Brain scans reveal first objective measure of physical pain

For the first time, scientists have identified how much pain people feel by looking at images of their brains.

Neurologic signature for physical pain across multiple systems. Image courtesy: Tor Wager, Colorado University

The findings, published in the New England Journal of Medicine, may lead to the development of methods doctors can use to objectively quantify patients' pain.

Currently, pain intensity is usually based on patient self-reports, using an intensity scale from one to 10. Objective measures of pain could confirm those subjective reports and provide clues about how the brain registers different types of pain.

The new research also may set the stage for using brain scans to objectively measure anxiety, depression, anger or other emotional states.

"Right now, there's no clinically acceptable way to measure pain and other emotions other than to ask a person how they feel," said Tor Wager of the University of Colorado and lead author of the paper.

"These findings demonstrate that it is possible to accurately predict people's experience of an incredibly complex psychological state - an emotional response - based on neural activity alone," said University of Michigan psychologist Ethan Kross, a faculty associate at the U-M Institute for Social Research and a co-author of the study. "They raise the tantalising possibility that it may be possible to predict the experience of other types of complex emotional responses, for example, depression or anxiety, using similar methods."

Neurologic signature for physical pain across multiple systems. Image courtesy: Tor WagerWager, Kross and colleagues used data-mining techniques to comb through images of brains taken when the subjects were exposed to multiple levels of heat, ranging from pleasantly warm to painfully hot.

"We found a pattern across multiple systems in the brain that is diagnostic of how much pain people feel in response to painful heat," Wager said.

Initially, the researchers expected that pain signatures would be unique to each individual. If that were the case, a person's pain level could only be predicted based on past images of his or her individual brain.

But, instead, they found that the signature was uniform across people. This uniformity allowed researchers to accurately predict how much pain the applied heat caused each person, with no prior brain scans needed as a reference point.

The scientists also demonstrated that the signature was specific to physical pain. Past studies have shown that social pain can look very similar to physical pain in terms of the brain activity it produces. For example, a prior study conducted by Kross, Wager and colleagues showed that the brain activity of people who have just been through a relationship breakup - and who were shown an image of the person who rejected them - is similar to the brain activity of someone feeling physical pain.

But when Wager's team tested to see if the newly defined neurologic signature for heat pain would also pop up in the data collected earlier from the heartbroken participants, they found that the signature was absent.

"Although social and physical pain are clearly related in terms of the specific areas of the brain they recruit, distinct patterns of neural activity or neural signatures may predict each type of experience," Kross said.

Finally, the scientists tested to see if the neurologic signature could detect when an analgesic was used to dull the pain. The results showed that the signature registered a decrease in pain in subjects given a painkiller.

The results of the study do not yet allow physicians to quantify physical pain, but they lay the foundation for future work that could produce the first objective tests of pain by doctors and hospitals. To that end, Wager, Kross and colleagues are already testing whether the neurologic signature varies with different types of physical pain.

The research team also included Mathieu Roy and Choong-Wan Woo of the University of Colorado, Lauren Atlas of New York University and Martin Lindquist of Johns Hopkins University

The study was funded by the National Institute on Drug Abuse, the National Institute of Mental Health and the National Science Foundation.

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