I would like to start this post with a challenge. Can you get through this entire post WITHOUT feeling itchy? I know I couldn't even write the first line. And I'm not alone. Itch is contagious. Watching someone else scratch can make you itch, and you should try to get through a lecture on a skin condition. I wonder how dermatologists can take it.
What IS an itch? The clinical definition is that it's an "unpleasant sensation associated with the urge to scratch". Ok, then. Itching is a very important part of clinical diagnosis, from things like poison ivy to allergies to severe use of methamphetamine. In addition, there is a psychological disorder of severe itch which can be both disfiguring and incredibly distressing.
But where does it come from and why do we itch? There's an obvious evolutionary reason (OMG a spider on my arm getitoffgetitoffgetitioff!!!!), but what about social itch? We know about the neurobiological "itch matrix", which involves areas of the brain associated with touch and somatosensory processing, the premotor areas (for scratching), the anterior insula, prefrontal cortex, thalamus, and cerebellum. From a combination of all of these areas (accompanied, of course, by other things like the visual areas to process seeing the spider on your hand), you get an itch and a scartching response, and other involved areas (like the insula and cingulate) may help make it unpleasant enough for you to want to deal with it. All of these areas are also associated with the processing of other stimuli, like touch and pain, which may contribute to the sensation of itch.
But all this was itch induced by things like histamine injected under the skin (the very concept makes me wince) while the patient was in an fMRI (no record of if the poor sap was allowed to scratch). But what about social itch? The contagious kind you get from watching other people?
Holle et al. "Neural basis of contagious itch and why some people are more prone to it" PNAS, 2012.
The authors of this study were particularly interested in contagious itch was associated not just with activity in areas already associated with itch, but with areas containing "mirror neurons", neurons originally discovered in monkeys, which fire both when you perform an action, and when you see others performing it. Some scientists have suggested that activity in mirror neurons may be correlated with things like empathy, though this is disputed, as there's no real proof that these neurons are responsible for things like "emphathy". But the authors here want to make the case for the basis of contagious itch behavior.
To answer this question, the authors of this study took 33 people, and showed them videos of someone either scratching a body part (upper arm, lower arm, or chest) or tapping that same body part with the flat of the hand, and asked them whether they felt itchy afterward. They then took another 18 people and looked at them with fMRI in the same conditions. These people were instructed NOT to scratch (yikes).
You can see that the itch was contagious. They got significant effects compared to the control (but considering the itchiness was rated on a 0-7 scale, contagious itching isn't really uncontrollably horrid, only about a 2). Still, 64% of people scratched. The people most likely to rate themselves as itchy after watching were also the most likely to scratch themselves (they also found a correlation with neuroticism).
When they put another set of patients into the fMRI and showed them the videos, they got nice activity in the "itch matrix", including the insula, the premotor cortex, the somatosensory cortex, the thalamus, etc. Keep in mind though, they this activity is relative. There is a lot of activity going on in the brain during any activity, and during this one the authors would have to have smoothed over the activity associated with watching a video (so visual cortex and processing there), hearing the fMRI making loud clacking noises (areas in the temporal lobe associated with sound processing), somatosensory activation from the pad they were touching as they lay in the fMRI, the processing associated with thinking about the task, and all the other activities in the brain that are constantly going to keep you breathing, your heart beating, and you conscious of the fact that you're in an fMRI doing a study on itch. So when we say that there was activity in the "itch matrix", it's not activity alone, it's what left over when you are deliberately comparing only those areas to those same areas during the control condition.
What's interesting about this is that social itch, just watching someone else scratch, produces activity that is similar to activity when you yourself itch (like with histamine injections). It's a real itch all right.
In addition to the itch matrix, they authors found that higher levels of activation in the somatosensory areas were associated with higher itch intensity. The authors hypothesize that this means that these areas had mirror neurons reacting to the videos of itch, though there is, of course, no way to prove that in this study, and I'm really not sure. There's no proof that the mirror neurons themselves were activated (fMRI certainly isn't that detailed), or that mirror neurons, when activated in humans, with increase thing like empathy for shared itch (though studies in monkeys suggest that mirror neurons can fire in response to observed behaviors like this, there's no proof that this results in "empathy"). But the finding that the itch matrix is activated in response to someone else scratching is both interesting and could be important in the future for those who suffer from constant itch.
As to why some people are more prone to it, the author saw a correlation between the degree of itch experienced following the videos (associated with neuroticism), and itch matrix activation. But they only got a correlation in one of the areas they were looking at, and with 18 subjects, I'm not sure that you can state that increased sympathy to itching is a sign of increased neuroticism or vice versa.
But what I take away from this study is that itching is one very contagious phenomenon, and that that's no fake sympathetic itch you're feeling. The itch is just as real from watching someone else scratch, even if nothing if there.
Author's final note: I once knew a guy who was trying to completely stop scratching itches. Like, physically stop himself from it ever. He considered it the next step in total self control. I say you can't stop the biology.
Holle H, Warne K, Seth AK, Critchley HD, & Ward J (2012). Neural basis of contagious itch and why some people are more prone to it. Proceedings of the National Academy of Sciences of the United States of America, 109 (48), 19816-21 PMID: 23150550