CU-Boulder pain imaging study

President Bill Clinton claimed to be able to feel people's pain, but now scientists are closer to being able to actually see it, as the result of a new study led by the University of Colorado.

Although the research does not yet enable doctors to quantify physical pain, it could pave the way for future study that may produce the first objective medical tests of pain.

A reliable, objective measure of pain -- instead of people merely giving subjective accounts such as rating their discomfort on a scale of one to 10 -- could confirm patients' self-reported pain and produce new insight into how the brain generates different types of physical discomfort.

The findings of the study, led by Tor Wager, an associate professor of psychology and neuroscience at CU, were published Wednesday in the New England Journal of Medicine.

Using functional magnetic resonance imaging of 114 brains, researchers were able to detect a distinct neurologic pain "signature" -- registering in clusters of yellow -- indicating pain that subjects felt from having varying levels of heat applied to their arm.

"We have a little hot plate that goes on the forearm, and we can control exactly how hot it gets, and for how long," Wager said. "We can have precise control over the pain people feel.

"That gives us a ground truth; for example, if we do a 48-degree stimulus, every person says 48 is more painful than 47. We can use that and count on it, and say, 'Let's find a brain pattern that can predict that effect.' The idea, broadly speaking, is we can establish a pattern where we know what the ground truth is, and that gives us some leverage to test in cases where we aren't sure."

Wager and his associates found a neurologic signature for pain that was not unique to each individual. Instead, the signature was transferable across different subjects, allowing scientists to assess how much pain each person was being caused by the applied heat with between 90 and 100 percent accuracy.

People often speak of the "pain" of a social discomfort, such as being rejected in a romantic relationship, and scientists know that such an experience actually does activate many of the same brain regions as physical pain.

However, Wager and his colleagues were able to discriminate in the neuroimaging between physical pain and emotional pain.

Also, they found that the strength of the visible signature was substantially reduced when the subject had been given an analgesic.

Wager conceded in the conclusion to the published study that its findings were limited to only assessing pain elicited by "noxious heat in healthy persons," and that further study is needed to determine whether the brain signature can also assess clinical pain -- that is, pain stemming from sickness, disease, injury or other causes.

And, Wager added, "We don't yet really know how well we could take two people in chronic pain and say who is really in more pain," Wager said. "That's the kind of test we could develop and are pursuing, but we haven't gotten there yet."

Wager and his colleagues, including scientists from New York University, Johns Hopkins University and the University of Michigan, are now testing how the neurologic signature performs when applied to other types of pain.

The New England Journal of Medicine, in an editorial on the study, stated that the research results "require cautious evaluation" for several reasons -- including the fact that its parameters to date only encompassed cutaneous pain, and therefore might not apply to pain in the context of disease or clinical circumstances.

The editorial concluded that neuroimaging may help clinicians assess some clinical symptoms, but that, "Being doctors ... we may ultimately have to acknowledge that 'pain is pain' and can only be reported by the patient."

