Researchers suggest that the brain is hardwired to respond to other people's scratching.
In simple terms, social contagion is the spread of behavior or attitudes between individuals.
Yawning is considered a socially contagious behavior; when another person yawns, we tend to reciprocate the action. Itching is another behavior that is socially contagious.
"Sometimes even mentioning itching will make someone scratch," notes principal study investigator Zhou-Feng Chen, Ph.D., of the Washington University Center for the Study of Itch.
"Many people thought it was all in the mind, but our experiments show it is a hardwired behavior and is not a form of empathy," he adds.
Chen and colleagues recently reported their results in the journal Science.
To reach their findings, the researchers studied the brain activity of mice as they engaged in contagious scratching.
First, the team placed the rodents in an enclosure with a computer screen, on which a video was shown of another mouse scratching. Within seconds, the mice placed in the enclosure also began scratching.
"This was very surprising because mice are known for their poor vision," says Chen. "They use smell and touch to explore areas, so we didn't know whether a mouse would notice a video. Not only did it see the video, it could tell that the mouse in the video was scratching."
Watching others scratch sends out 'itch signals' in the brain
On assessing the brain activity of these mice after viewing the video, they noticed that their suprachiasmatic nucleus (SCN) - an area of the brain involved in regulation of the sleep-wake cycle - showed increased activity.
When the researchers analyzed the brain activity of the mice as they engaged in contagious scratching, they found that the SCN released a substance called gastrin-releasing peptide (GRP). In previous research, Chen and colleagues found that GRP is involved in the transmission of "itch signals" between the brain and spinal cord.
On blocking GRP or the brain cell receptor that the substance binds to, the researchers found that the rodents did not reciprocate scratching in response to seeing other mice scratch.
According to the researchers, these findings suggest that mice have no control over their scratching when seeing other rodents scratch.
"The mouse doesn't see another mouse scratching and then think it might need to scratch, too," says Chen. "Instead, its brain begins sending out itch signals using GRP as a messenger."
The team believes that these results help to shed light on the neural mechanisms that underlie contagious itching.
"It's an innate behavior and an instinct. We've been able to show that a single chemical and a single receptor are all that's necessary to mediate this particular behavior.
The next time you scratch or yawn in response to someone else doing it, remember it's really not a choice nor a psychological response; it's hardwired into your brain."
Zhou-Feng Chen, Ph.D.