Cortical plasticity as a basis of phantom limb pain: Fact or fiction?
Introduction
There is considerable evidence that phantom limb pain is related to changes in the somatotopic map in primary sensory and motor cortex, although causal evidence is lacking and it has been debated whether maladaptive cortical plasticity or preserved function of the representation of the limb contribute to pain (cf., Flor et al., 2006, Flor et al., 2013, Makin et al., 2013a, Makin et al., 2015). In this review, we present evidence for the central changes related to phantom limb pain and discuss their interaction with peripheral factors. In addition, we evaluate the role of methodological aspects of assessing cortical reorganization, type of experimental task (sensory, motor or both) and the role of body perception and use-dependent plasticity. We also address the role of psychological factors and how they relate to phantom pain. A better understanding of how these factors interact could help to understand differences between studies and could advance the analysis of mechanisms of phantom limb pain. Finally, we review some training interventions for phantom limb pain, aiming at inducing changes in the perception of the phantom limb and we discuss their contribution to our current understanding of phantom pain.
Section snippets
Perspectives on the neural basis of phantom limb pain
Neural plasticity is generally viewed as an adaptive learning process enabling the cortex to redistribute computational resources to focus on brain regions containing behaviorally relevant information. For example, the cortical representation of the ventral body surface is expanded in nursing rats (Xerri et al., 1994). Map expansions also occur in humans following extensive sensory and sensorimotor training (e.g., Merzenich et al., 1990, Recanzone et al., 1992, Elbert et al., 1995, Molina-Luna
Context matters
It is important to emphasize conceptual differences across studies in assessing cortical reorganization following amputation. While some studies used various types of phantom movements (imagined, executed, or a combination), other studies used innocuous stimulation at body sites represented adjacent to the former hand area, movement of adjacent body parts such as the mouth or illusory movement, such as that related to mirrored movements.
For example, in a functional magnetic resonance imaging
Cortical plasticity and phantom limb pain: clinical implications
Several lines of evidence based on models of plasticity in the “affected” sensorimotor cortex suggest that reorganization processes occurring after amputation are reversible, offering potential for therapy. For instance, a recent study aimed to relieve phantom pain in brachial plexus avulsion patients by restoring functional integrity of the phantom limb representation in the sensorimotor cortex using a brain machine interface and real-time magnetoencephalography (MEG) (Yanagisawa et al., 2016
Conclusion
Different models have been proposed to explain the neural basis of phantom pain. We propose that topographic shifts and preserved representations are not necessarily mutually exclusive; however, clear causal evidence in support of either model of the neural basis of phantom pain is still lacking. Furthermore, the role of peripheral changes in the maintenance of phantom pain needs to be clarified also in light of computational models that suggest that central and peripheral changes are
Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft (SFB1158/B07) awarded to H.F. and J.A. and an Advanced Grant (230249) from the European Research Council awarded to H.F. The authors declare no conflict of interest.
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