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Sleep plays a crucial role in forming new memories. It enables us to learn and organize acquired knowledge and experiences, crystallizing them into stable memories. While investigating these mechanisms, scientists from Rice University and the University of Michigan discovered that neurons in the hippocampus—a brain region critical for memory—not only replay past events but can also anticipate future experiences.
The findings are published in the scientific journal Nature.
The Leading Role of the Hippocampus
It has long been known that the hippocampus, an essential brain region for memory and navigation, consolidates new experiences into stable memories during sleep. The new research advances this understanding further, showing that certain neurons can anticipate future events.
We can see these other changes occurring during sleep, and when we put the animals back in the environment a second time, we can validate that these changes really do reflect something that was learned while the animals were asleep.
Caleb Kemere, a neuroscientist at Rice University and co-author of the study
Researchers trained rats to run a maze and monitored the neurons in their hippocampus. By analyzing sharp wave ripples—a pattern of neuron activation associated with memory consolidation—they observed how individual neurons formed spatial representations. This innovative approach allowed the team to assess where the rats “dreamed” of being, even while they slept.
Observing Neurons in Action
For the first time, scientists have been able to observe individual neurons stabilizing spatial representations during periods of rest. Sleep is essential for memory and learning. That was evidenced by improved memory test results following sleep compared to periods of wakefulness or sleep deprivation. Two decades ago, researchers discovered that neurons in the brains of sleeping animals replay their exploratory trajectories. This finding confirmed the idea that sleep helps crystallize new experiences into stable memories.
However, the new study indicates that there is more to the story.
We imagined that some neurons might change their representations—reflecting the experience we’ve all had of waking up with a new understanding of a problem
Caleb Kemere, a neuroscientist at Rice University and co-author of the study
Neurons Can Anticipate Future Experiences
One of the key innovations of the study was the development of a statistical machine learning approach. This method allowed researchers to determine where the animal “dreamed” of being and to use these positions to assess the spatial tuning process of each neuron.
The method confirmed that the spatial representations formed during new experiences remain stable for most neurons for several hours of sleep after the experience. However, a significant breakthrough was the ability to track neuron activity even without stimulus.
The thing that I loved the most about this research and the reason that I was so excited about it is finding that it’s not necessarily the case that during sleep the only thing these neurons do is to stabilize a memory of the experience. It turns out some neurons end up doing something else.
Caleb Kemere, a neuroscientist at Rice University and co-author of the study
These changes were observed during sleep, and when the animals were returned to their environment, the changes reflected what they had learned during rest.
This research is significant because it represents direct observation of neuroplasticity during sleep. Most studies of plasticity examine brain reorganization during wakefulness when stimuli are present.
The study showcases advancements in neuroscience made possible by technological progress in neural probes and computational power based on machine learning. At the same time, researchers emphasize the importance of continued support for such groundbreaking scientific research.
