Has Your Friend Got an Itch? You Might Too
A new study reveals why some people may be more susceptible to contagious itching than others
--by Alexia Severson
Feeling the urge to scratch an itch after watching someone else scratch may be attributed to neuroticism, as well as neural activity in several regions of the brain, according to a new study conducted by researchers in the United Kingdom.
The team sought to understand why some people are more prone to visually contagious itching than others. They found that while itching was contagious among most of the volunteers, the degree of contagion is tied to differences in neuroticism, or the tendency to experience negative emotions. Itching is not linked to empathy, a willingness to take someone else’s viewpoint and to respond compassionately, as determined by personality questionnaires.
The authors also found a correlation between participants’ tendency to scratch and neural activity in several brain regions previously identified as part of an ‘itch matrix,' reporting activity in three regions: the left BA44, BA6, and the primary somatosensory cortex.
The Expert Take
To determine why some people are particularly susceptible to itching when they observe others scratching, Henning Holle and colleagues administered personality questionnaires and functional magnetic resonance imaging (fMRI) tests to 51 healthy volunteers who were shown short video clips of people either scratching or tapping parts of their arms or chests.
The first important finding of the study is that social contagion of itching is a normal human behavioral response. When participants were free to scratch, 64 percent did so at least once. This puts itching on par with other types of socially contagious behavior, such as laughter and yawning. Participants who experienced stronger feelings of itchiness during the experiment tended to spontaneously scratch themselves more often when free to do so.
Researchers also identified regions in the brain that support the subjective experience of itching, observing that watching someone scratch activated the same set of brain regions associated with feelings of itching induced by an irritant, such as histamine.
Source and Method
Of the 51 participants in this study, eighteen took part in the fMRI procedure, while the remainder completed the behavioral ratings and questionnaires outside of the scanner.
Short video clips were created in advance, showing either body scratching or a control movement (tapping). Scratching consisted of continuous scraping of the target site using four curled fingers of one hand. Five different target sites were used: left forearm, left upper arm, chest, right forearm, and right upper arm. The control videos showed continuous tapping of a target site.
The total stimulus set comprised 20 videos exhibiting two conditions: scratching versus control (tapping).
According to a New York Times article published in 2009, “Scratching relieves itch by quieting nerve cells,” by Benedict Carey, scientists have argued that “itching is most likely related to grooming, and evolved to protect animals against some toxic plants, as well as insects, along with the diseases they can transmit, like malaria, yellow fever, and river blindness.” But the biology of what causes an itch in the first place remains a mystery.
According to that same article, about “50 diseases leave people in a misery of itching which usually cannot be treated.” And while the study by Holle and colleagues does not provide any solutions for chronic itching, researchers believe their findings could help uncover the neural basis of persistent and compulsive itching in people without underlying dermatological conditions.
While researchers are still trying to pinpoint the exact cause and necessity of itching, in a study published online in 2009 in Nature Neuroscience, researchers found that the spinal cord plays a significant role in producing the sensation of itching and the feeling of relief after scratching in healthy individuals.
Another study, published in the European Journal of Pain, looked at the similarities and differences between pain and chronic itching. Researchers found that itching and pain exhibit corresponding patterns of central sensitization, and that the knowledge of these similarities and differences could lead to itch-preventing therapeutic approaches.
Similar to the study discussed above, a study published in the British Journal of Dermatology in 2011 also examined the motives behind contagious itching in human behavior. Researchers in this study investigated whether exposure to visual cues for itching can induce or intensify itching in healthy subjects and patients with eczema. They found that patients with eczema reported a higher itch intensity and scratched more frequently while watching itch videos, even in the presence of mock itch stimuli, concluding that “interpersonal social cues can dramatically alter the subjective sensory experience of itch.”