Summary: A new study reveals how the brain reprocesses and refines memories during sleep, particularly those related to spatial learning. Researchers tracked rats’ hippocampal neuron activity for up to 20 hours and found that memory patterns first echoed the learning phase and then gradually shifted to match the recollection phase upon waking.
This reorganization, observed during non-REM sleep, involved a dynamic change in the neurons representing reward locations—some stopped firing while others became active. These changes not only reinforced the memory but may also free up neurons for storing new information.
Key Facts:
- Non-REM Sleep Role: Memory reactivation and optimization occurred during non-REM sleep, while REM sleep seemed to counteract this effect.
- Representational Drift: Neuronal activity shifted from mimicking the learning phase to the recall phase, helping refine memory storage.
- Neural Efficiency: After sleep, fewer neurons were involved in recalling reward locations, suggesting an optimization of memory representation.
Source: IST
A good night’s sleep helps us remember recently learned information, ‘engraving’ our memories. This is also true for animals, as remembering for example the location of food resources is essential for their survival.
Scientists can examine this role of sleep in the lab by training lab mice or rats about their environment using various memory tasks. In such experiments designed for spatial learning, the animals must learn and subsequently remember the location of food rewards in mazes.
Despite extensive research aimed at understanding the neuronal mechanisms that favor learning, memory formation, and recollection, many questions about these essential brain functions remain unanswered.
Now, researchers from Professor Jozsef Csicsvari’s group at the Institute of Science and Technology Austria (ISTA) have probed the key roles of sleep stages in optimizing memory recollection.
They wirelessly measured neuronal activity patterns in rat brains for up to 20 hours of sleep, considerably extending previously reported measurement times.
“We showed that the neuronal assemblies in the early stages of sleep reflect recently learned spatial memories. However, as sleep progresses, neuronal activity patterns gradually transform into those seen later, when the rats awaken and remember the locations of their food rewards,” says Csicsvari.
Mapping—and remembering—reward locations
Past work showed that a cortical brain area called the hippocampus is important both for exploring and maintaining routes in an environment (called spatial navigation), and for spatial learning.
Hippocampal neurons keep track of the animal’s location by firing at specific locations, thus forming a cognitive map of the environment. Animals use this map to navigate in space while updating it during learning.
In this process, the reward locations play an instrumental role, becoming disproportionately represented on the animals’ cognitive map.
Following spatial learning, the hippocampus plays an important role in enhancing memory during sleep.
It does so by reactivating recently learned memory traces. Previously, the Csicsvari group showed that the more often a specific reward location is reactivated during sleep, the better the animal remembered that location when they woke up.
On the other hand, when the team blocked the reactivation of a specific reward memory, the animals were unable to recall the respective location.
Reorganizing neuronal patterns during sleep engraves memories
While scientists could so far only examine the reactivation of spatial memories in shorter sleep periods of two to four hours, the team now achieved such experiments during long overnight sleep. Using wireless recordings, they monitored neuronal activity in the hippocampus for up to 20 hours while the rats rested and slept after a spatial learning paradigm.
“Our findings were unexpected. We showed that the activity patterns of neurons linked to the reward locations reorganized during the long sleep,” says ISTA PhD graduate Lars Bollmann, one of the study’s co-first authors.
Indeed, when a given reward location was reactivated, not all the neurons that represented that location remained active in the entire sleep. While some did—the ISTA researchers called them a “stable subgroup”—, others stopped firing during later sleep stages. But at the same time, a new group of neurons started to fire gradually.
“Most surprisingly, we showed that while the pattern of firing neurons in the early stages of sleep echoed the neuronal activity in the learning phase, this pattern later evolved to mirror the neuronal activity when the rats woke up and remembered where the rewards were located,” adds Bollmann.
Thus, the team not only observed a drift in neuronal activity patterns during sleep in the frame of spatial learning but also linked it to the process of memory reactivation.
Thus, they shed light on how sleep helps keep memories fresh. In addition, they showed that this reorganization happens during non-rapid eye movement (non-REM) sleep, while REM sleep counteracts it.
Freeing neurons up for new memories?
What could be the role of this phenomenon—called “representational drift”—that occurs in sleep?
“We can only speculate in this regard,” says Csicsvari.
“It is possible that memory representations must be formed quickly during learning but that such representations are not optimal for long-term storage. Therefore, a process may take place in sleep that optimizes these representations in sleep to reduce brain resources to store a specific memory.”
In support of this hypothesis, the researchers observed that fewer neurons were linked to a given reward location after sleep than before. Therefore, some neurons get freed up to take in newer memories.
“Any new memories must find a way to be integrated into existing knowledge. Frequent repetitions of the new memories as well as partial change in the neuronal code may thus help optimize their integration into existing memory representations,” concludes Csicsvari.
The present research was conducted at the Institute of Science and Technology Austria (ISTA) by the recent ISTA PhD graduate Lars Bollmann and former ISTA postdoc Peter Baracskay (co-first authors) together with former ISTA postdoc Federico Stella, currently an Assistant Professor at Radboud University, Donders Institute for Brain, The Netherlands, and ISTA Professor Jozsef Csicsvari (co-corresponding authors).
About this sleep and memory research news
Author: Andreas Rothe
Source: IST
Contact: Andreas Rothe – IST
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Sleep stages antagonistically modulate reactivation drift” by Jozsef Csicsvari et al. Neuron
Abstract
Sleep stages antagonistically modulate reactivation drift
Hippocampal reactivation of waking neuronal assemblies in sleep is a key initial step of systems consolidation. Nevertheless, it is unclear whether reactivated assemblies are static or whether they reorganize gradually over prolonged sleep.
We tracked reactivated CA1 assembly patterns over ∼20 h of sleep/rest periods and related them to assemblies seen before or after in a spatial learning paradigm using rats.
We found that reactivated assembly patterns were gradually transformed and started to resemble those seen in the subsequent recall session.
Periods of rapid eye movement (REM) sleep and non-REM (NREM) had antagonistic roles: whereas NREM accelerated the assembly drift, REM countered it.
Moreover, only a subset of rate-changing pyramidal cells contributed to the drift, whereas stable-firing-rate cells maintained unaltered reactivation patterns.
Our data suggest that prolonged sleep promotes the spontaneous reorganization of spatial assemblies, which can contribute to daily cognitive map changes or encoding new learning situations.
