Perception
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Pain synaesthetes experience pain in a presensitised region when observing or imagining another person in pain. We conducted an upper-limb embodiment study using a modified rubber-hand illusion in which lower-limb amputees originally participated as control subjects for the upper-limb amputees. While we found all subjects experienced topographic illusory sensations, we also serendipitously found that lower-limb amputee pain synaesthetes experienced pain or a motor response in their phantom leg when the embodied hand was threatened (e.g., with a retractable knife, mousetrap, or syringe) or submitted to high-frequency stimulation (e.g., vibration). ⋯ We found no association between pain synaesthesia and empathy scores. On the basis of related literature we suggest that pain synaesthetes likely experienced phantom-leg pain because (a) the motor system was already engaged during visual capture; (b) threatening stimuli, to which they are hyper-vigilant, triggered avoidance or 'escape' motor schemata; and (c) there could be no feedback confirming that initiated motor schemata for the phantom limb were successfully performed. Ultimately, we have further defined this new condition, synaesthesia for pain, as not only having a sensory pain component, but also a key motor component, manifesting itself in avoidance, contraction, and withdrawal 'actions'.
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The bodily boundaries in amputees may seem to be more malleable than in non-amputees, given the propensity for a phantom limb to embody a mirror-reflected hand. However, in the present investigation, in which phantom-limb illusions within body space are induced and manipulated, we found that perceiving phantom sensations and illusory embodiment does not require amputation. Surprisingly, in the present modified rubber-hand illusion, we found that simultaneous stroking or stimulation of the participant's target hand was not necessary to induce illusions of embodiment and corresponding perceptual illusions. ⋯ In this model, passive observation of the hand in the mirror triggers body representations (body image and body schema), most likely through activation of the posterior parietal cortex and temporo-parietal junction. Activity in these regions heightens awareness of peripersonal space and increases tactile sensitivity, and may subsequently enhance perception of illusory touch and embodiment. Furthermore, sense of embodiment may be more apparent to the participant when the hand is threatened; however, embodiment may even be strengthened when the motor system is engaged, evoking motor schemata to support the more easily induced perceptual embodiment via body image.