Sleep is essential for many aspects of brain health, but new research shows how it may be crucial for preserving emotional memory. Many people believe that the brain shuts down during sleep, but this new study suggests that groups of neurons continue working, imprinting memories into the brain.
The University of Michigan studied how memories associated with a specific sensory event are formed and stored in mice. They specifically examined how a fearful memory formed in relation to a particular visual stimulus.
The study published in Nature Communications found that neurons activated by the visual stimulus were kept active during sleep. Sleep was vital to their ability to connect the fear memory to the sensory event.
Previous studies have shown that regions of the brain that are highly active during intensive learning tend to show more activity during sleep afterward. However, it was still unclear whether this “reactivation” of memories during sleep has to happen to store the memory of newly learned material fully.
“Part of what we wanted to understand was whether there is communication between parts of the brain that are mediating the fear memory and the specific neurons mediating the sensory memory that the fear is being tied to. How do they talk together, and must they do so during sleep? We would really like to know what’s facilitating that process of making a new association, like a particular set of neurons, or a particular stage of sleep,” said Sara Aton, senior author of the study. “But for the longest time, there was really no way to test this experimentally.”
New Research Tools
There are now tools available for researchers to genetically tag cells that are activated by an experience during a specific window of time. By focusing on a specific set of neurons in the primary visual cortex, researchers were able to create a visual memory test. This test was used on a group of mice which included showing them a neutral image and then expressing genes in the visual cortex neurons activated by the image.
Alton and her team tested whether they could instigate the memory of the image stimulus by selectively activating the neurons without showing the image. When the neurons were activated and were paired with a mild foot shock, they found that the mice would be afraid of images that looked like the ones the cells encode.
When the mice had disrupted sleep, when shown an image and given a foot shock, there was no fear associated with the visual stimulus. Those with regular sleep learned to fear the specific visual stimulus that had been paired with the foot shock.
In the absence of sleep, the brain seems to manage to process being afraid, but there may be a lost connection on what the fear should be about. Researchers believe these findings could help with how anxiety and post-traumatic stress disorder are understood.