I itch, pass the mirror

That the sense of body ownership can be so quickly and easily, yet profoundly alters still amazes and intrigues me.

Here, the New Scientist online reports on an interesting finding relating itch

Scratch a mirror image of your itch to bring relief

Next time you struggle to resist an itchy rash or insect bite, you could find relief in the mirror.

Perception of our own bodies can be easily manipulated using tricks such as the rubber hand illusion, which fools people into thinking a rubber hand is their own. Reflecting someone’s limb in a mirror has also been used to treat phantom limb pain.

Now Christoph Helmchen and his colleagues at the University of Lübeck in Germany have shown that a similar mirror illusion can fool people into feeling relief from an itch, even when they scratch the wrong place.

The team injected the right forearms of 26 male volunteers with itch-inducing chemical histamine. Because the injection creates a red spot, they painted a corresponding dot on the opposite arm so both looked identical.

One of the researchers then scratched each arm in turn. Unsurprisingly, scratching the itchy arm produced relief, while scratching the other one did not.

Sweet relief

Next, they placed a large vertical mirror in front of the itchy arm, blocking off the subject’s view of their right arm and reflecting back the non-itchy one in its place . They asked the volunteers to look only at the reflected limb in the mirror, whilst a member of the team again scratched each arm.

This time the participants felt relief when the unaffected, reflected arm was scratched.

Although the effect was relatively weak – the relief from mirror scratching is about a quarter of that from scratching the real itch – the study shows that visual signals to the brain can override messages from the body if there is a mismatch between them.

Moseley et al (2012) proposed a “cortical body matrix” which Lorimer wrote about over at BiM. The notion of the cortical body matrix is that of “A network of multisensory and homeostatic brain areas…a dynamic neural representation that not only extends beyond the body surface to integrate both somatotopic and peripersonal sensory data, but also integrates body-centred spatial sensory data and then integrates the whole lot with homeostatic and motor functions.”

The 2012 article is a brilliant synthesis and review of a lot of research carried out in the field – worth having a dig through the articles page over at BiM, around 2012.

– Tim Cocks

www.noigroup.com

Moseley GL, Gallace A andSpence C (2012) Bodily illusions in health and disease: Physiological and clinical perspectives and the concept of a cortical ‘body matrix’. Neurosci. Biobehav. Rev. 36 (2012) 34–46.