For the first time, scientists can see how the brain registers our memories while we sleep

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Do you toss and turn at night, yearning for a good night's sleep? According to the <a href="https://www.cdc.gov/sleep/index.html" target="_blank">US Centers for Disease Control and Prevention,</a> one third of American adults have a sleep disorder that can affect their lives in serious ways.

But the exact way the brain stores our memories is misunderstood.

Now for the first time, tiny microelectrodes planted in the brains of two people with epilepsy show how brain neurons fire during sleep to “replay” our short-term memories in order to move them to a more permanent store. The study has been published Tuesday in Cell Reports magazine.

“This study is fascinating,” said Dr. Richard Isaacson, who heads the Alzheimer’s Prevention Clinic at Weill Cornell Medicine and New York-Presbyterian Hospital.

“Despite decades of research, it is not yet clear how” short-term “memories are stored to become” long-term “memories that can be remembered later,” said Isaacson, who was not involved in the study.

“Using a brain-computer interface is an exciting way to study memory as it can record patterns of brain cell activity and then look for exact patterns later on,” he added.

Tracking individual neurons

The study was conducted at BrainGate, an academic research consortium made up of Brown University, Stanford University and Case Western Reserve University.

Braingate has spent the last dozen or more years developing brain-computer interfaces that allow people with amyotrophic lateral sclerosis (ALS) and other neurological diseases, brain injury or limb loss to be able to use brain signals to move their computer cursors, robotic arms and other assistive devices to communicate and control their world.

“The neurons are tiny. They are about 10 microns in size,” said computational neuroscientist and study author Beata Jarosiewicz. “And macroelectrodes approved for human use, such as deep brain stimulation electrodes, are too large to record the individual spiking activity of each neuron.”

But in Braingate, surgeons have implanted a series of small electrodes on the upper part of the brain of two people with sensory and motor paralysis, thus allowing the person to “think” about moving the hand in one direction. By mapping the way neurons behave during thought, the decoder can translate thought into word or action via connected prosthetic limbs and assistive robotic devices.

“Different neurons have different preferred directions,” said Jarosiewicz, who was an assistant research professor at BrainGate at the time of the study.

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“Some increase their fire rates every time the person wants to move their hand upward; others when the person wants to move left or right,” he said. “And we can tell from the activation pattern across all neurons which direction the person wants to move his hand.”

Playing Simon Says

In the study, the two people with implanted devices were asked to take a nap in which their neuronal activity was recorded as the basis. So everyone played a sequenced game modeled after the hit 80’s electronic game Simon, in which players are asked to repeat the same order of light movements that the game had just shown.

Instead of moving their arms to follow Simon, of course, the two people used their minds to repeat the game’s actions while their neural activity was being recorded. Eventually, the two were asked to rest and take a nap.

The results showed that during those naps neuron activity was identical to that recorded while the two subjects were actually playing. This meant that their brains continued to play Simon after they fell asleep, reproducing the same patterns in their brain at the neuronal level.

“Fully understanding how a memory is stored in the brain can help unlock secrets for optimal cognitive function,” said Isaacson, who is a trustee of the McKnight Brain Research Foundation, which funds research into new treatments and prevention of neurological diseases.

The use of such technology to study brain movements during sleep is why “this study is unprecedented,” said Jarosiewicz. “There aren’t many scenarios where a person would have a multielectrode array positioned in his brain, where the electrodes are small enough to be able to detect the trigger activity of individual neurons.”

The procedure is obviously rare. In fact, Braingate has implanted only 12 of these devices in the consortium’s work history, Jarosiewicz said.

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“This supports the idea that in order to optimize memory function and learning, people should prioritize restful activities – especially adequate sleep – to keep our” engines “at peak performance,” he said. Isaacson said.

There are some home monitoring devices, said Isaacson, such as straps and rings worn on the wrist, which claim to track the different sleep stages that may be most important for memory consolidation.

Hopefully, he said, “future studies will help clarify which specific stages of sleep – deep sleep, REM sleep – where memory replay occurs most frequently.”

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