Why We Itch When Other People Scratch: An fMRI Study

The study supports a growing body of literature that suggests “your perception is indeed your reality,” Napadow said.

We know from research, and from everyday life, that an itch can be contagious. We tend to feel itchy when we see people scratch—or even, as studies have shown, when we hear them talk about an itch.

What we don’t fully understand, from a neuroscience perspective, is why.

In other words: Studies have confirmed that itch can be induced and modulated by cognitive and emotional factors, as well as by placebo and nocebo effects (a nocebo is essentially a negative placebo, an aversive response to an inert stimulus). But the brain activity underlying this has, thus far, remained elusive.

A team of researchers at the MGH Martinos Center for Biomedical Imaging and collaborators at other institutions recently looked into this question of the imagined itch in patients suffering from chronic itch. In a paper published online last month in the journal Allergy, they reported what they found.

The study looked at a cohort of patients clinically diagnosed with atopic dermatitis, a type of inflammation that leads to swollen and cracked skin. The patients received two saline skin pricks: one in which they thought the saline was actually an allergen that would cause itchiness and the other where they knew the saline was a simple drop of water. During each, the researchers performed fMRI scans of what was going on in the patients’ brains.

The results demonstrated that the nocebo—the saline thought to be an allergen—produced a greater itch sensation than the open saline control, with greater fMRI signal increases in brain regions associated with motivational, attention and cognitive processing.

Notably, these responses correlated with responses in near identical brain regions to a real allergen observed in the same patients. This suggested that the brain circuitry activated by the real allergen also comes into play with the imagined, nocebo-induced itch.

The upshot? The study supports a growing body of literature that suggests “your perception is indeed your reality,” said Vitaly Napadow, an Associate Professor of Radiology at Harvard Medical School, a Martinos Center investigator and corresponding author of the Allergy paper. If you think you feel itchy after a skin prick with saline—as opposed to a real allergen—then your brain responds in the same way it would for the allergen.

“Our brains have an amazing capacity to recreate the world around them,” he said, “even without the afferent stimulus we think is necessary to produce a certain sensation.”

Beyond such questions of perception and reality, the findings also have important clinical implications. They suggest, Napadow said, that brain-based therapies can be used to effectively down-regulate itch perception in chronic itch patients, to a greater degree than previously thought.  The investigators are now looking into further studies in which they would assess the potential of specially tailored cognitive / affective therapies for this end.

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