Perceptual abnormalities in amputees: phantom pain, mirror-touch synaesthesia and referred tactile sensations

It is often reported that after amputation people experience "a constant or inconstant... sensory ghost... faintly felt at time, but ready to be called up to [their] perception" (Mitchell, 1866). Perceptual abnormalities have been highlighted in amputees, such as sensations in the phantom when being stroked elsewhere (Ramachandran et al., 1992) or when observing someone in pain (Giummarra and Bradshaw, 2008). This thesis explored the perceptual changes that occur following amputation whist focusing on pain, vision and touch. A sample of over 100 amputees were recruited through the National Health Service. Despite finding no difference in phantom pain based on physical amputation details or nonpainful perceptual phenomena, results from Paper 1 indicated that phantom pain may be more intense, with sensations occurring more frequently, in amputees whose pain was triggerinduced. The survey in Paper 2 identified a group of amputees who in losing a limb acquired mirror-touch synaesthesia. Higher levels of empathy found in mirror-touch amputees might mean that some people are predisposed to develop synaesthesia, but that it takes sensory loss to bring dormant cross-sensory interactions into consciousness. Although the mirror-system may reach supra-threshold levels in some amputees, the experiments in Paper 3 suggested a relatively intact mirror-system in amputees overall. Specifically, in a task of apparent biological motion, amputees showed a similar, although weaker, pattern of results to normalbodied participants. The results of Paper 4 showed that tactile spatial acuity on the face was also largely not affected by amputation, as no difference was found between the sides ipsilateral and contralateral to the stump. In Paper 5 cross-modal cuing was used to investigate whether referred tactile sensations could prime a visually presented target in space occupied by the phantom limb. We conclude that perception is only moderately affected in most amputees, but that in some the sensory loss causes normally sub-threshold processing to enhance into conscious awareness.