Humans and other social animals suffer from something called contagious itching. It's when you see somebody else scratching and suddenly need to scratch yourself, too. Now, a group of scientists led by Zhou-Feng Chen at the Center for the Study of Itch at Washington University in St. Louis have discovered that mice have the same problem. In the process, the scientists may have identified what causes some contagious behaviors.

Once Chen and his colleagues had established that mice have contagious itching just like other mammals, they wanted to know more. How did such behavior arise in the neurocircuitry of mouse brains? One possibility was that it was a kind of empathetic reaction to mice they knew well or that it was a response to smelling or touching an itchy mouse. But they found that mice could catch itches from strange mice—and even from mice they saw on video (see video above). So clearly all they needed was to see another itchy mouse to get itchy themselves.

Chen and his team then dove straight into the neurobiology of the mice. Brain scans of the scratching mice revealed activity in a region called the suprachiasmatic nucleus, whose neurons emitted a chemical called gastrin-releasing peptide (GRP). By switching GRP on and off in mouse brains, the researchers discovered that it governed only contagious scratching. Mice with blocked GRP still got itchy, but they didn't start scratching when they saw other scratching mice.

Then the researchers wondered if they could induce contagious scratching even when there were no other mice nearby to catch it from. So they injected GRP directly into their subjects' brains and also used brain implants that stimulated electrical signaling in the suprachiasmatic nucleus. Both types of treatment caused the mice to engage in frantic scratching, as if they had just seen one of their fellow rodents scratching themselves. In a paper for Science, Chen and his colleagues suggest that this means contagious scratching is a kind of involuntary response, hardwired into the brain.

University of Sussex neuroscientist Anil Seth, who was not involved in the research, told The Guardian that "very deep and evolutionary ancient mechanisms might be involved."

What kind of evolutionary pressures would create such a fundamental process in the brain? "It's possible that when a lot of mice are scratching, maybe it warns other mice that this is a place that has a lot of insects, and you'd better start scratching before it is too late," Chen told Live Science.

He and his colleagues write that their work also "may have implications for our understanding of neural circuits that control socially contagious behaviors." In other words, it's possible that there are other socially contagious activities in animals that could be induced by brain implants.

It's hard to deny that this sounds like the first step toward some kind of Matrix-like dystopia, where brain implants fool us into thinking we're eating tasty steaks. But that's unlikely. Human behavior is far more complex than what we see in mice. Still, this research has revealed something fascinating about how mammal brains work. With a little neurochemical intervention, scientists can induce social reflexes in mice who are entirely alone.

Science, 2017. DOI: 10.1126/science.aak9748

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