Scratching an itch is bad for the skin. What if you can relieve an itch on one arm by scratching the other arm? A team of neurologists examined whether they can use mirrors to achieve this. Lu and Tirth discuss this tingly study on S01E06 of the Recreational Science podcast (timecode 28:14):
Tirth: This study is about itch, a very fundamental sensation. It’s called “Itch Relief by Mirror Scratching. A Psychophysical Study”. It was published in 2013 in PLoS ONE. As you know, it’s a great journal.
Lu: One of my favorites.
Tirth: The experimenters are based out of Germany, the University of Lubeck.
Lu: Good institution.
Tirth: So there’s this concept called phantom limb syndrome. Patients who have had amputations of their arms or strokes or other traumatic brain injury, who lose the use of one arm, may still get pain and other sensations in that arm. This is very frustrating because they can’t relieve the sensation by moving the arm, and in fact the pain may be “present” on the arm even when the arm is not actually there. One of the things that was developed in the 80s or 90s, I think, was mirror therapy, where patients put the unaffected arm in a mirrored box and move it. This tricks their brain into thinking that the affected arm is being moved and it gives them some relief.
Lu: I see.
Tirth: Here, the experimenters are testing this idea with itch. The main goal of the study is to see if you induce an itch on one arm, can you relieve that itch by scratching the other arm, while using mirrors to fool the brain into thinking that the itchy arm was being scratched? To study this, the experimenters recruited 26 male participants and induced itch on their right arm by injecting it with histamine, which causes itch.
Lu: Okay. They couldn’t have used like a feather to tickle the arm?
Tirth: Ah no, that’s not very rigorous. And then on the other arm, on the left arm, they actually made a red mark to mimic histamine injection. So, if the participant is looking at their right and left arms, they would look roughly the same. There are four experimental conditions. In all four conditions, the experimenters induced itch on the right arm. The difference is which arm they scratched and whether or not a mirror was used.
Lu: Okay.
Tirth: For the first condition, they scratched the right arm without using any mirrors. And the participants rated their itch on a scale before and after scratching.
Lu: Okay, straightforward condition.
Tirth: For the second condition, they scratched the left arm, again without using any mirrors, so the participants can see that their left arm is being scratched.
Lu: They’re scratching the left arm in the exact same spot where they injected the histamine on the right arm?
Tirth: Yes. For the third condition, they scratched the left arm with a mirror placed between the right and left arms. The participant’s head is on the left side of the mirror, so their right arm is hidden.
Lu: So when they try to look at their right arm, they’re actually seeing a reflection of the left arm.
Tirth: That’s correct. So when they scratched the left arm, the participants, looking at the mirror, perceive that the right arm is being scratched. And then for the fourth condition, they scratched the right arm, but the participant’s head is on the right side of the mirror. So when they look at the mirror, they think their left arm is being scratched.
Lu: Okay, very systematic.
Tirth: Before I get into the results, do you want to guess which condition you think gave the best relief from itch?
Lu: Condition number one.
Tirth: Right.
Lu: Because they caused the itch on the right arm and scratched it without mirrors.
Tirth: And sure enough, that was the most effective at itch relief.
Lu: I’m guessing the least effective is scratch the left without a mirror.
Tirth: Yes, absolutely. So the question is then, if they scratch the left, but then using mirrors, trick the brain into thinking they’re scratching the right (condition three), does that cause relief?
Lu: Maybe a little bit, I’m guessing.
Tirth: That’s exactly right. Compared to scratch the left without a mirror, scratch the left with a mirror made the participants think that there was some relief from itch on the right arm. But what’s interesting to me is scratch the right with a mirror, so that they think the left is being scratched…’
Lu: The fourth condition.
Tirth: Yeah, this caused slightly lower itch relief than scratch the right without a mirror. Even though in both conditions, the right arm is being scratched.
Lu: I see. So they were tricked into thinking the left arm was being scratched a little bit.
Tirth: Yes, a little bit.
Lu: I see. Interesting. What’s the conclusion?
Tirth: Well, the conclusion here is that just like with phantom limb syndrome, using mirrors to trick your brain also works for a sensation like itch.
Lu: It seems to me… so parakeets cannot recognize themselves in the mirror. When they see themselves in the mirror, they think it’s another bird. Is that basically what they’re trying to get at with humans?
Tirth: No, no, no. Not humans. The right arm. The right arm is looking at the left arm like, yeah, it’s getting confused.
Lu: Uh-huh. You don’t recognize that it’s a reflection of your right arm. You think it’s your left arm.
Tirth: Yeah.
Lu: Humans, susceptible to trickery by mirrors. And in what context is this useful?
Tirth: Well, there are many people out there with chronic dermatological and allergic conditions, psoriasis, eczema, so on and so forth. And if patients keep scratching the itch, they can cause skin breakdown and infections.
Lu: Okay. So you scratch the other arm instead. You spread out the scratching to both of your arms to lighten the scratch load on the effected arm.
Tirth: Both arms, yeah.
Lu: I see. So people with psoriasis on both arms are just out of luck.
Tirth: That’s right. Maybe they should get one of those fake arms.
Lu: Oh, that’s a good idea. And also does this only works for arms? They haven’t shown if this works for relieving itch on legs, the torso or the face.
Tirth: No they haven’t.
Lu: Probably not the face.
Tirth: That’s for future studies.
Article citation:
Helmchen et al., 2013. Itch Relief by Mirror Scratching. A Psychophysical Study. PLoS One. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0082756
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