A clinical and neurophysiological investigation of acquired synaesthetic pain in amputees

Pain typically describes an experience resulting from injury to one’s own body. In the current thesis, we investigate a newly identified pain phenomenon where the experience of pain is triggered by the observation or imagination of injury to another: ‘synaesthetic pain’. The earliest accounts of synaesthetic pain, an intriguing phenomenon of which little is known, were serendipitously identified in a sample of amputees who experience pain in their absent limb(s). The broad aim of this thesis was to provide the first systematic clinical and neurophysiological investigations of acquired synaesthetic pain experienced following amputation. Four main studies are reported. The first is an investigation of the incidence and characteristics of synaesthetic pain in a sample of amputees. Participants completed a brief questionnaire on phantom limb pain experiences, including whether their phantom limb pain was triggered by observing or imagining pain in another. Of the total sample of 74 amputees, around 16 % responded positively to this question, indicating that synaesthetic pain is experienced in a relatively large number of amputees. In the second study, electroencephalography (EEG) was used to investigate neurophysiological responses to pain observation in amputees who experience synaesthetic pain. Participants also completed selected behavioural measures in order to explore aspects of interpersonal differences (e.g., empathy), and their relationship to synaesthetic pain. We found that amputees who experience synaesthetic pain exhibit a unique neurophysiological response to observed pain, compared to controls, perhaps reflective of changes in inhibitory processing. No differences were observed between amputees who experience synaesthetic pain and controls on selected behavioural measures. The third study used transcranial magnetic stimulation (TMS) to investigate pain observation in amputees who experience synaesthetic pain. Here we found enhanced motorevoked potential response to observed pain in pain synaesthetes, compared to controls, that was not specific to the site of observed injury. No differences were observed between amputees who experience synaesthetic pain and controls on selected behavioural measures. There was, however, a trend towards significance within the pain synaesthete group for reduced response to observed pain and greater scores on a subscale of an empathy measure. The final study was a TMS investigation of motor cortical excitability and inhibition in acquired pain synaesthetes compared to controls. Although differences were observed between groups, we did not observe differences between amputees who experience synaesthetic pain and the amputee control-group who do not experience synaesthetic pain. When combining the two amputee groups, however, increased cortical facilitation was observed compared to healthy controls. This may reflect changes in motor cortex excitability related to neuropathic pain in amputees. This thesis describes the very first attempt at obtaining a clinical description of synaesthetic pain in amputees, and the investigation of its neurophysiological underpinnings. The findings indicate acquired synaesthetic pain in amputees is experienced in a substantial portion of individuals following amputation. Moreover, there appear to be unique neurophysiological responses to pain observation in amputees who experience synaesthetic pain. These findings have significant implications for our understanding of synaesthetic pain, and, more generally, for synaesthesia, social cognition, and pain.