For some people, hearing about a bug bite or a rash is enough to cause them to furiously dig at their own unaffected skin. Even some doctors who treat people with shingles report feeling itchy after witnessing their patients scratching. And we'd bet many of you readers are feeling itchy right this very second.
It seems humans commonly catch itches from one another, but scientists hadn’t proven it—until now. Researchers found that itching is contagious much like yawning and laughing.
“With itching, there [was] only anecdotal evidence that watching [a person] itch induces itching,” explains Henning Holle, a lecturer in the psychology department at the University of Hull in England. “We wanted to know whether contagious itch would effect everyone.”
Holle asked 51 adults to take a personality test that ranks the Big Five personality traits of openness, conscientiousness, extroversion, agreeableness and neuroticism. Then using functional magnetic resonance imaging, or fMRI (a technique that detects brain activity by monitoring blood flow in the brain), the subjects watched either a video where someone was scratching herself or tapping her arm or chest. The fMRI allowed the researchers to see the subjects’ brain activity as they responded to the video images.
Two-thirds of the people who saw the scratching video scratched themselves. This finding mirrors what experts know about other socially contagious behaviors such as yawning and laughing: most of us "catch" yawns and laughter.
“Most people tend to experience contagious itch—some are more prone to it than others,” Holle explains. “I was really surprised by the amount of people who spontaneously scratched.”
Watching an itch sparks activity in the anterior insular, primary somatosensory area, and the prefrontal and premotor cortices. These regions, part of the itch matrix, also activate when a person actually feels an itch, meaning that watching someone scratch makes the brain think it is experiencing an itch.
After establishing that itching spreads socially, Holle wondered what caused it. He suspected itch might spread because of empathy. There is some evidence that people feel pain empathetically: When someone sees a family member receive an electric shock, the observer also feels pain (as this is in a lab setting the people aren’t actually receiving the shock). It turns out that people who exhibit more empathy do not scratch more than those who are less compassionate. But people who are more neurotic, those who experience the biggest mood swings and exhibit anxiety, depression, jealousy, and guilt, are more susceptible to contagious itch than others. The neural activity in the prefrontal cortex reinforces the self-reported data indicating that people with neurotic tendencies are more likely to catch an itch.
“This introspective awareness might explain why people are more prone [to contagious itch],” he says.
The paper appears in the journal Proceedings of the National Academy of Sciences.