When you fall asleep, it’s easy to think your brain shuts down, but new research shows that groups of neurons activated during previous learning will continue to hum, carve Deepen the memories into your brain.

UM researchers studied how memories related to a particular sensory event are formed and stored in mice. In a study conducted before the coronavirus pandemic and recently published in the journal Nature Communications, researchers examined how fear memory formation is related to a specific visual stimulus.

They found that not only did the neurons activated by the visual stimulus continue to be more active during the next sleep, sleep was also important for the ability to connect fear memories to the event. their senses.

Previous research has shown that brain regions that are active during deep learning tend to be more active during later sleep. But what is not clear is whether this memory “reactivation” during sleep needs to occur in order to fully store the contents of the newly learned material.

“Part of what we wanted to find out is whether there is communication between the parts of the brain that mediate the fearful memory and the specific neurons that mediate the temporary memory the fear is dealing with. attached to it or not. How do they talk to each other, and what do they do while they sleep? We really wanted to know what facilitated the process of making a new association, like a specific group of neurons or a specific stage of sleep,” said Sara Aton, senior author of the study. Researcher and professor at UM, Department of Molecular, Cellular and Developmental Biology. “But for the longest time, there’s really no way to test this experimentally.”

Now, researchers have the tools to genetically tag cells activated by an experiment over a specific period of time. Focusing on a specific group of neurons in the primary visual cortex, Aton and the study’s lead author, graduate student Brittany Clawson, created a visual memory test. They showed a group of mice a neutral image and expressed the genes in the visual cortex neurons that were activated by the image.

To verify that these neurons recognized this neutral image, Aton and her team tested whether they could activate the visual memory agent by selectively activating the neurons. neurons without showing them the picture or not. When they activated the neurons and combined that activation with a mild shock in the leg, they found that the animals would then fear visual stimuli that looked similar to the images the cells encode. . They found the opposite was also true: after combining a visual stimulus with a shock to the leg, their animals would then become fearful as the neurons reactivated.

“Basically, the principle of visual stimulation and this principle of completely artificial activation of neurons produced the same response,” Aton said.

The researchers found that when they interrupted sleep after showing the animals a picture and giving them a slight leg jerk, there was no sense of fear associated with the visual stimulus. These disrupted sleepers became afraid of specific visual stimuli that were associated with foot shock.

“We found that these mice actually became fearful of every visual stimulus we showed them,” says Aton. From the moment they entered the room where the visual stimuli were present, they seemed to know there was a reason to feel fear, but they did not know specifically what they were afraid of.”

According to Aton, this may suggest that, in order for them to make the correct association between fear and a visual stimulus, they must reactivate the neurons that encode that stimulus in the sensory cortex. sensation in sleep. This allows a memory to become concrete when there is a visual clue. The researchers think that at the same time, that sensory cortex must communicate with other brain structures, in order to associate the sensory aspect of memory with the emotional aspect.

Aton says their findings may have implications for understanding anxiety and post-traumatic stress disorder.

“For me, this is a clue that, if you associate fear with some very specific event during sleep, sleep disruption can affect this process. When you’re sleep-deprived, the brain seems to be processing that you’re afraid, but you can’t link that to what specifically you’re afraid of,” Aton said. “That detailed process can go wrong with PTSD or generalized anxiety disorder.”

References:

Brittany C. Clawson, Emily J. Pickup, Amy Ensing, Laura Geneseo, James Shaver, John Gonzalez-Amoretti, Meiling Zhao, A. Kane York, Femke Roig Kuhn, Kevin Swift, Jessy D. Martinez, Lijing Wang, Sha Jiang, Sara J. Aton. Causal role for sleep-dependent reactivation of learning-activated sensory ensembles for fear memory consolidation. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-21471-2

Source: Sleep is vital to associating emotion with memory, study finds

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