Merely seeing someone else scratch activates brain centers involved in the itch response, suggesting the observation makes one itchy.
But this response doesn't apply to everyone. Those study participants who were more neurotic (a tendency toward negative emotions) were more likely to experience itch contagion. Surprisingly, the researchers found empathy (a willingness to take another's viewpoint) did not correlate with the phenomenon.
"Before it was only anecdotal that people experience contagious itch," lead study author Henning Holle, a research fellow at the University of Sussex, told LiveScience. "There's a general tendency for people to experience contagious itch."
Catching an itch
To see where this happens in the brain, Holle and colleagues used functional magnetic resonance imaging (fMRI) machines to scan the brains of participants who watched silent videos of people either scratching or tapping themselves. (MRI scans show blood flow to active areas of the brain.)
Several regions of the brain already known to be involved in the itch response (both in "feeling itchy" and the related scratching behavior) lit up during the scratching videos. These included the premotor cortex, which influences motor activities, and the insula, a region behind the temples that activates when people experience empathy. However, during the tapping videos, these same brain centers didn't light up.
The researchers included psychological tests for the 51 study participants and found that empathetic people did not have heightened levels of itch contagion. Past research has suggested another contagious behavior, yawning, may be heightened for friends and family, suggesting contagious yawns may stem, in part, from empathy.
Participants who scored high on neuroticism were significantly more likely than others to experience itch contagion, the researchers found.
"It's a very nice study, it complements some of our previous observations, and it clearly demonstrates a brain mechanism that [is] involved in this behavior," Gil Yosipovitch, a dermatologist at Wake Forest Baptist Medical Center who studies itch contagion, told LiveScience.
"Even animals that are less intellectually sophisticated - monkeys are to be compared to little babies - they were able to behave similarly," said Yosipovitch, who with colleagues this year showed itch contagion in primates.
Yosipovitch cautioned that the brain centers and personality traits behind itch contagion may be more complicated than the current study suggests, and warrant further investigation.
"I wouldn't be surprised that other areas involved in cognitive aspects and memory would be highly involved," Yosipovitch wrote in an email. "There are possibly other explanations for this mechanism, to put the eggs all in the basket of neuroticism (negative feelings), I'm not so sure."
Memory may play a role, for example, people may associate itching with a disastrous bug-infested camping trip or a part of a back rub given by a loving partner. However,the experiments did not show any clear role of memory.
"We would expect to see activity in memory-driven areas such as the hippocampus, for instance, which we did not find," Holle said.
Study's itchy limitations
The researchers aren't able to rule out that some of the brain response may come from the active and conscious suppression of itching, Holle said.
The researchers also warned that brain-imaging studies show correlations, but not necessarily the causes of itch contagion.
The videos had no sound, and auditory cues of scratching could play a possible role in itch contagion, Holle said, which could lead to future experiments.
The research may help explain why some patients have unusual tactile perceptions with no simple explanations, such as chronic itchers who feel ants in their veins or crawling sensations on their skin.
One possibility could be that these patients have hyperactivated contagious itch centers in their brains, Holle said.
"Itching has a pleasurable aspect to it, it's a double-edged sword so to speak. On the one hand, it's unpleasant; if you start scratching it is also rewarding at the same time. That is what I find interesting about it, and that's why I really want to do more research on it," Holle said.
The study is detailed online today (Nov. 12) in the journal the Proceedings of the National Academy of Sciences.