Do animals have "mirror neurons" that spark aggression?

Mirror neurons are clusters of brain cells that activate both when we perform an action and when we observe a similar action performed by others. Since their discovery in primates in the 1990s, mirror neurons have generated much interest both in science and popular culture, rising (often unfounded) speculation that they might underlie humans’ ability to mimic, empathize, and even develop culture or appreciate art. 

Now, a team of researchers led by Stanford University has found that, in male mice, mirror neurons located in the hypothalamus – an evolutionary ancient part of the brain – play a fundamental role in structuring aggressive behaviors. The experts discovered that the same neurons become activated when mice are fighting and when they are watching a fight. These findings hint at a more primal origin for these types of neurons than previously thought.

“Aggression in the wild is rarely a private affair,” said study senior author Nirao Shah, a professor of Psychiatry and Behavioral Sciences at Stanford. “Aggression is usually not only to defeat the other animal, but also to tell others in the vicinity, ‘Hey, I’m the boss.’ It’s a public display.”

In previous research, Professor Shah and his colleagues traced aggression in male mice to a cluster of brain cells located in the ventromedial hypothalamus subsequently dubbed “the rage center.” Now, by using precise imaging techniques to record activity in the rage center in both mice engaged in a fight and those only witnessing a fight, the scientists found that these neurons are also sensitive to aggression between other mice.

Most research on mirror neurons has focused on those located in the cortex – the most evolutionary advanced part of the brain – so their discovery in the hypothalamus was quite shocking. Another surprising aspect was that these aggression-mirroring neurons seemed to be triggered by sight in observers, whereas in fighting mice, they were triggered by the smell of pheromones. 

Finally, the scientists found that these neurons appeared innately tuned to aggression, even in mice that never engaged in or witnessed aggressive behavior, and not only contributed to sensing aggression, but also to enable it.

According to the experts, the fact that aggression-mirroring neurons exist in such a primitive area of the brain suggests that may have been conserved across evolution, from mice to humans. “It suggests that we might have the same neurons, and maybe they encode some qualities of aggression in ourselves,” Shah concluded.

The study is published in the journal Cell. 

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By Andrei Ionescu, Earth.com Staff Writer

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