Distinguishing my hand from a fake rubber hand on a table should be easy. It also should be easy to see the difference between my face and someone else's face in a mirror. But sometimes, this can be tricky. Sometimes the boundary between self and other can blur and even vanish completely.

Check out the rubber hand illusion for a breakdown in the self-object boundary. In the rubber hand illusion, first you place your hand and arm on a table. Then an experimenter hides your hand behind a curtain so you can't see it. Next a rubber hand is placed on the table in front of you. You can see the fake rubber hand, but not your real hand (see a picture of the set up to the side of this post). So far it's easy to see that the rubber hand is a fake. But then the experimenter strokes both your hand and the fake rubber hand with a paint brush. In the critical condition, the two hands are stroked synchronously - that is, every time your hidden hand is stroked, you see the fake rubber hand being stroked. In the control condition, the stroking is out of sync. When synchronously stroked for a few minutes, you will start to see the rubber hand as your hand. Weird! You feel the stroking occurring at the location of the rubber hand you see. If you close your eyes and point at your hand, you'll point to the rubber hand. Really weird! If the experimenter threatens or harms the rubber hand, you feel that your hand has been threatened or harmed (experimenters have really abused that rubber hand - they've bent fingers backwards, stabbed at it, and hit it with a hammer). You get sweaty palms and show similar neural responses just as if your real hand was harmed. Now that is really weird. (To see a video demo of the basic illusion, click here.) That is the rubber hand illusion - you start to see and treat the rubber hand as if it is your hand.

Rubber Hand Illusion Set Up

When Botvinick and Cohen first demonstrated the illusion, they argued that vision is crucial for identifying one's body. You see where your hands and feet are as much, if not more, than you feel where they are. We see objects that move with us or that we move as being part of us (thus I described in an earlier post how I am my toothbrush and my car). The cool thing is that the boundary between the self I perceive and the world around me is malleable. I can see a rubber hand as being my hand, as being part of my body, as part of me.

In a paper published this year, Paladino and colleagues extended this type of illusion to identifying with another person - seeing that person as part of the self. Similar to the rubber hand illusion, they stroked a person's cheek while that person watched a video of someone else being stroked on the cheek. After a few minutes of being synchronously stroked by paintbrushes, the participants rated that other person as being more like them and they reported feeling a little like they were seeing a video of themselves. Perhaps the beginnings of an out-of-body experience.

Paladino and colleagues argued that this may contribute to how people identify with others. We move together. We have the same experiences. We respond the same way. Similar actions + similar experiences = similar people.

I'm reminded of the classic comedy gag in which someone acts like another person in a mirror. I'm sure this has been too many times to count. I love the version with Lucille Ball and Harpo Marx (check out the Lucy and Harpo video here or a really old version with Groucho Marx). Harpo is not sure if he is seeing himself in a mirror or if it is a different person. It's funny that he can be confused. How can he fail to recognize that the person in the mirror isn't himself? If the person in the mirror moves in sync with him, the illusion may be easy to understand. It is him.

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