How do you convince someone that a finger they can't see or feel – one they don't even know is there – is actually part of their body? Turns out it's all in the wrist.
The technique is a spin on the rubber hand illusion, developed almost 15 years ago. To perform the original trick, sit someone at a table and somehow hide one of their hands from their view. Then put a corresponding rubber hand on the table in front of them and stroke it while also stroking the real hand unseen. Bizarrely, they will often feel that the rubber hand is their own.
Besides being a cool party trick, this illusion revealed a novel insight into how the brain develops its sense of "owning" body parts. It quickly led to treatments for conditions in which that sense is disrupted, such as phantom limb syndrome.
Since then, the illusion has been tested thoroughly to find exactly what is needed for it to occur. We now know that the trick works using a rubber hand with a different colour skin to the participant and even without a rubber hand at all. You can do it just by making a person think you're going to stroke their hand. It's even been done in virtual reality.
The theory emerging from these experiments is that if two different senses – like sight and touch – both suggest a rubber hand is yours, then your brain is convinced.
Now, Simon Gandevia from Neuroscience Research Australia in Sydney has shown that you can induce the illusion using sensory information from receptors in muscles only.
In Gandevia's new version of the illusion, a participant first has both hands anaesthetised. Then the volunteer puts both hands under a screen so that they cannot see what is happening to them.
Under the screen is a finger-sized tube that can swivel, connected to a fake finger 12 centimetres above (see diagram). A researcher puts the participant's right index finger into the tube, then presses the participant's left thumb and index finger onto the fake finger. The experimenter then manipulates the participant's left thumb and finger to wiggle the fake finger back and forth, thereby also moving the real right index finger.
After a few minutes of wiggling, the illusion takes hold: most participants say that they feel their left hand is gripping their right index finger and moving it.
Because the hands are anaesthetised and it's all hidden from view, the only sensory information the participant gets comes from receptors in the muscles that move the fingers, which are located near the wrist. These receptors tell you where your fingers are when you're not looking.
What's the right hand doing?
When Gandevia disconnected the link between the fake finger and the real one – so the movements didn't match – the illusion disappeared. This shows the matching movement of both hands, sensed in muscle receptors, is what's generating the illusion, he says.
A better understanding of what sensory inputs generate our brain's map of the body should lead to treatments for conditions where that map is wrong, says Gandevia. "Altering any of those signals might help you put the body image back together or distort it in a way that you might want clinically."
The study shows how flexible our sense of the body is, says Jonathan Cole from the University of Bournemouth in the UK. He says that different sensory inputs will be used in different contexts, and depending on what information the brain is receiving. "What one has to do is lots of differing experiments to see what the brain uses to 'fill in' information about the body under which circumstance," he says.
Journal reference: Journal of Physiology, in press
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