A full night’s sleep strengthens memory by reinforcing newly learned information. This process is crucial for animals as well, as recalling the location of food sources is vital for survival. Researchers study this function of sleep in the lab by training mice and rats with different memory tasks to explore their environment.
In spatial learning experiments, animals must locate and remember food rewards within mazes. While extensive research has explored the neuronal mechanisms behind learning, memory formation, and recall, many fundamental questions remain unanswered.
Now, a team led by Professor Jozsef Csicsvari at the Institute of Science and Technology Austria (ISTA) has investigated how different sleep stages optimize memory recall. Using wireless recordings, they monitored neuronal activity in rat brains for up to 20 hours—significantly longer than previous studies.
“We found that in the early stages of sleep, neuronal activity reflects recently learned spatial memories. As sleep continues, these patterns gradually shift, resembling those observed when the rats wake and recall the locations of their food rewards,” Csicsvari explains. The findings are published in Neuron.
The Hippocampus and Cognitive Mapping
Previous research has shown that the hippocampus, a cortical brain region, is essential for both spatial navigation—exploring and maintaining routes—and spatial learning. Hippocampal neurons track an animal’s position by firing at specific locations, creating a cognitive map of the environment. Animals rely on this map to navigate while continuously updating it as they learn. Notably, reward locations become disproportionately represented on these cognitive maps, playing a crucial role in the learning process.
After spatial learning, the hippocampus strengthens memories during sleep by reactivating recently acquired memory traces. The Csicsvari group previously demonstrated that the more frequently a reward location is reactivated during sleep, the better the animal recalls it upon waking. Conversely, when researchers blocked the reactivation of a specific reward memory, the animals were unable to remember that location.
Scientists previously studied spatial memory reactivation only in short sleep periods, but this study extended observations to 20 hours using wireless recordings.
“Our findings were unexpected. Neuronal activity linked to reward locations reorganized during long sleep,” said ISTA Ph.D. graduate Lars Bollmann.
Neuronal Shift During Sleep
Some neurons remained active, forming a “stable subgroup,” while others stopped firing, and new ones gradually took over. Early sleep patterns mirrored learning-phase activity but later resembled wakeful recall.
The study linked this shift in neuronal activity to memory reactivation, showing that non-REM sleep supports reorganization, while REM sleep counteracts it.
Scientists previously studied spatial memory reactivation only in short sleep periods, but this study extended observations to 20 hours using wireless recordings.
“Our findings were unexpected. Neuronal activity linked to reward locations reorganized during long sleep,” said ISTA Ph.D. graduate Lars Bollmann.
Some neurons remained active, forming a “stable subgroup,” while others stopped firing, and new ones gradually took over. Early sleep patterns mirrored learning-phase activity but later resembled wakeful recall.
The study linked this shift in neuronal activity to memory reactivation, showing that non-REM sleep supports reorganization, while REM sleep counteracts it.
Read the original article on: Medical X Press
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