While it’s common knowledge that a restful night rejuvenates our energy, a recent study from Cornell University reveals it also refreshes another crucial aspect: our memory.
When we learn or encounter new experiences, neurons in the hippocampus, a brain area essential for memory, become activated. During our sleep, these neurons begin to replay the same activity patterns, which aids the brain in cementing those memories for later use in a broader area known as the cortex. However, how can we continue to learn new things throughout our lives without exhausting our pool of neurons?
A newly published study titled “A Hippocampal Circuit Mechanism to Balance Memory Reactivation During Sleep” in Science indicates that at certain stages of deep sleep, specific portions of the hippocampus become inactive, which permits these neurons to reset.
“This process may enable the brain to efficiently reuse the same neurons for acquiring new knowledge the following day,” stated Azahara Oliva, an assistant professor in neurobiology and behavior and the corresponding author of the paper.
The hippocampus is organized into three sections: CA1, CA2, and CA3. While CA1 and CA3 are well-researched and involved in forming time and space-related memories, CA2 is less understood. The recent study discovered that this region is responsible for the silencing and resetting of the hippocampus during sleep.
To investigate, the researchers placed electrodes in the hippocampi of mice to monitor neuronal activity during both learning and sleep. They found that, as the mice slept, neurons in the CA1 and CA3 regions displayed the same activity patterns as those formed during daytime learning. However, the researchers were curious about how the brain manages continual learning without overburdening its neurons.
“We discovered other sleep states in the hippocampus where activity ceases,” Oliva explained. “The previously active CA1 and CA3 areas became completely quiet, indicating a memory reset, which is triggered by the CA2 region.”
Pyramidal neurons are believed to be the crucial active neurons required for learning functionality. In contrast, a different type of neuron, called interneurons, comes with various subtypes. The study’s researchers found that the brain comprises parallel circuits regulated by these two types of interneurons—one that influences memory and another that facilitates memory resetting.
The researchers are optimistic that they now possess the means to enhance memory by adjusting the consolidation mechanisms, which could be particularly useful when dealing with memory impairments seen in conditions like Alzheimer’s disease. Furthermore, they have gathered evidence suggesting pathways to potentially eliminate negative or traumatic memories, aiding in the treatment of issues such as post-traumatic stress disorder.
This findings help clarify why sleep is essential for all animals—not just for memory consolidation, but also for resetting the brain to ensure optimal function while awake. “Our research demonstrates that memory is a constantly evolving process,” Oliva noted.
This study received funding from the National Institutes of Health, a Sloan Fellowship, a Whitehall Research Grant, a Klingenstein-Simons Fellowship, and a New Frontiers Grant.
