A well-known sensory illusion could help amputees acquire a sense of touch in, and ownership over, their prosthesis. Henrik Ehrsson (Karolinska Institute, Sweden) and colleagues showed that six out of 18 amputees experienced a strong sensation of touch from a rubber prosthesis during a version of the rubber-hand illusion. This is the effect whereby a rubber hand is placed where a participant's hand might be and is stroked in time with the stroking of the real but concealed hand, and observing this provokes a sensation of touch in the location of rubber hand.

The researchers demonstrated the illusion in amputees by stroking their stump in synchrony with stroking a rubber hand. The effects were measured by questionnaire and a pointing task, which required the participants to indicate where in space they experienced sensation from the stroking. The researchers also demonstrated the participants' sweat levels increased when the rubber hand was stabbed with a needle.
The strength of the illusion wasn't as great as has been shown with people without an amputation, but this is likely to be due to unavoidable constraints on the way the illusion was performed. For example, the illusion in its classic form is typically strongest when the rubber hand and real hand are stroked in the exact same areas - an impossibility with the amputee participants (although the researchers did exploit the fact some participants experienced sensations in a 'phantom' hand when specific parts of their stump were touched).

Ehrsson's team say it should be possible 'to design prosthesis equipped with tactile sensors in the fingertips that can be connected to an array of tactile simulators on the stump that would reproduce the present illusion in everyday usage... Every time the finger of the prosthesis touched an object a tactile stimulation would be delivered instantaneously to the stump, thereby tricking the multisensory brain into experiencing the sensation of touch from the artificial finger.' The researchers added that the illusion could also have psychological value by provoking a sense of ownership over the false limb.

The precise mechanism by which the illusion worked is not known. One possibility is that the correlation of visual information from the rubber hand with somatosensory information from the stump leads to re-mapping in multisensory areas of the brain, similar to the somatotopic reorganisation postulated to underlie the experience of the illusion in healthy people.

--Christian Jarrett