Phantom limb pain occurs to some degree in over 90% of limb amputees, and may be severe. Some theories of the pathogenesis of phantom pain suggest it occurs due to activation of pain systems from a conflict between visual feedback and proprioceptive representations of the amputated limb. Because of this, mirror systems have been used, often successfully, to treat phantom pain via activation of mirror neurons in the hemisphere of the brain that is contralateral to the amputated limb, which may then suppress the release of pain neuron activation that (for unknown reasons) occurs when there is a lack of activation of contralateral sensory and motor areas.
The "mirror box" was invented to facilitate such treatment. It consists of a box in which the patient may view a mirror image of their existing limb in a context which makes it appear to be the missing limb restored. Wikipedia notes that "in a mirror box the patient places the good limb into one side, and the stump into the other. The patient then looks into the mirror on the side with good limb and makes "mirror symmetric" movements, as a symphony conductor might, or as we do when we clap our hands. Because the subject is seeing the reflected image of the good hand moving, it appears as if the phantom limb is also moving. Through the use of this artificial visual feedback it becomes possible for the patient to "move" the phantom limb, and to un-clench it from potentially painful positions." Mere imagery of using the amputated limb does not work--the body must be "fooled" into an active perception that the amputated limb is being used normally once more. This is akin to the difference between imagination and illusion in our perceptions, with different brain activation between the two.
In the study below, augmented reality with the amputated limb appearing restored on the computer video screen, rather than in a mirror of the other limb, was shown in a single patient to work in pain relief in a patient in whom the mirror box had failed. Electrodes on the stump were used to control a video simulation of the missing limb. If this proves also to work in other amputees, it would indicate that one not need move the healthy limb to activate mirror neurons, and indeed since residual stump activation was used, one may not need to use the mirror neurons at all, but rather properly activate the appropriate zones of contralateral primary motor and sensory cortex for the amputated limb region of the body (that part of the homunculi of the brain). In that case, the mirror neurons activated in the mirror box treatment program are just used as a pathway to contralateral cortical activation, as a means, not a end.
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ABSTRACT
Max Ortiz-Catalan, Nichlas Sander, Morten B. Kristoffersen, Bo Håkansson and Rickard Brånemark
Treatment of phantom limb pain (PLP) based on augmented reality and gaming controlled by myoelectric pattern recognition: a case study of a chronic PLP patient
Front. Neurosci., 25 February 2014 | doi: 10.3389/fnins.2014.00024
A variety of treatments have been historically used to alleviate phantom limb pain (PLP) with varying efficacy. Recently, virtual reality (VR) has been employed as a more sophisticated mirror therapy. Despite the advantages of VR over a conventional mirror, this approach has retained the use of the contralateral limb and is therefore restricted to unilateral amputees. Moreover, this strategy disregards the actual effort made by the patient to produce phantom motions. In this work, we investigate a treatment in which the virtual limb responds directly to myoelectric activity at the stump, while the illusion of a restored limb is enhanced through augmented reality (AR). Further, phantom motions are facilitated and encouraged through gaming. The proposed set of technologies was administered to a chronic PLP patient who has shown resistance to a variety of treatments (including mirror therapy) for 48 years. Individual and simultaneous phantom movements were predicted using myoelectric pattern recognition and were then used as input for VR and AR environments, as well as for a racing game. The sustained level of pain reported by the patient was gradually reduced to complete pain-free periods. The phantom posture initially reported as a strongly closed fist was gradually relaxed, interestingly resembling the neutral posture displayed by the virtual limb. The patient acquired the ability to freely move his phantom limb, and a telescopic effect was observed where the position of the phantom hand was restored to the anatomically correct distance. More importantly, the effect of the interventions was positively and noticeably perceived by the patient and his relatives. Despite the limitation of a single case study, the successful results of the proposed system in a patient for whom other medical and non-medical treatments have been ineffective justifies and motivates further investigation in a wider study.
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