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Individual differences |
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A mirror box is a box with two mirrors in the center (one facing each way), invented by Vilayanur S. Ramachandran to help alleviate "phantom limb" pain, in which patients feel they still have a limb after having it amputated.
Based on the observation that phantom limb patients were much more likely to report paralyzed and painful phantoms if the actual limb had been paralyzed prior to amputation (for example, due to a brachial plexus avulsion), Ramachandran and Rogers-Ramachandran proposed the "learned paralysis" hypothesis of painful phantom limbs (Ramachandran & Blakeslee 1998). Their hypothesis was that every time the patient attempted to move the paralyzed limb, he or she received sensory feedback (through vision and proprioception) that the limb did not move. This feedback stamped itself into the brain circuitry through a process of Hebbian learning, so that, even when the limb was no longer present, the brain had learned that the limb (and subsequent phantom) was paralyzed. Often a phantom limb is painful because it is felt to be stuck in an uncomfortable or unnatural position, and the patient feels he or she cannot move it.
In order to retrain the brain, and thereby eliminate the learned paralysis, Ramachandran and Rogers-Ramachandran (Ramachandran, Rogers-Ramachandran & Cobb 1995) created the mirror box. The patient places his or her 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 unclench it from potentially painful positions. Because this visual feedback elicits kinesthetic sensations, Ramachandran and Rogers-Ramachandran (Ramachandran & Rogers-Ramachandran 1996) refer to this as a kind of visual-kinesthetic synesthesia, although this is true only in the broadest sense of the term.
Repeated training in some subjects has led to long-term improvement (Ramachandran & Blakeslee 1998), and in one exceptional case, even to the complete elimination of the phantom limb between the hand and the shoulder (so that the phantom hand was dangling from the shoulder). Subsequently, the use of the mirror box has been extended to rehabilitation of hemiparesis, or paralysis one side of the body, due to stroke (Altschuler et al. 1999) and to rehabilition of spatial neglect (Ramachandran et al. 1999). These extensions have met with moderate success, but further research is required to evaluate their clinical effectiveness.
Recently, Herta Flor and colleagues have performed functional neuroimaging experiments to explore the neural reorganization that leads to phantom limb pains. They find that telescoping phantoms lead to greater cortical reorganization (and associated pain) (Karl et al. 2004), while use of the mirror reverses these changes, and thereby leads to a reduction in pain (Flor et al. 2006). For example, in phantom limbs due to arm amputation, Flor and colleagues find that the distance the representation of the lip has been displaced predicts the degree of pain. However, with the mirror box, the representation of the lip is no longer displaced relative to its counterpart in the other (non-affected) hemisphere. That is, use of the mirror box can eliminate the remapping associated with phantom limb pains.
- Altschuler, E. L., Wisdom, S. B. & Stone, L., et al. (1999), "Rehabilitation of hemiparesis after stroke with a mirror.", Lancet, no. 353(9169), pp. 2035-2036
- Karl, A., Muhlnickel, W. & Kurth, R., et al. (2004), "Neuroelectric source imaging of steady-state movement-related cortical potentials in human upper extremity amputees with and without phantom limb pain", Pain, vol. 110, no. 1-2, pp. 90-102
- Flor, H., Diers, M. & Christmann, C., et al. (2006), "Mirror illusions of phantom hand movements. Brain activity mapped by fMRI", NeuroImage, vol. 31, pp. S159
- Ramachandran, V. S. , Altschuler, E. L. & Stone, L., et al. (1999), "Can mirrors alleviate visual hemineglect?", Medical Hypotheses, vol. 52, no. 4, pp. 303-305
- Ramachandran, V. S. , Rogers-Ramachandran, D. C. & Cobb, S. (1995), "Touching the phantom.", Nature, no. 377, pp. 489-490
- Ramachandran, V. S. & Rogers-Ramachandran, D. C. (1996), "Synaesthesia in phantom limbs induced with mirrors.", Proceedings of the Royal Society of London, no. 263(1369), pp. 377-386
- Ramachandran, V. S. & Blakeslee, S. (1998), Phantoms in the brain: Probing the mysteries of the human mind., William Morrow & Company, ISBN 0-688-15247-3
- Ramachandran, V. S. & Hirstein, W. (1998), "The perception of phantom limbs: The D.O. Hebb lecture.", Brain, vol. 9, no. 121, pp. 1603-1630
- V.S. Ramachandran's website
- WNYC - Radio Lab: Where Am I? (May 5, 2006) downloadable segment of radio program looks at historical examples and a present-day case of phantom limbs
- Ramachandran's Reith Lecture on Phantom Limbs
- Mirror box therapy web site
- Mirror glasses web site
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