The Rubber-Hand Illusion and Representation Using Similarity (1)

Glenn Carruthers
photographed by Patrick Wilken

Here's something cool we can do: we can make people feel like they experience touch in a lifeless prosthetic hand and even that the prosthesis is their own hand. Not only is this pretty cool in itself, but it actually can be used to tell us quite a bit about the mind. We like this illusion, called the rubber hand illusion, because it lets us alter the experience of a hand being a part of one's body. Before the illusion was discovered this really only occurred following stroke or other forms of traumatic injury to the brain, in a delusion know as somatoparaphrenia sufferers of which claim that one of their body parts belongs to someone else.

The discovery of the illusion gave us a way to experiment on this kind of experience. We also like the illusion because it involves, as we will see, the use of information from various different senses to produce a representation of a single object. It thus gives us another way of experimenting with how the senses interact. More than this it gives us a way to experimentally manipulate an aspect of self-consciousness which I call “the sense of embodiment”. Others call this, misleadingly I have argued, “the sense of ownership”. Here it doesn't really matter what we call it, what matters is that it is the experience of a body part (or anything else for that matter) being a part of oneself.

To experience the illusion, in its simplest form, you sit with one hand on a table, e.g. your left, and behind a screen so you can't see it. A prosthetic hand is placed in front of you, in a place where you could get your left hand. An experimenter then strokes your left hand in synchrony with the prosthetic, while you watch the prosthetic. After a few seconds to minutes most of you (around about 80%) will come to experience the touch as if it's coming from the rubber hand and experience the rubber hand as if it where your own. More precisely subject's give increased agree ratings to questionnaire items like “It felt the rubber hand was my own hand” than when the seen and felt stroking is asynchronous. In contrast control questions like “it felt like I had two hands” don't change between these conditions.

If we can work out how this illusion occurs it will help us understand the sense of embodiment, which is the bit I'm particularly interested in. So, why might you feel a sense of embodiment for the rubber hand in this, and similar, set ups? Previously I have suggested that the synchrony of the stimulus to the real and rubber hand causes the rubber hand to be represented in a manner sufficiently similar to the real hand that it has some of the same effects on processing as a representation of the real hand can have. In essence the representation of the rubber hand is enough like a representation of a real hand that it can play a similar role in your mind. Put more intuitively (and to treat things a little too much in terms of absolutes) the rubber hand is mistaken for a real hand and so you think about it in similar ways – including eliciting a sense of embodiment in it.

You'll hear more about the rubber hand illusion in my second post next week.