Amputees who experience "phantom limb pain" could find relief with a 3D virtual reality system that creates the illusion that they can once again control their missing limb.

Stephen Pettifer, from the University of Manchester in the UK, developed the system with colleagues Craig Murray and Toby Howard.

The person using the system wears a head-mounted display and operates a controller with their remaining limb. The system lets them control a virtual arm or a leg, which is shown in 3D in place of their missing limb.

Many people who lose a limb experience the sensation that it is still attached to their body. This is also often accompanied by a feeling of pain in the missing appendage – a condition that can be very severely debilitating.

Brain rewiring

Phantom limb pain is thought to relate to a "rewiring" of different parts of the brain associated with physical stimulus. Signals from other parts of the body may inadvertently be fed into parts of the brain that relate to a missing limb.

Drugs can be used to treat the condition, but Vilayanur Ramachandran, based at the University of California, San Diego, US, also discovered that the illusion of a missing limb can help reduce pain. Ramachandran developed a "mirror box" that creates the visual illusion of two hands for people who actually only have one (see They do it with mirrors).

Pettifer's team tested their high-tech version of the mirror box on five patients suffering from severe phantom limb pain. Each was asked to use their virtual limbs to perform simple tasks, like catching a virtual ball or stepping on highlighted squares. Four out of the five patients said that pain decreased after using the system, although the amount of pain relief was not quantified.

Complete illusion

The researchers believe that the technique may work by somehow activating nerve pathways in the brain that were linked to the missing limb, but that became dormant after its loss.

Brain areas tied to the remaining limb may try to compensate for the dormant side, perhaps creating phantom limb pain. The virtual system may prevent this compensation by reactivating the dormant areas, they speculate.

Pettifer admits that the results are preliminary but says the experiment shows promise. "These were all people who have stubborn conditions," he told New Scientist.

He adds that the virtual reality system may have advantages over the mirror box because it creates a more complete illusion for a user.

The researchers intend to test their system on more patients to discover which patients it is most effective for.

Details of the research will appear in a forthcoming edition of the International Journal of Disability and Human Development.