The ‘rubber hand illusion’ is where we can be fooled into feeling a sensation in a fake hand. A group of researchers have used this same technique with arm amputees and found that they can induce sensations that seem to be located in the rubber hand even in people who have had their real hand amputated.
The study has just been published online by the neurology journal Brain, and it could have important implications for the development of prosthetic limbs that can relay touch sensations which could seem to be experienced in the mechanical fingers.
The study is from the same team that recently hit the headlines with their virtual reality ‘body swapping’ study, which, like the ‘rubber hand illusion’, is based on the same general principal.
This is the now widely replicated finding that when we see a fake but convincing body part being touched, and we feel a genuine sensation on the actual body part, our brain ‘moves’ the sensation to where the fake body part is.
The ‘body swapping’ study used camera trickery to do this – each person had a camera by their eyes but had goggles which displayed what the person sitting opposite saw. When they shook hands, each person saw themselves from the other person’s perspective and with the genuine touch from the handshake, it produced the illusion that the person was located ‘inside’ their opposite.
However, the ‘rubber hand illusion’ is a much simpler way of producing a similar effect. It requires that you sit with your real hand out of sight, under the table perhaps, and a rubber arm placed on top of the table as if it were in the natural position of your limb.
When both the real hand and the false hand are touched in an identical way, such as the little finger being stroked with a pen, the sensation seems to be located in the rubber hand, despite the fact you know it to be fake and you’re aware your real hand is under the table. There’s a video of it online if you want to see an example of the set-up.
This is obviously a little difficult to do with people who have had their arm amputated, but the researchers used the same procedure but stroked the stump of the amputated limb.
Probably because this stimulates the remaining nerve fibres, the same illusion was triggered, and the sensation ‘moved’ to the rubber hand.
To check the effect wasn’t just the participants saying what the experimenters wanted to hear, they wired the participants up to a skin conductance measure – something known to increase when people are stressed.
They then stabbed the rubber hand with a syringe. When this happened after the illusion was induced, the stress response was significantly greater, indicating that the effect was real and compelling enough to increase anxiety.
Interestingly, the illusion was weaker in people who had their hand amputated for longer periods. This is likely due to the fact that the mapping of how brain areas represent body parts slowly rearranges after amputation.
It continues rearranging over time and areas previously used to represent the hand start to be used for representing other existing body parts, making the illusion less compelling. This also explains why phantom limbs often fade or ‘warp’ over time.
UPDATE: I’ve just noticed that Scientific American has a good brief article on the rubber hand illusion that appeared this month.
UPDATE TWO: Neurophilosophy also takes a look at the study and actually does a better job than me!
Link to open-access study from Brain.