You're not imagining the pain. But your brain might be behind it, nonetheless. For the first time, it is possible to distinguish between brain activity associated with pain from a physical cause, such as an injury, and that associated with pain linked to your state of mind.
A fifth of the world's population is thought to experience some kind of chronic pain – that which has lasted longer than three months. If the pain has no clear cause, people can find themselves fobbed off by doctors who they feel don't believe them, or given ineffective or addictive painkillers.
But a study led by Tor Wager at the University of Colorado, Boulder, now reveals that there are two patterns of brain activity related to pain. One day, brain scans could be used to work out your relative components of each, helping to guide treatment.
"Pain has always been a bit of a puzzle," says Ben Seymour, a neuroscientist at the University of Cambridge. Hearing or vision, for example, can be traced from sensory organs to distinct brain regions, but pain is more complex, and incorporates thoughts and emotions. For example, studies have linked depression and anxiety to the development of pain conditions, and volunteers put in bad moods have a lower tolerance for pain.
So does this mean we can think our way into or out of pain? To find out, Wager and his colleagues used fMRI to look at the brain activity of 33 healthy adults while they were feeling pain. First, the team watched the changing activity as they applied increasing heat to the volunteers' arms. As the heat became painful, a range of brain structures lit up. The pattern was common to all the volunteers, so Wager's team called it the neurologic pain signature.
The group then examined whether the volunteers could control the pain by thought alone. "We asked them to rethink their pain, either as a blistering heat, or as a warm blanket on a cool day," Wager says. Although the volunteers couldn't change the level of activity in the neurologic pain signature, they could alter the amount of pain they felt. As they did this, a distinct set of brain structures linking the nucleus accumbens and ventromedial prefrontal cortex became active (PLoS Biology, doi.org/x55).
"It's a major finding," says Vania Apkarian at Northwestern University in Chicago. "For the first time, we've established the possibility of modulating pain through two different pathways."