Disturbing the Body Image
These findings fit perfectly with the current view of how the brain constructs a mental representation of the body. In the case of supernumerary phantom limb, the parts of the brain that relay body image-related sensory information to the SPL have been starved of oxygen. Cell death occurs, so the SPL is deprived of some the information it normally processes. This perturbs SPL function, and so distorts the body image. In this particular case, the brain's representation of the left arm has been duplicated, and incorporated into the mental scheme of the body.
In BIID, the situation is apparently reversed: the body image is missing a representation of the affected limb. This body image distortion is almost certainly congenital—the body image is probably “hard-wired” in the brain during development, because children born with missing limbs sometimes experience phantom limb syndrome. Thus, the brains of apotemnophiles apparently fail to generate a representation of the affected limb, because of some aberrant developmental mechanism. The limb has never been a component of the body image, so the afflicted person grows up thinking that it feels "wrong.”
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His latest book is How We Decide.