Thinking of something else is a time-honored method for coping with pain. Indeed, psychologists have demonstrated repeatedly that what you think about can modulate the pain you experience. But what's less clear is how exactly that effect plays out in the body. In a study published today in Current Biology, neuroscientists have found that distraction does more than merely divert your mind; it actually sends signals that bar pain from reaching the central nervous system.
"This study connects two important fields of pain research," says lead author Christian Sprenger, a physician and neuroscientist at the University Medical Center Hamburg–Eppendorf in Germany. "There are many studies describing the sensitization processes of the spinal cord. On the other hand, it is well known that certain psychological factors are good predictors of the development of pain."
Sprenger and his colleagues told 20 male volunteers they would be participating in an experiment that would study concentration and memory. Each subject, while undergoing functional magnetic resonance imaging (fMRI) to map their neural activity, used a computer screen to take a memory test called an "n-back test." In such a test, subjects recall a specific letter either one or two letters back from the end of a series. As initial sessions confirmed, remembering a letter two-back is more challenging than a letter one-back. Researchers gave volunteers either the one- or two-back test so that they could study the nervous system under two levels of cognitive load.
While taking the test, each subject received a burning sensation on the forearm, courtesy of a heating element that reached a little over 47 degrees Celsius—hot enough to hurt but not enough to damage the skin. After completing the test and heat stimulus session, each man rated the sensation of pain on a scale of 0 (no pain) to 100 (unbearable). On average, the amount of heat should have produced a pain level of about 60. In line with previous studies, individuals taking the tricky two-back test described less pain than those taking the simpler one-back test.
But the effect wasn't just in their heads. The fMRI revealed differences in each group taking place in their spinal cords. For those unfortunate subjects taking a one-back test, pain signals successfully reached nerve fibers along the spinal cord, creating the experience of pain. But in subjects preoccupied by the more challenging test, pain receptors were blocked. The researchers concluded that the brain sends some kind of signal to the rest of the body, which enables the nervous system to ignore most pain and focus on the task at hand.
Intrigued, the researchers decided to repeat the test but add an extra variable. As some individuals received an injection of harmless saline, others received naloxone, a drug that blocks the action of opioids. Those receiving saline showed the same pattern as before as did subjects taking a one-back test with naloxone. For subjects taking a two-back test with naloxone, however, the pain-relieving effects of a distracted brain diminished by 40 percent. The finding suggests the concentrating brain doesn't just divert attention from the agony, it also triggers a release of opioid-based compounds.
"I think this is very novel," says Tor D. Wager, a psychologist and neuroscientist at the University of Colorado at Boulder. "It has not been clear whether attention works at an output stage or modifies a higher level or even interferes with memory for pain." Wager, who is also studying the interaction of working memory and pain, explains that Sprenger's study is nearly unprecedented in its look at the spinal cord.
Sprenger and his colleagues—whose previous work examined the placebo effect—another brain–body mystery—believes distraction is just one of many cognitive cases in which the brain modulates pain. He adds that their research is a first step. Much of the pain-response process seen in this study remains unknown: current functional magnetic resonance image technology can focus on either the brain or the spinal cord, but not both, and fine details—such as precisely how naloxone interacts with the nervous system and opioids—are invisible.
The study, however, does provide a candidate mechanism for the brain's inhibition of the pain response, a mechanism that can be measured and further studied, eventually leading to clinical applications such as therapies. Meanwhile, the next time you have a toothache, rather than pop some acetaminophen, consider a brainteaser.