint experiences on a neural map - rather like pinning the location of a restaurant or store on Google maps.