• Outstanding research
While the dream may seem like a rest moment, the body is really involved in a variety of processes, including the creation of new memories. The research shows that the new memories we make are reactivated during sleep for several days after they are created for the first time. So how does the brain avoid mixing old and new memories during sleep?
A new study funded by the National Health Institutes discovered patterns in the activation of old and new memories during sleep that maintain these separate memories. Their findings offer key information about memory and learning.
Pupila size as a marker of different sleep states
Previous research in humans and animals have shown that changes in pupil size are a score of changes in brain states, including sleep -related those. In this study, Cornell’s University Antonio Fernández-Ruiz, Ph.D. , Azahara Oliva, Ph.D. And colleagues wondered if these sleep states help the brain avoid the interference of memory and overlap.
The researchers first examined how mice students change while sleeping. The mice sleep with their eyes open, which makes it easier for researchers to track these changes. They discovered that mice pupils shrunk during ReM sleep (rapid ocular movement), but alternated between reduction and expansion during sleep NO Rem.
Non REM dream is the time when memories are consolidated. Find a different pattern in the changes of the pupil between the REM and the Non REM confirmed to the researchers that the behavior of the pupil can be used to identify different states of sleep.
Discovering the structure of the sleepy sleep
To understand the activity of the brain during the non -REM sleep, the researchers examined the activity of neurons in the hippocampus (a part of the brain involved in the formation of memory). The researchers sought to see if the neuronal activity related to the activation of old and new memories was linked to changes in the size of the pupil. They also used optogenetics, a technique that uses light to control the activity of neurons, to see what happened when neuronal activity was interrupted during the specific pupil states.
They discovered that:
- Neurons associated with old memories were activated more frequently during the great states of the pupil, while neurons associated with new memories were activated more frequently during the small states of the pupil.
- When interrupting the neuronal activity, while the students were small mice avoided to retain the information they had just learned, but did not affect the information previously learned.
- The interruption of neuronal activity during the great states of the students did not affect the retention of freshly learned information.
This suggests that during the non -REM sleep, the new memories are reactivated during the small states of the pupil and the oldest memories during the great states of the pupil, keeping them separate.
Why are these findings important?
For the first time, researchers have determined that non -REM sleep has several different substitute and that they replace separate the repetition of old and new memories. These findings help explain how new and old memories are prevented from being mixed by the brain when they are activated during sleep.
Mental health disorders are often associated with memory deficits that can be attributed to sleep disorders. A deeper understanding of how memories are formed during sleep could help shed light on why these memory problems occur and suggest ways to reverse or prevent them.
References
Chang, H., Tang, W., Wulf, AM, Nyasulu, T., Wolf, Me, Fernández-Ruiz, A. and Oliva, A. (2025). Sleep microstructure organizes memory repetition. Nature, 637 (8048), 1161–1169. https://doi.org/10.1038/s41586-024-08340-w
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