There is nothing simple about the brain, or about the perception of self and other. In this case, though, at least one thing is clear: The premotor cortex is directing the misperception.
The premotor cortex, located in the motor cortex of the frontal lobe, plays a central role in body ownership. It guides the recognition that this arm is mine, the arm intertwined with mine is his, and the hammer I'm holding is an external object.
The rubber hand illusion has provided evidence of how the premotor cortex uses data to determine ownership. It apparently relies on multiple channels of sensory input, and it weights those channels as more or less reliable in making its decision.
The study plays with the senses of vision and touch: Visually, the subject can't see her own hand, but she can see the rubber hand, which is about where her hand would be if it were there; the fake hand looks a lot like hers; and it's being stroked. Tactilely, she feels her own, hidden hand being stroked, and it feels just like the stroking she can see.
The premotor cortex takes this in. (Positional input plays a role here, too, which is why the hand needs to be placed in a realistic location; the subject needs to perceive it as a viable stand-in for her own hand.) It integrates the visual data and the tactile data, which don't quite match up, and attempts to make sense of it all.
When faced with discrepancies between visual and tactile input, the brain tends to place higher value on the visual [source: Kruglinski]. The result in the rubber hand illusion goes something like this: I see a hand that looks a bit like mine, and I see someone stroking it. I feel my own hand being stroked. The hand I see being stroked must be my hand. That the subject has no tactile sense of the rubber hand is, in effect, tossed out of the equation.
Beyond explaining the brainwork behind a really cool party trick, this insight into the brain's integration and management of multiple, possibly contradictory sensory inputs might open the door to a greater understanding of a wide range of diseases with symptoms of body- and self-related distortions, including schizophrenia, phantom-limb syndrome, body dysmorphic disorder, anorexia and, of course, stroke [sources: Science Daily, Ravilious]. It could eventually tell us precisely why a particular stroke victim may no longer recognize her right arm as her own -- and maybe even how to put her perceived body back together.
For more information on body ownership, distortions of self, and a whole bunch of related illusions, check out the links on the next page.