Synesthesia, the confusion of the senses, has fascinated artists and writers for centuries. However, the first medical description of a synesthete, who experienced “coloured hearing,” was not until 1812.
Now 61 different types of synesthesia have been identified, and new forms continue to regularly pop up in the scientific literature. The most common type is grapheme-colour synesthesia, in which an affected individual experiences letters and numbers as shaded or tinged with colour. There are also such things as “flavour-colour,” “sound-touch,” and “vision-temperature” synesthetes.
Interestingly, many of the more recently identified synesthesias involve emotion, a characteristic that has not traditionally been thought of as a primary “sense” but can be intimately linked to the senses. An example is synesthesia for pain, a condition in which the synesthete has so much empathy for the physical pain in others that the pain is actually experienced when it is observed. This variety of synesthesia was recently reviewed in a paper by Bernadette M. Fitzgibbon and colleagues that describes possible cognitive and neural mechanisms underlying the phenomenon.
Synesthesia for pain has mainly been found in rare phantom limb patients who have had an amputation but still experience sensations in the absent limb. This quality differentiates synesthesia for pain from many other forms of developmental synesthesia in that synesthesia for pain is acquired by experience. Although most of us (hopefully all of us?) feel empathetic towards those suffering from pain, it is thought that some phantom limb patients experience ‘uber-empathy’ for pain due to rewiring and reshaping of their brains.
The neural system that Fitzgibbon’s group mainly discuss as a possible underlying mechanism is of course our great unified theory of neuroscience: the mirror neuron system. Neuroimaging studies of non-synesthetes reveal that certain areas of the brain are active when pain is felt: some areas involving emotion/affective processing, some areas involving cognition/evaluation, and other areas involving the actual sensation of pain. When non-synesthetes observe pain being inflicted on another, the only activated component of the pain matrix is the emotional one. This finding has been interpreted as evidence for a mirror system of empathy (although this is hotly debated in the literature).
So the non-synesthetes feel bad for those experiencing pain but they don’t actual feel that pain (validating the claim “you’ll never understand my pain!!”).
Fitzgibbon’s group hypothesize that pain synesthetes have a “disinihibited” mirror empathy system, such that observation of pain elicits activity in all components of the pain matrix, including the areas involving the sensation of pain. This has not yet been tested, but neuroimaging studies could provide insight to the understanding of synesthesia for pain and possible treatments for this surely gruelling condition (imagine not being able to watch Scarface without feeling as though you are being dismembered by a chainsaw).
Interestingly – although this is pure speculation - further study of non-synesthetes could potentially help us understand synesthesia for pain. We all know people who cringe or feel tingly at the sight of blood. I know that I sometimes get goosebumps when viewing gory scenes in movies. Is this a less severe form of synesthesia for pain? Can these reactions be reduced with certain experiences, such as repeated exposure? Or with brain-altering interventions such as rTMS?
The synesthete may be viewed as abnormal, but perhaps we underestimate the interrelatedness of the senses that exists in us all.
Fitzgibbon BM, Giummarra MJ, Georgiou-Karistianis N, Enticott PG, & Bradshaw JL (2010). Shared pain: from empathy to synaesthesia. Neuroscience and biobehavioral reviews, 34 (4), 500-12 PMID: 19857517