The brain has a "mind map" of body parts. This sense of "self", of knowing which bit of the universe is you and which is not, is very important. If an arm or leg is amputated, but your "mind map" thinks the appendage is still there, you will experience phantom limb pain.
I came across an article in the April/May 2011 issue of the Australian magazine "Cosmos", page 24, titled "Ever wanted a third arm? It's all in your head" written by Benjamin Skuse, which goes the other way.
"Swedish scientists have managed to convince people that they own three arms, or that they're the size of a Barbie doll, using tricks in a laboratory setting.
"The research addresses some of the oldest philosophical and psychological questions about the relationship between body and mind, and has potential applications in prosthetics and robotics.
" "We demonstrated that the body image is much more flexible then previously thought, even allowing healthy participants to experience awareness of an extra third arm," said study author Arvid Guterstam from the Swedish medical university Karolinska Institutet.
"The experiment reported in PLos ONE involved sitting a participant at a table with a rubber prosthetic arm placed next to the right arm. Touching the subject's right hand and the rubber hand with two small brushes at corresponding locations, the scientists stimulated a feeling of owning the prosthetic arm by synchronising the strokes.
" "Instead of choosing to experience only one hand as your own, we surprisingly, found that the brain accepts both right hands as part of the body image," said Guterstam. On going projects question whether the perceived body can be shrunk to the size of a barbie doll, or if the brain can accept a body of a different sex. "This might have important bearing on the development of advanced prosthetics, where the patient can experience and control an extra arm," Guterstam said."
I seem to recall accounts from my reading in which people believed their body size had varied during an unusual experience. I'll try and locate some references on this, for a future post.
The full article may be read here.