Avid Guterstam, Valeria Petkova and Henrik Ehrsson form the Karolinska Institutet, Sweden, gave a group of 154 volunteers the rather odd sensation of having three arms.
First, the volunteers were seated at a table with a realistic looking right arm carefully positioned next to their real right arm. The experimenter then stroked and touched both right arms (real and rubber) in an identical manner. You can see this part of the experiment in the picture below. This stimulation caused the subjects to experience both right arms as belonging to their own bodies.
Third-arm-illusion experiment conducted by Arvid Guterstam.
Credit: © Henrik Ehrsson.
The next part of the experiment makes me wonder whether the volunteers knew what they were in for. To test how completely the subjects associated the rubber arms with their own bodies, the scientists proceeded to threaten each arm with a kitchen knife. This somewhat bizarre methodology was accompanied by physiological stress tests. The volunteers were just as freaked out by the thought of the experimenters stabbing their fake hands as their real ones, indicating that they really did feel like they possessed three biological arms.
The scientists see two future benefits to tricking the brain in this manner. We could offer stroke or amputee patients synthetic arms that would feel just like the ones they lost. We could also provide extra arms for people who are too busy to manage with just the two.