My Third Hand

…or, How a Scientist Threatened Me With a Knife.

As some of you may recall there was a study published a little while ago about some researchers from Karolinska Institute in Stockholm adding a new twist to the classic rubber hand experiment. I feel compelled to write about it here because of two reasons: 1. I know Arvid Guterstam, one of the researchers, and 2. I was one of the test subjects.

The classic version involves the test subject hiding her hand underneath a table while the experimenter stimulates her hand as well as a visible rubber prosthetic, for instance with a brush, to create an illusion of ownership of the prosthetic. Basically the experiment is all about tricking the brain into thinking a prosthetic is part of the body. What Arvid Guterstam and his colleagues at KI did differently was leaving the real hand visible on the table. So the test subject would see two right hands in front of her, one real, one rubber.

Both hands were stimulated with brushes, while the test subject — hell, I — was asked to focus my attention on the rubber hand. Then, at random intervals, the researcher — ok, Arvid — would bring out a huge scary kitchen knife and threaten to stab one of my hands.

My response was measured by skin conductivity on my left hand and there was also a questionnaire afterwards. In a press release from KI, Arvid says:

“What one could expect is that only one of the hands is experienced as ones own, presumably the real arm. But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of the body image, and the subjects experience having an extra third arm.”

In fact most test subjects had a very weird, uncanny feeling about it — like the brain wanted to incorporate the third hand into its body image but still felt there was something strange about the whole ordeal. Personally I must admit I can’t quite remember how I felt, as it’s quite a long time ago. Though I do remember that the kitchen knife was really scary, and Arvid said I reacted beautifully when it came to skin conductivity. Insert joke about sweaty palms here.

The classic experiment gave us hope that prosthetics can be fully incorporated into the body image of amputees. The new experiment opens for whole new possibilities: Maybe in the future we could adopt extra limbs as our own? I for one would love to have four arms! Just imagine the sophisticated levels of procrastination you could attain if you didn’t have to release the mouse and keyboard to scratch yourself and reach for the chocolate!

Finally, I can definitely recommend signing up to be research subjects if you have the opportunity. Getting to see how science is actually done is great fun (at least if you’re not the person who has to brush/threaten the hands of 154 people), and it’s far easier to get other people interested in a subject when you can talk about it from personal experience instead of academic interest. Plus, this particular bit of research was covered by Scientific American. Which sort of means I’m famous by proxy!

Cross-posted at

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  1. That experiment sounds kinda cool, Felicia.
    How does one find out about opportunities to be part of such experiments?

  2. @QuestionAuthority: I would suggest checking the websites of local psychology departments. They usually have some type of website set up for students to see what projects are accepting participants.

  3. @QuestionAuthority: I actually don’t really know the best way, I’ve gotten to do it through contacts. But @Siveambrai‘s idea sounds good to me.

    @Rei Malebario: Not to rain on your parade, but considering the illusion didn’t work if the rubber limb looked dissimilar from your own hand (like if it was replaced with a foot), I find it hard to believe we’ll be able to incorporate ridiculously long prehensile tails into our body image anytime soon. ;) But it WOULD be cool. I wouldn’t even have to get out of my chair to make new tea, I could just send my tail to the kitchen!

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