Evidence that human brains replay our waking experiences while we sleep

May 5, 2020

Science Daily/Cell Press

When we fall asleep, our brains are not merely offline, they're busy organizing new memories -- and now, scientists have gotten a glimpse of the process. Researchers report in the journal Cell Reports on May 5 the first direct evidence that human brains replay waking experiences while asleep, seen in two participants with intracortical microelectrode arrays placed in their brains as part of a brain-computer interface pilot clinical trial.

During sleep, the brain replays neural firing patterns experienced while awake, also known as "offline replay." Replay is thought to underlie memory consolidation, the process by which recent memories acquire more permanence in their neural representation. Scientists have previously observed replay in animals, but the study led by Jean-Baptiste Eichenlaub of Massachusetts General Hospital and Beata Jarosiewicz, formerly Research Assistant Professor at BrainGate, and now Senior Research Scientist at NeuroPace, tested whether the phenomenon happens in human brains as well.

The team asked the two participants to take a nap before and after playing a sequence-copying game, which is similar to the 80s hit game Simon. The video game had four color panels that lit up in different sequences for the players to repeat. But instead of moving their arms, the participants played the game with their minds -- imagining moving the cursor with their hands to different targets one by one, hitting the correct colors in the correct order as quickly as possible. While the participants rested, played the game, and then rested again, the researchers recorded the spiking activity of large groups of individual neurons in their brains through an implanted multi-electrode array.

"There aren't a lot of scenarios in which a person would have a multi-electrode array placed in their brain, where the electrodes are tiny enough to be able to detect the firing activity of individual neurons," says co-first author Jarosiewicz. Electrodes approved for medical indications, like those for treating Parkinson's disease or epilepsy, are too big to track the spiking activity of single neurons. But the electrode arrays used in the BrainGate pilot clinical trials are the first to allow for such detailed neural recordings in the human brain. "That's why this study is unprecedented," she says.

BrainGate is an academic research consortium spanning Brown University, Massachusetts General Hospital, Case Western Reserve University, and Stanford University. Researchers at BrainGate are working to develop chronically implanted brain-computer interfaces to help people with severe motor disabilities regain communication and control by using their brain signals to move computer cursors, robotic arms, and other assistive devices.

In this study, the team observed the same neuronal firing patterns during both the gaming period and the post-game rest period. In other words, it's as though the participants kept playing the Simon game after they were asleep, replaying the same patterns in their brain at a neuronal level. The findings provided direct evidence of learning-related replay in the human brain.

"This is the first piece of direct evidence that in humans, we also see replay during rest following learning that might help to consolidate those memories," said Jarosiewicz. "All the replay-related memory consolidation mechanisms that we've studied in animals for all these decades might actually generalize to humans as well."

The findings also open up more questions and future topics of study who want to understand the underlying mechanism by which replay enables memory consolidation. The next step is to find evidence that replay actually has a causal role in the memory consolidation process. One way to do that would be to test whether there's a relationship between the strength of the replay and the strength of post-nap memory recall.

Although scientists don't fully understand how learning and memory consolidation work, a cascade of animal and human studies has shown that sleep plays a vital role. Getting a good night's sleep "before a test and before important interviews" is beneficial for good cognitive performance, said Jarosiewicz. "We have good scientific evidence that sleep is very important in these processes."

https://www.sciencedaily.com/releases/2020/05/200505121711.htm

What makes memories stronger?

Researchers shed new light on how the brain solidifies important memories

April 29, 2019

Science Daily/VIB (the Flanders Institute for Biotechnology)

A team of scientists at NeuroElectronics Research Flanders (NERF- empowered by imec, KU Leuven and VIB) found that highly demanding and rewarding experiences result in stronger memories. By studying navigation in rats, the researchers traced back the mechanism behind this selective memory enhancement to so-called replay processes in the hippocampus, the memory-processing center of the brain. These important findings provide new insights into one of the most enigmatic brain features: memory consolidation.

When we experience something important, we usually remember it better over time. This enhanced memory can be the result of stronger memory encoding during the experience, or because of memory consolidation that takes place after the experience. For example, experiences that turn out to be very rewarding have been found to lead to stronger and longer-lasting memories.

