Enhanced corticospinal response to observed pain in pain synesthetes

Observing noxious injury to another’s hand is known to induce corticospinal inhibition that can be measured in the observer’s
corresponding muscle. Here, we investigated whether acquired pain synesthetes, individuals who experience actual pain when observing injury to another, demonstrate less corticospinal inhibition than
do controls during pain observation, as a potential mechanism for the experience of vicarious pain. We recorded motor-evoked
potentials (MEPs) induced at two time points through transcranial magnetic stimulation while participants observed videos
of a hand at rest, a hypodermic needle penetrating the skin, a Q-tip touching the skin, and a hypodermic needle penetrating
an apple. We compared MEPs in three groups: 7 amputees who experience pain synesthesia, 11 nonsynesthete amputees who experience
phantom limb pain, and 10 healthy controls. Results indicated that the pain synesthete group demonstrated significantly enhanced
MEP response to the needle penetrating the hand, relative to the needle not having yet penetrated the hand, as compared with
controls. This effect was not observed exclusively in the same muscle where noxious stimulation was applied. We speculate
that our findings reflect a generalized response to pain observation arising from hyperactivity of motor mirror neurons not
involved in direct one-to-one simulation but, rather, in the representation of another’s experience.