How can a big scare lead to nightmares?

Have you ever had a nightmare after experiencing a particularly frightening event during the day? Researchers from the New York University Neuroscience Institute studied the mechanisms behind this process of emotional memory reactivation. In their experiment, they exposed rats to a harmless experience with the help of a keyboard cleaner. So why do we have nightmares after experiencing a frightening situation?

Just like humans, rats also store what are known as cognitive maps. The term was introduced in 1948 by E.C. Tolman who argued that rodents didn't just learn responses (turn right or left, go up or down), but were capable of building mental maps of their environment; in other words of the world they experience. The different contexts encountered are processed by different groups of neurons in the hippocampus. In order to consolidate memory and long-term storage, a similar mechanism goes to work during their sleep. Obviously, a similar process occurs in humans, though it has only been explained quite recently (see in particular the works of A. Schapiro’s team at Harvard Medical School).

The research team composed of G. Girardeau, I. Inema and G. Buzsaki wanted to know if these mental maps could also include the animal’s feelings, and particularly fear. They set up the following experiment: rats were placed at a specific location within a maze and received a puff of air in the face (using a keyboard cleaner). While painless, the experience is unpleasant and upsetting to the rodents who subsequently slow down and then run very quickly when they reach the fateful place in the maze. While the rats mapped out this passage, the scientists recorded neuronal ensembles in the hippocampus and basolateral amygdala (BLA). The same monitoring was carried out during their sleep. Specifically, when the rats go to sleep, the fear center of their brain is reactivated and they "relive" the arrival at this unsettling location in the maze.

This study shows that our memories aren’t just pieces of information, but are in fact linked to an emotional context. Obviously, there’s no way to tell if the rats had nightmares, but as the authors of the study point out: if the same thing goes on in people, it might lead to nightmares, “it has been fairly well documented that trauma leads to bad dreams.”
Source: G. Girardeau, I. Inema and G. Buzsaki, “Reactivations of emotional memory in the hippocampus-amygdala system during sleep”, in Nature Neuroscience, Sept. 2017

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