Scratching An Itch Activates Pleasure Centers In The Human Brain: Study

scratching dog
You may be told not to scratch, but there's a reason why it's so indescribably wonderful. Takashi Hososhima, CC BY-SA 2.0.

Doctors always tell you, if you have an itch don’t scratch it: you could break the skin, cause an infection or make yourself even itchier. But if scratching an itch is not the proper response, why is it so wonderful? Is there something in our brains compelling us to scratch, even though our better judgment tells us no?

In their new study, researchers from the Temple University School of Medicine (TUSM) sought to study just that. Publishing their findings in the Journal of Investigative Dermatology, researchers examined brain activity in chronic itch patients to see if there was an award system associated with scratching.

“Chronic itch is a major symptom in dermatological diseases such as atopic eczema and psoriasis and a bothersome symptom in other diseases like end-stage renal disease,” said Dr. Hideki Mochizuki, assistant professor of dermatology at TUSM, in a recent press release. Him and his team felt inspired to study this side effect’s reward system in the brain because of the curious way scratching can quickly turn from satisfaction to pain. “Despite being pleasurable at first, ongoing scratching can lead to an increase in the intensity of the itch as well as pain and permanent skin damage,” Mochizuki said. “That is why it is important to understand the cerebral activity that may be inducing this pathological scratching behavior.”

Yes, there is the potential to become a pathological scratcher, but why? In order to find out, researchers recruited 10 chronic itch patients along with 10 healthy participants to undergo fMRI imaging while they scratched an itch. To induce the itch, researchers administered cowhage (a certain plant) to the right forearms of all participants. While examining brain activity with the fMRI, researchers discovered that chronic itch patients had an increase in brain activity in areas associated with motor control, and a motivation to act. These areas specifically were the supplementary motor area, premotor cortex and primary motor cortex. The researchers also found that in the healthy patients, areas involved in the reward circuit, such as the striatum, cingulate cortex, caudate nucleus, and orbitofrontal cortex had a more significant increase in activity than in the chronic itch patients.

The latter discovery builds on previous studies led by study coauthor Dr. Gil Yosipovitch, which found an association between cerebral mechanisms of scratching and the brain’s reward circuit. Yosipovitch says that TUSM’s new study, however, is the first to investigate these mechanisms further in patients with chronic itch.

“Our findings may enable us to identify and advance the understanding of the brain network underlying the itch-scratch cycle in chronic itch patients,” Mochizuki said. “This understanding could lead to new therapies for these patients.”

So no, you’re not crazy; there is something inherently pleasurable about scratching an itch. And the next time your doctors throw around the term pathological scratcher, you can tell them it’s not your fault. Your brain is just wired that way.  

Source: Mochizuki H, Lin A, Kraft R, et al. Scratching Induces Overactivity in Motor-Related Regions and Reward System in Chronic Itch Patients. Journal of Investigative Dermatology. 2015.

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