While the study was done in mice, it might offer some insight about the long-observed connection between sleep and memory in humans.
People's memories and learning abilities suffer when sleep-deprived--contrary to the hopes of college students pulling all-nighters everywhere. But this is the first time that scientists have observed new learning-related brain connections forming during deep sleep.
They were able to see exactly how each new skill created a new pathway in the brain. And the study offers some evidence suggesting it might be important to sleep right after learning something new--instead of hours later.
A Departure From Previous Research
These results were unexpected.
Past studies have shown that learning a new skill strengthens internal connections, or synapses, between brain cells. But normally, researchers have seen that the number of synapses decreases during sleep, so they've theorized that there is some sort of "paring down" effect, where trimming the connections somehow allows the brain to store more information, according to an article by neuroscientists David R. Euston and Hendrick W. Steenland published alongside the study.
But those past studies generally did not look at the direct link between a specific set of new skills and sleep that followed immediately after, and in the new research, a team from NYU's Langone Medical Center observed something different than what had been seen before. If mice were allowed to sleep immediately after learning a new skill, the connections that showed learning in their brains continued to grow. But if they weren't allowed to sleep, their brains formed far fewer connections.
"Perhaps," Euston and Steenland suggest, "synapses that are forged during recent experiences undergo strengthening whereas synapses representing more distant memories undergo downscaling."
While sleep is known to be a time when you essentially clear the cobwebs accumulating in your brain, those hours are also crucial for storing new information and new pathways, letting you reinforce something you've just learned.
The New Experiment
The NYU researchers engineered mice to have a fluorescent protein in their brains so they could observe the growth of little spikes called dendrites on neurons, or brain cells. More of those dendrites mean more synapses, or stronger connections between neurons, which shows learning.
They taught the mice to run on a spinning rod, training them to keep their balance while running forward. Later, the researchers had them run forward again, or taught them a new trick: running backwards. With each new skill learned, a new pathway of brain connections was created.
This was significant because it showed that each new skill learned was associated with one particular new set of connections in the brain. "Imagine a tree that grows leaves on one branch but not another branch," said Dr. Wen-Biao Gan of NYU, one of the authors of the study, in the press release. "When we learn something new, it's like we're sprouting leaves on a specific branch."
The Effect Of Sleep
When the researchers repeated the experiment, they allowed some mice to sleep after the running lessons and forced others to stay awake another eight hours.
The sleep-deprived mice grew fewer new connections in their brains, and were less likely to keep any new connections they did form. (The researchers checked up on their brains 24 hours later.)
The mice who slept did much better. They continued to form new connections while sleeping, along the same pathway that was associated with their new skill.
What's more, the new connections seem linked to better skill retention.
When the mice were tested again--first one day later, and then again five days later--they all did better with their new tricks than they had the first time. But the performance improvement was twice as great in the ones who had slept (and formed more new connections).
In the article accompanying the study, neuroscientists Euston and Steenland note that this result suggests that sleep does more than solidify something that's been learned: It helps give it "staying power."
Even when the sleep-deprived mice were able to later sleep and receive an additional training session, they never formed as many neural connections related to that particular skill as the mice who had been allowed to sleep after learning the skill the first time.
What This Means
Although studies have suggested for some time that sleep helps crystallize memory, this is the first time that researchers have been able to see how that works. They think that the mice may have been able to form additional connections during sleep because they were able to sleep immediately after training.
Since the mice were tested on motor skills, it's too soon to say whether the results would hold for other types of learning as well. And since this is the first time this process has been shown, it will need to be replicated--ideally in humans. But the Science study suggests new avenues for investigation and will have important implications for researchers trying to understand the relationship between learning, memory, and sleep.