The Marble Hand Illusion


Moshe Feldenkrais said "we move in accordance with our self image." A new study provides some cool evidence that our self-image is easily modified, and that the modifications have physiological consequences. The illusion was created quite simply. Researchers repeatedly struck someone's hand very gently with a tiny hammer. Each time the hand was struck, the natural sound of the hammer on the hand was replaced with the sound of a hammer striking marble. After a while, subjects felt that their hands were heavier, stiffer, harder, less natural, and generally weird.

Further, when subjects were presented with a threatening stimulus in the form of a needle nearby the hand, there was an enhanced galvanic response. This reminds me of the effects of a "rubber hand illusion" which results in the neglected hand becoming colder and more subject to inflammation.

I find these illusions interesting not just because they are fun party tricks. (Which they are.) But also because they tell us something about how we form perceptions about the body.

We tend to assume that we hear only with our ears, see with our eyes, taste with our tongue, and feel pain with our nociceptors. But in fact our perceptions are formed through a process of multisensory integration. This means that, for example, sensory information from the eyes can change how something tastes, or that sensory information from the ears can affect how something feels.

The perceptions of threat which lead to pain are also created through a process of multisensory integration. As Lorimer Moseley puts it:

once a danger message arrives at the brain, it has to answer a very important question: “How dangerous is this really?” In order to respond, the brain draws on every piece of credible information previous exposure, cultural influences, knowledge, other sensory cues – the list is endless.

The marble illusion study is evidence that the list of inputs the brain uses to form an image of the body is long indeed. Apparently, the brain even considers what a hand sounds like in determining how it should feel.

Thus, the self image is a plastic construct that is modified in surprising ways. The image is built from information and can change when new information is received. The new information might arrive through the eyes, ears, nose, skin, joints, muscles, through memories or past experience. Or by what a therapist tells a client about the state of their body.

What kind of an image does someone start to form about their body if they hear from an authority figure that their discs can slip, that they have the neck of a seventy year old, that their core is unstable, that they have adhesions and scar tissue? How does the image change if they are provided information indicating that their body is flexible, strong, safe, healthy, and coordinated? You never know!

If you find this topic interesting and would like more information on what illusions can tell us about pain, here are some other articles that address this subject.  

The Thermal Grill Illusion

Review of Lorimer Moseley Conference Part Three

The Rubber Hand Illusion

More on Pain and Illusions

Watch Your Back: Mirrors Reduce Back Pain