The particular pleasure of scratching an itch on the ankle

It's only in recent times that scientists have discovered there are dedicated nerve pathways for communicating the sensation of itch. This troublesome skin signal provides us with a mixed experience. The prickly discomfort of an itch can be agonising. Yet to scratch an itch is one of life's great pleasures. In fact, it often seems that the more intense the itch, the more unreachable its source, then the greater the ultimate pleasure that's derived from finally reaching and clawing at it.

Now the aptly named Gil Yosipovitch and his colleagues have performed one of the first comparisons to see if itches are itchier on some body parts than others. They also investigated whether scratching itches in some places brings more satisfaction than others.

The researchers used cowhage spicules to induce itchiness on either the forearm, ankle or the back of 18 healthy volunteers (10 women; mean age 34). After the spicules were applied, each volunteer indicated from one to ten the intensity of the itch every 30 seconds for five minutes. The itch was then scratched by an experimenter using a cytology brush (a brush with stiff bristles and an elongated handle). The participants again indicated every 30 seconds the intensity of the itch and the pleasure derived from the scratching.

The main findings were that itches were perceived as more intense on the ankle and back, as compared with the forearm. Similarly, scratching was more pleasurable on the ankle and the back than on the forearm. The greater the itch intensity on the forearm and ankle, the more pleasure came from the scratching. Meanwhile, for the back and forearm, as the itch subsided, the pleasure from scratching faded. By contrast, scratching an ankle itch continued to provide pleasure even after the itch had been relieved. "The pleasurability of scratching the ankle appears to be longer lived compared to the other two sites," the researchers said.

"The present study uncovers a topographical relationship between itch attenuation by scratching and the accompanying pleasurability in healthy individuals," they concluded. "Future studies should also examine the scratching pleasurability associated with other itchy areas such as the scalp ...".

Why should itches and scratches be experienced differently on different parts of the body? Yosipovitch and his colleagues discussed a number of potential mechanisms, including differences in nerve density between body regions and regional variations in levels of neuropeptides known to be involved in itch induction. Prior research suggests the pleasure derived from scratching is associated with deactivation in brain areas - such as the anterior cingulate cortex - that are involved in the unpleasantness of itch, and the concomitant activation of other areas - such as the putamen - involved in the anticipation of reward.

How do these findings match your own experience? Do you find ankle itches particularly satisfying to scratch?
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  bin Saif, G., Papoiu, A., Banari, L., McGlone, F., Kwatra, S., Chan, Y., and Yosipovitch, G. (2012). The Pleasurability of Scratching an Itch: A Psychophysical and Topographical Assessment. British Journal of Dermatology DOI: 10.1111/j.1365-2133.2012.10826.x [h/t @mocost]. 

Post written by Christian Jarrett for the BPS Research Digest.

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