Mental itching is a real head-scratcher

Two recent studies show that an itch can be caused just by watching someone else scratch, or by looking at pictures of itchy things.

The first involved 51 participants watching videos of others scratching, while scanning their brains to see which parts were activated. Interestingly, watching someone else scratch switched on the same areas of the brain as scratching yourself (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, vol. 109, no. 48, pp. 19816-19821, doi: 10.1073/pnas.1216160109).

It’s tempting to think this is related to empathy, but in fact those most susceptible to induced itching were the more neurotic members of the group. So it appears to have more to do with one’s own hang-ups than an ability to put oneself in others’ shoes.

Close-up of a bull ant, found in bushland surrounding Swifts Creek, Victoria (click to embiggen)
Bull ant from the genus Myrmecia, found in bushland surrounding Swifts Creek, Victoria – actual size is around 25 millimetres. Are you feeling itchy yet? (Photo by Fir0002/Flagstaffotos)

The second study had 30 participants looking either at itch-related pictures, like ants, fleas or skin conditions, or neutral images, like butterflies or healthy skin. Once again, there was a strong correlation between feelings of itchiness or actual scratching, and the nature of the images (Lloyd DM, Hall E, Hall S & McGlone FP 2012, “Can itch-related visual stimuli alone provoke a scratch response in healthy individuals?”, British Journal of Dermatology, published online 22 November 2012, DOI: 10.1111/bjd.12132).

The researchers of this study concluded that this shows evidence of the involvement of so-called ‘mirror neurons’ – those brain cells that respond equally to performing a task and watching someone else perform the task – but they point out that the fact that only pictures of stimuli were involved, it’s more than just mimicking behaviour.

Either way, it seems that the urge to scratch can be as much a matter of perception as it is of the presence of a genuine irritant.

(This story aired on 29 November 2012 – you can listen to the podcast.)

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