Neuronal Principles of Mirror Therapy for the Treatment of Chronic Pain
The perception of one's own body requires the involvement of several sensory modalities, and the integration of somatosensory and visual input is considered to be the prerequisite for higher-order body experiences. The sense of self has been shown to be altered in chronic pain states such as phantom pain after the amputation of a limb, which might at least partly be caused by the co-occurring reorganization of the body representation in primary somatosensory cortex. Thus, the cortical representation of the affected body part is invaded by adjacent functional areas, which represents dysfunctional neuronal plasticity caused by the discontinuation of sensory input. These alterations reduce cortical responsiveness contralateral to the affected limb  and are further associated with pain intensity , suggesting that a correction of the body representation might likewise alleviate chronic pain. Thus, it has been shown that eliciting the illusion of two intact limbs by performing a multi-week training of movements in front of a mirror was sufficient to alleviate painful sensations in several chronic pain states, while executing movements without a mirror had no effect . Even in chronic pain patients, mirror therapy has been shown to be sufficient to cause normalization in the cortical representation of the body in primary somatosensory cortex which is positively correlated with effectiveness . It is believed that the multisensory input re-activates the former limb representation which causes the re-reorganization of the cortex. However, the perception of the patients plays an important role in this process, since the ability to relate the mirrored image to the affected limb significantly predicts the effect size of mirror therapy. Further, the presence of perceived distortions in the affected limb in terms of shape, length, or mobility significantly reduces effectiveness, emphasizing the importance of body perception and the integration of sensory input for this kind of therapy. By implementing recent developments in the field of virtual and augmented reality techniques , the mirror therapy approach could be modified to solve potential discrepancies between perceived and mirrored limb in order to maximize the effectiveness. In conclusion, the mirror therapy approach represents a promising procedure to alleviate chronic pain, which is not restricted to amputees but also other pain disorders accompanied by a distorted body representation, such as complex regional pain syndrome or fibromyalgia.
 Chan BL, Witt R, Charrow AP, Magee A, Howard R, Pasquina PF, Heilman KM, Tsao JW (2007). Mirror therapy for phantom limb pain. N Engl J Med 257:2206-7.
 Diers M, Christmann C, Koeppe C, Ruf M, Flor H (2010). Mirrored, imagined and executed movements differentially activate sensorimotor cortex in amputees with and without phantom limb pain. Pain 149:296-304.
 Flor H, Elbert T, Knecht S, Wienbruch C, Pantev C, Birbaumer N, Larbig W, Taub E (1995). Phantom-limb pain as a perceptual correlate of cortical reorganization following arm amputation. Nature 375: 481-4.
 Foell J, Bekrater-Bodmann R, Diers M, Flor H (2014). Mirror therapy for phantom limb pain: brain changes and the role of body representation. Eur J Pain 18:729-39.
 Trojan J, Diers M, Fuchs X, Bach F, Bekrater-Bodmann R, Foell J, Kamping S, Rance M, Maaß H, Flor H (2014). An augmented reality home-training system based on the mirror training and imagery approach. Behav Res Methods 46:634-40.