For many years, neuroscientists believed they had identified a specific pattern of brain activity acting as a kind of "signature" for pain in the brain. Recently this so-called "pain matrix" has been called into question, and a new study by British researchers may have shattered the myth once and for all.
The pain matrix is actually a cluster of regions in the brain that prior imaging studies indicated are involved in processing pain perception, including the posterior insula and the anterior cingulate cortex. This has been so broadly accepted that the signature pattern has been used to declare that emotional pain (like social rejection) and physical pain are the same thing, as far as the brain is concerned. The argument goes that something like a bad romantic breakup has the same effect on brain activity as spilling a hot cup of coffee on your shirt.
More recent studies have cast doubt on those conclusions, however. And now researchers at the University of Reading and University College London have concluded that this cluster of regions in brain is not specific to pain. It also responds to loud noises, bright lights, a strong non-painful touch (like a firm handshake), and yes, social rejection. They describe their findings in a new paper published today in JAMA Neurology.
"I wouldn't say that's it's wrong to say that the 'pain matrix' is involved in processing pain," lead author Tim Salomons (University of Reading) told Gizmodo. "What's wrong is the idea that it is specific to pain—in other words, that when you observe this pattern, you can just assume that person is in pain."
Most of these studies employ functional magnetic resonance imaging (fMRI). Unlike conventional medical MRI, which creates a static image of the brain similar to an x-ray, fMRI monitors the brain in action. When enough neurons fire together in response to a given stimulus, blood flow increases to those parts of the brain involved in processing that input. The fMRI detects this as slight increases in blood oxygen levels—the so-called BOLD response–in those different regions. The resulting gorgeous full-color images make for terrific eye candy, but they aren't actually real-time snapshots of the brain in action; rather, they're visualizations of statistical data.
So how can scientists know for sure if the pattern they're seeing is really an indicator for pain (or any other type of cognitive process)? The gold standard is cognitive neuroscience studies that involve patients with existing brain damage, according to Bradley Voytek, a neuroscientist at the University of California, San Diego, who was not involved with the study. So if you want to prove that a particular cluster of brain regions encodes pain, you must first determine that patients with damage to those regions can no longer feel pain.