"One of the ways in which our brains consolidate memories is by mentally reliving the experience," explains Prof. Fabian Kloosterman, whose research is aimed at unravelling memory processing in the brain. "In biological terms, this boils down to the reactivation or replay of the neuronal activity patterns associated with a certain experience. This replay occurs in hippocampal-cortical brain networks during rest or sleep."

The question Kloosterman and his team at NERF set out to answer was whether the positive effect of rewards on hippocampal replay extend beyond the time of the experience itself and thus could further support enhanced memory consolidation.

Rewards and challenges

To find answers, the researchers trained rats to learn two goal locations in a familiar setting. One of the goals was a large reward -- nine food pellets -- while the other goal location only had a single food pellet on offer as a small reward. "Perhaps unsurprisingly, we found that rats remembered better the location where they found the large reward," says Frédéric Michon, PhD student in the Kloosterman lab, who conducted the experiments. "But we also observed that this reward-related effect on memory was strongest when the food pellets were located in places that required more complex memory formation."

Replay for better memory

To assess the contribution of replay brain activity after the actual experience, the researchers disrupted this particular signaling network, but only after the rats got a chance to discover the reward locations. Michon: "Mirroring our earlier findings, we observed that memory was impaired only for the highly rewarded locations, and in particular, when the rewards were at challenging locations."

In sum, the researchers could demonstrate that hippocampal replay, occurring after initial learning, contributes to the consolidation of highly rewarded experiences, and that this effect depends on the difficulty of a task. "A relatively simple experimental setting with rats and food pellets can teach us a lot about memory," says Kloosterman. "Our results demonstrate that replay contributes to the finely tuned selective consolidation of memories. Such insights could open future opportunities for treatments that help to strengthen memories, and could also help us understand memory decline in diseases such as dementia."

https://www.sciencedaily.com/releases/2019/04/190429111840.htm

Interrupted sleep impairs memory in mice

July 27, 2011

Science Daily/Stanford University Medical Center

With the novel use of a technique that uses light to control brain cells, researchers have shown that fragmented sleep causes memory impairment in mice.

Until recently scientists have been unable to tease out the effects on the brain of different yet intertwined features of sleep. But these investigators were able to overcome that problem and come to their findings by using the novel method, known as optogenetics, to manipulate brain cells to affect just one aspect of sleep.

The study shows that "regardless of the total amount of sleep, a minimal unit of uninterrupted sleep is crucial for memory consolidation," the authors write in the study that will be published online July 25 in the Proceedings of the National Academy of Sciences.

While the study does not reach any conclusions about the amount of sleep needed to avoid memory impairment in humans, it does suggest that memory difficulties in people with apnea and other sleep disorders are likely connected to the compromised continuity of sleep caused by such conditions.

http://www.sciencedaily.com/releases/2011/07/110725152844.htm

Sleep selectively stores useful memories: brain evaluates information based on future expectations

February 1, 2011

Science Daily/Society for Neuroscience

After a good night's sleep, people remember information better when they know it will be useful in the future, according to a new study in the Feb. 2 issue of The Journal of Neuroscience. The findings suggest that the brain evaluates memories during sleep and preferentially retains the ones that are most relevant.

"Our results show that memory consolidation during sleep indeed involves a basic selection process that determines which of the many pieces of the day's information is sent to long-term storage," Born said. "Our findings also indicate that information relevant for future demands is selected foremost for storage."

Some, but not all, of the volunteers were allowed to sleep between the time they learned the tasks and the tests. As the authors expected, the people who slept performed better than those who didn't. But more importantly, only the people who slept and knew a test was coming had substantially improved memory recall.

The researchers also recorded electroencephalograms (EEG) from the individuals who were allowed to sleep. They found an increase in brain activity during deep or "slow wave" sleep when the volunteers knew they would be tested for memory recall.

"The more slow wave activity the sleeping participants had, the better their memory was during the recall test 10 hours later," Born said. Scientists have long thought that sleep is important in memory consolidation. The authors suggest that the brain's prefrontal cortex "tags" memories deemed relevant while awake and the hippocampus consolidates these memories during sleep.

http://www.sciencedaily.com/releases/2011/02/110201172603.htm