The strength of the signature response scaled with increasing temperatures and correlated well with each person's reports of increasing pain as researchers cranked up the temperature from a toasty but painless 44.3°C to a scalding 49.3°C-akin to a hot cup of coffee, Wager says. Based solely on the intensity of the signature, the team could distinguish whether participants had experienced painful heat or nonpainful warmth with 93% accuracy. The neural reading could also predict which of two painful temperatures hurt more for the subjects, as reported Thursday in The New England Journal of Medicine.
When participants received a pain-relieving chemical called remifentanil, the signature response subsided-even during trials in which people believed that they had received a placebo. The volunteers did not show the pain signature response while anticipating a painfully hot sensation or remembering a previous bout of pain. Those responses were also notably absent as people viewed photographs of their recent exes, a painful social experience that activates some of the same brain areas as physical pain.
"This paper is a quantum leap for the field of imaging pain and hopefully the basis for other groups to go forward," says David Borsook, a pain specialist at Boston Children's Hospital and Harvard Medical School. He predicts that brain scans could one day help doctors track patients' symptoms as they try different treatments or assist researchers in comparing the efficacy of experimental pain drugs.
For now, more work is needed to determine how well Wager's methods can distinguish other kinds of pain such as dull, throbbing aches and stabbing pains, says Robert Coghill, a neuroscientist at Wake Forest University in Winston-Salem, North Carolina. "We need some really hard evidence," that the tests can be used broadly, but he says this study makes a good start. "It's definitely a tour de force."