Neuroscientists at the Swedish Karolinska Institutet performed an experiment on healthy adult volunteers, in which they placed a realistic rubber right arm on a table next to the subject's real right arm, and covered it with a sheet, so the subject could not tell at a glance which arm was theirs at a glance. Then, as the volunteer watched, both of the right arms were rubbed with a small brush simultaneously. One would expect that a person would only feel one brush, the one touching their real hand. Instead, the brain gets confused, and resolves the issue by accepting both right hands as real. The person perceives having three arms, instead of two. They even reacted when either right hand was threatened with a kitchen knife, regardless of whether or not the hand was real.
It's a promising result for prosthetics and other medical applications, and those who are often at risk. If a limb can be accepted into the body image, as a person's own, then it can be controlled, theoretically, just as easily as a real limb. It's also just an interesting nuance to our understanding of how the mind interprets physical experiences. It's a fascinating demonstration of how little we really understand our own perception, and how flexible our brains can be.
Source: Science Daily-Scientists Create Illusion of Having Three Arms
*Apologies to those who haven't read Douglas Adams' Hitchhiker's Guide to the Universe, or who don't like it.