Virtual Reality as a Rehabilitative Technology for Phantom Limb Experience

A pilot study

The relationship between phantom limb pain, prosthesis use and psychological well-being is an intimate one. For instance, significant correlations have been observed between adjustment to amputation and pain (Katz, 1992; Marshall, Helmes and Deathe, 1992), with adjustment to amputation less likely as levels of pain increase. Amputees with PLP are less likely to use a prosthetic limb (Dolezal, Vernick, Khan, Lutz and Tindall, 1998). In a survey of 685 lower-limb amputees, Knight and Urquhart (1989) found 38% of the sample did not use a prosthesis with pain being cited as the primary reason. Non-prosthesis use often results in the restriction of normal activities (such as self-care, visiting friends and carrying out domestic work), and is associated with higher levels of depression (Williamson, Schulz, Bridges, and Behan, 1994). The problem of PLP then is large and pervasive in many amputees' lives.

While a range of pharmaceutical, surgical and psychological interventions are used to treat PLP, the success of these approaches is often limited and short-term. However, one promising development in this regard was reported by Ramachandran (1993). Ramachandran (1993) created a mirror box made by placing a vertical mirror inside a cardboard box with the top removed, in which the amputee places their remaining anatomical limb inside and views a reflection in the visual space occupied by their phantom limb. He reports anecdotal evidence that the box was able to induce in patients vivid sensations of movement originating from the muscles and joints of their phantom limb. For some patients their phantom limb pain was relieved and others were able to gain control over 'paralysed' phantoms'. The mirror box has also recently been used with similar success with lower-limb amputees, where viewing a reflection of an anatomical limb in the phenomenal space of a phantom limb resulted in amputees reporting a significantly greater number of movements of their phantom limb than with attempted movement alone (Brodie, Whyte and Waller, 2003). Ramachandran himself recognized that a more controlled study was required to determine if this was a placebo effect or the direct result of providing visual feedback with the mirror. While the above work and theory indicates that the mirror box may be an effective treatment for negative phantom limb experience, as yet there are no controlled studies which have explored the number and lengths of mirror box sessions necessary to effect change, how long such change lasts for, which types of amputation and phantom limb phenomenology respond best, psychological variables which predict who will respond best to such therapy, and any potential negative responses to mirror box therapy.

Blakemore, Wolpert and Frifth (2002) explain the mirror box phenomenon in terms of a central nervous system internal forward model in which the body and its interaction with the world are represented. The forward model predicts the sensory consequences of motor commands whenever movements are made. This means that the normal experience of a limb is based upon a predicted rather than an actual state. In the absence of a limb motor commands are still made, so that if a prediction of movement is made then movement will be experienced in a phantom limb. However, because the limb does not actually move there is a discrepancy between these predicted and actual states. With time the forward models will adapt to this situation, so that movement is no longer experienced in a phantom even when motor commands to do so are issued. Therefore, when Ramachandran found that a mirror-box was able to restore voluntary movement of a phantom limb, then, according to Blakemore, Wolpert and Frifth (2002), this was because the forward models were updated. The efference copy produced in parallel with the motor commands generates changes in the predicted position of the amputated limb that matched what the amputee had seen in the mirror.

The above work and theory on the mirror box suggest that other visual therapies that work in similar ways may also relieve phantom limp pain as well as increasing volitional movement in phantom limbs. One example of this would be virtual reality (VR) technology, such as the combination of an immersive head- mounted display, instrumented peripheral devices and computer graphics. The proposed research is intended to build upon the insights of Ramachandran's mirror box by producing a similar phenomenon using virtual technologies. VR offers an opportunity to provide a visual representation of the amputee's whole body, including their phantom limb. Unlike the mirror box, which confines participants' limbs to a narrow spatial dimension, VR enables complex hand-eye coordination, and both fine and gross motor movements of the fingers, hand and arm, and toes, feet and legs. Users of such virtual limbs can engage in tasks made impossible by the mirror box, such as pegboard tasks, racket games, ball games, etc. It is hope that such VEs will prove to be a therapeutic treatment for phantom limb pain, as well as aiding successful prosthesis use.

This project aims to produce virtual facsimiles of amputees' phantom limbs; to obtain appropriate measurements that enable conclusions to be reached about the efficacy of VR in the treatment of phantom pain; and to obtain appropriate measurements that enable conclusions to be reached about the efficacy of VR in decreasing body image dissatisfaction and encouraging and enabling successful prosthesis use.