The rubber-hand illusion

Neuroscience is to some extent an exercise in reverse engineering the brain. Reverse engineering means trying to understand how a complex device works without knowledge of its design or access to the owner’s manual. One of the basic tools of reverse engineering is to disable one part of the machine and then see what effect that has on the machine’s function.

In trying to understand how the brain works, scientists can’t just disable parts of people’s brains and then see what happens. But they can do some harmless tricks to cause the brain to malfunction a little. This is why neuroscientists like studying illusions. An illusion is basically a failure of the brain to perform one of its most basic tasks – creating a subjective experience which corresponds to reality. So by understanding how and why the brain can fail in this function, you can better understand how the function is performed.

The rubber hand illusion reveals some interesting facts about the way the brain creates the self image. It also might make a fairly entertaining party trick, provided that you have decided to bring a rubber hand to a party. Here’s how it works.

The Illusion
The victim of the illusion places both hands down on a table. The left hand is placed on the outside of a screen to block it from view, and a rubber hand is placed on the inside of the screen in a place that makes it look roughly like the left hand. So the subject looks down at the table and sees his right hand and a rubber hand where the left would normally be. The real left hand is out of sight.

Then the illusionist strokes the rubber hand and the hidden hand at the same time on the same finger. So the subject now sees the rubber hand being stroked on the ring finger while at the same time feeling her left hand being stroked on the ring finger. After a few strokes the subject gets the uncanny feeling that the rubber hand belongs to her. In fact, the subject will flinch when the rubber hand is threatened. So, the brain makes a big mistake by basically taking ownership of something it does not own. What does this tell us about how the brain creates the self image?