Phantom sensations are experienced by almost every person who has lost their hand in adulthood. This mysterious phenomenon spans the full range of bodily sensations, including the sense of touch, temperature, movement, and even the sense of wetness. For a majority of upper-limb amputees, these sensations will also be at times unpleasant, painful, and for some even excruciating to the point of debilitating, causing a serious clinical problem, termed phantom limb pain (PLP). Considering the sensory organs (the receptors in the skin, muscle or tendon) are physically missing, in order to understand the origins of phantom sensations and pain the potential causes must be studied at the level of the nervous system, and the brain in particular. This raises the question of what happens to a fully developed part of the brain that becomes functionally redundant (e.g. the sensorimotor hand area after arm amputation). Relatedly, what happens to the brain representation of a body part that becomes overused (e.g. the intact hand, on which most amputees heavily rely for completing daily tasks)? Classical studies in animals show that the brain territory in primary somatosensory cortex (S1) that was “freed up” due to input loss (hereafter deprivation) becomes activated by other body part representations, those neighboring the deprived cortex. If neural resources in the deprived hand area get redistributed to facilitate the representation of other body parts following amputation, how does this process relate to persistent phantom sensation arising from the amputated hand? Subsequent work in humans, mostly with noninvasive neuroimaging and brain stimulation techniques, have expanded on the initial observations of cortical remapping in two important ways. First, research with humans allows us to study the perceptual consequence of remapping, particularly with regards to phantom sensations and pain. Second, by considering the various compensatory strategies amputees adopt in order to account for their disability, including overuse of their intact hand and learning to use an artificial limb, use-dependent plasticity can also be studied in amputees, as well as its relationship to deprivation-triggered plasticity. Both of these topics are of great clinical value, as these could inform clinicians how to treat PLP, and how to facilitate rehabilitation and prosthesis usage in particular. Moreover, research in humans provides new insight into the role of remapping and persistent representation in facilitating (or hindering) the realization of emerging technologies for artificial limb devices, with special emphasis on the role of embodiment. Together, this research affords a more comprehensive outlook at the functional consequences of cortical remapping in amputees’ primary sensorimotor cortex.
Tamar Makin and London Plasticity Lab
Karim Fouad, Abel Torres-Espín, and Keith K. Fenrich
Spinal cord injury results in a wide range of behavioral changes including impaired motor and sensory function, autonomic dysfunction, spasticity, and depression. Currently, restoring lost motor function is the most actively studied and sought-after goal of spinal cord injury research. This research is rooted in the fact that although self-repair following spinal cord injury in adult mammals is very limited, there can be some recovery of motor function. This recovery is strongly dependent on the lesion size and location as well as on neural activity of denervated networks activated mainly through physical activity (i.e., rehabilitative training). Recovery of motor function is largely due to neuroplasticity, which includes adaptive changes in spared and injured neural circuitry. Neuroplasticity after spinal cord injury is extensive and includes mechanisms such as moderate axonal sprouting, the formation of new synaptic connections, network remapping, and changes to neuron cell properties. Neuroplasticity after spinal cord injury has been described at various physiological and anatomical levels of the central nervous system including the brain, brainstem, and spinal cord, both above and below injury sites. The growing number of mechanisms underlying postinjury plasticity indicate the vast complexity of injury-induced plasticity. This poses important opportunities to further enhance and harness plasticity in order to promote recovery. However, the diversity of neuroplasticity also creates challenges for research, which is frequently based on mechanistically driven approaches. The appreciation of the complexity of neuronal plasticity and the findings that recovery is based on a multitude and interlinked adaptations will be essential in developing meaningful new treatment avenues.