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.
Monash Alfred Psychiatry Research Center, School of Psychology and Psychiatry, Monash University, Old Baker Building, Level One, Alfred Hospital, Melbourne 3004, Australia. bernadette.fitzgibbon@monash.edu
Bernadette M
BM
Fitzgibbon
Peter G
PG
Enticott
John L
JL
Bradshaw
Melita J
MJ
Giummarra
Michael
M
Chou
Nellie
N
Georgiou-Karistianis
Paul B
PB
Fitzgerald
D000328:Adult; D000704:Analysis of Variance; D016009:Chi-Square Distribution; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006225:Hand / Q000294:innervation; D006801:Humans; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000294:innervation; D010146:Pain / Q000473:pathology* / Q000503:physiopathology* / Q000523:psychology*; D010591:Phantom Limb / Q000473:pathology / Q000503:physiopathology / Q000523:psychology; D010775:Photic Stimulation; D011712:Pyramidal Tracts / Q000503:physiopathology*; D011930:Reaction Time; D012462:Saline Solution, Hypertonic / Q000008:administration & dosage; D011795:Surveys and Questionnaires; D050781:Transcranial Magnetic Stimulation
D012462:Saline Solution, Hypertonic