Why scratching brings relief

CHICAGO: Oh, it brings such blessed relief and now scientists can tell you why — scratching an itch temporarily shuts off areas in the brain linked with unpleasant feelings and memories.

"Our study shows for the first time how scratching may relieve itch," Dr Gil Yosipovitch, a dermatologist at Wake Forest University Baptist Medical Center in Winston-Salem, North Carolina, said in a statement.

Prior studies have shown that pain, including vigorous scratching, inhibit the need to itch. Yosipovitch and colleagues looked at what goes on in the brain when a person is scratched.

He and colleagues used a technique known as functional magnetic resonance imaging to see which areas of the brain are active during scratching. They scratched 13 healthy people with a soft brush on the lower leg on and off in 30-second intervals for a total of five minutes.

Scratching reduced activity in the anterior cingulate cortex and the posterior cingulate cortex — areas linked with pain aversion and memory. And the more intensely a person was scratched, the less activity they found in these areas of the brain.

"It's possible that scratching may suppress the emotional components of itch and bring about relief," Yosipovitch said. But they also found why one scratch often begets another.

Scratching increased activity in the secondary somatosensory cortex, a pain center, and in the prefrontal cortex, which is linked with compulsive behaviour.

"This could explain the compulsion to continue scratching," Yosipovitch said. The researchers noted that the study is limited because people were not scratching in response to an actual itch.

But they said understanding what goes on in the brain may lend clues about how to treat people tormented by chronic itch, including people with eczema and many kidney dialysis patients. The study, which appears online in the Journal of Investigative Dermatology, was paid for by the National Institutes of Health.

They scratched 13 healthy people with a soft brush on the lower leg on and off in 30-second intervals for a total of five minutes.

I wouldn't count on this study having much lasting value. This article calls the brush a “soft brush.” Others use “small brush.” The study itself said:

Scratching stimulus

Scratching was accomplished by study personnel repetitively moving a cytology brush (Medi-Pak 7-inch cytology brush; General Medical Corporation, Elkridge, MD) over the left leg. Uniformity was controlled by applying sufficient pressure to bend skin-facing brush bristles, so that the brush handle touched the skin surface. The bending force of the cytology brush was equivalent to approximately 29 g on a digital scale....

Each examinee underwent a training session in which study personnel applied the cytology brush. The examinee was asked whether the procedure was similar to their experience when they scratch their skin. All subjects reported that they perceived this procedure as closely simulating scratching.

http://www.nature.com/jid/journal/vaop/ncurrent/full/jid20083a.html

Now, if they had used the ENDS of the bristles, AND if the bristles were stiff, that would have simulated scratching. But they didn't. They used the SIDE of the bristles. The ends of the bristles would have contributed little to the “scratching.” They also “..touched the skin's surface...” with the “brush handle.”

Perhaps the brush handle touching the skin MIGHT have simulated “scratching,” the SIDE of the bristles could not, whether the bristles were stiff or soft. So then, why did the examinees report that it DID “simulate scratching?”

Put yourself in their place: You are a college student who needs the pittance offered to take part in an experiment. The big deal itch expert professor asks if it feels like scratching. If you say “no,” you (imagine that) you will annoy a professor, you imagine a $20 bill sprouting wings and flying out the window. It is not difficult to imagine that the examinee would not be completely candid.

Thus, the actual finding of this study is that sliding the side of the bristles of a cytology brush on skin (which is NOT experiencing itching) apparently causes activation and deactivation of certain parts of the brain. I think that it has no correlation with scratching an actual itch.

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