Brain : a journal of neurology
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Limb amputation results in plasticity of connections between the brain and muscles, with the cortical motor representation of the missing limb seemingly shrinking, to the presumed benefit of remaining body parts that have cortical representations adjacent to the now-missing limb. Surprisingly, the corresponding perceptual representation does not suffer a similar fate but instead persists as a phantom limb endowed with sensory and motor qualities. How can cortical reorganization after amputation be reconciled with the maintenance of a motor representation of the phantom limb in the brain? In an attempt to answer this question we explored the relationship between the cortical representation of the remaining arm muscles and that of phantom movements. ⋯ Interestingly, phantom limb movements that the patient could not produce voluntarily were easily triggered by TMS, suggesting that the inability to voluntarily move the phantom is not equivalent to a loss of the corresponding movement representation. We suggest that hand movement representations survive in the reorganized motor area of amputees even when these cannot be directly accessed. The activation of these representations is probably necessary for the experience of phantom movement.
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The loss of a limb leads to sensorimotor modifications that are frequently accompanied by the vivid experience that the missing limb is still present, and that it can be moved at will. Furthermore, amputees can clearly distinguish between phantom movements of the fingers and of more proximal joints, like movements of the elbow. This phenomenon raises the question of whether these specific phantom movement experiences are translated into differentiated activity within the remaining muscles. ⋯ Failed attempts to move the paralysed phantom limb always resulted in the same EMG pattern, no matter what type of phantom movement was attempted, while ischaemic nerve block reduced or eliminated the ability to voluntarily move the phantom limb and produced a dramatic reduction in the amplitude of stump muscle EMG activity. Our data suggest that the experience of phantom hand movement involves the activation of hand motor commands. We propose that preserved hand movement representations re-target the stump muscles to express themselves and that when these representations are voluntarily accessible they can instruct the remaining muscles to move in such a way as if the limb is still there.