After amputation, highly empathetic individuals may be predisposed to strengthening existing pathways between observed and felt touch, according to recent study results.
“For most people, observing touch activates the somatosensory system and may lead to behavioral facilitation/interference of felt touch, but this activity is not normally associated with conscious experiences of being touched. However, some people do report tactile experiences on their own body in response to observing touch on others and this has been termed mirror-touch synaesthesia,” the researchers wrote. “In this study, we explore whether amputees report tactile (or painful) sensations in response to observing touch on others. The visual stimuli were either non-painful or mildly painful but not strongly painful.”
Twenty-eight amputees and 31 control participants observed 67 videos of touch events. The control group viewed videos that did not mention or portray phantom limbs. All participants were asked whether they felt any actual tactile or painful experiences on their body while watching the videos.
When watching someone else being touched, almost a third of amputees reported a tactile sensation localized on the phantom limb or residual limb, irrespective of the body part shown. Study results showed that synaesthetic sensations were more intense when real bodies were observed vs. dummies or objects, as well as when the touch was mildly painful. Researchers found that although frequency, intensity and cause of phantom limb pain do not appear to determine whether an amputee will report mirror-touch sensations, amputees who do report it show greater empathetic emotional reactivity.
“Unlike normal-bodied mirror-touch synaesthetes, the sensations are not somatotopically mapped, but usually occur in the phantom limb or stump. Such acquired synaesthesia may be the consequence of an unmasking of usually inhibitory pathways and the formation of new synaptic connections caused by the sensory loss associated with amputation,” the researchers concluded. “By exploring the situations leading to sensations and characteristics of the experienced touch, the current study has enhanced our understanding of visuo-tactile interaction in amputees with phantom limb.”
Although the findings of this study are largely based on reports from amputees, the researchers do not attribute the responses simply as repeating what the amputees saw in the videos or an inclination to be compliant. Reports often deviated from what amputees viewed; amputees reported feeling a sensation in their phantom limb upon viewing an arm being touched, for example. A robust pattern of agreement among responders suggested that the sensations were in fact real and not caused by spontaneous fluctuations in phantom limb sensations.
Two causes of synaesthetic sensations in the affected limb of amputees are the presence of pain in the region and the loss of true sensation in that region. However, the researchers found no evidence that the intensity or frequency of phantom limb pain is related to the presence of synaesthetic experiences.
In fact, although the presence of mirror-touch synaesthesia in amputees was not predicted by characteristics of their phantom limb, the researchers found that emotional reactivity, which measures automatic emotional reactions directed toward other people, did predict mirror-touch syneasthesia.
“Our claim is not that high empathy is a necessary condition for mirror-touch syneasthesia to occur following an amputation, but rather that these individuals have a head start when it comes to modifying existing pathways linking observed touch to felt touch,” the researchers stated. — by Casey Murphy
Disclosure: This study was supported by a grant from the UK Economic and Social Research Council.