By Pat Anson, PNN Editor
Getting out of bed, taking a shower, doing the dishes and other simple chores can be painful experiences for someone with intractable chronic pain. But some of that pain may be self-fulfilling: Getting out of bed hurts because you expect it to.
That's the theory behind a new brain imaging study published in the journal Nature Human Behaviour, which found that false expectations about pain can persist even when reality demonstrates otherwise.
"We discovered that there is a positive feedback loop between expectation and pain," said senior author Tor Wager, PhD, a professor of psychology and neuroscience at the University of Colorado Boulder.
"The more pain you expect, the stronger your brain responds to the pain. The stronger your brain responds to the pain, the more you expect."
Wager and his colleagues recruited 34 people for a heat test to see if the expectation of pain can cause changes in neural mechanisms of the brain.
Participants were taught to associate one symbol with low heat and another with painful heat. Then, the subjects were placed in a functional magnetic resonance imaging (fMRI) machine, which measures blood flow in the brain as a proxy for neural activity.
For 60 minutes, subjects were shown the low or high pain cues (the symbols “Low” and “High” or the letters L and W), and then asked to rate how much acute pain they experienced as heat was applied to their forearms or legs. Unbeknownst to the participants, heat intensity was not actually related to the preceding cue.
The study found that when subjects expected more heat, brain regions involved in threat and fear were more active as they waited for the heat to be applied. Regions involved in the generation of pain were also more active when they received the stimulus.
The result? Participants reported more pain with high-pain cues, regardless of how much heat they actually got.
"This suggests that expectations had a rather deep effect, influencing how the brain processes pain," said lead author Marieke Jepma, PhD, a researcher in Wager's lab who is now a researcher at Leiden University in the Netherlands.
Many subjects also demonstrated a high degree of confirmation bias -- a tendency to learn from things that reinforced their beliefs, while discounting those that didn’t. If they expected high pain and got it, they might expect even more pain the next time. But if they expected high pain and didn't get it, nothing changed.
"You would assume that if you expected high pain and got very little you would know better the next time. But interestingly, they failed to learn," said Wager.
Researchers say the study was the first to demonstrate the dynamics of a feedback loop between pain expectations and neural mechanisms that cause pain. Although the test only involved short-term acute pain, researchers say the findings may help explain why chronic pain can linger long after damaged tissues have healed.
"Our results suggest that negative expectations about pain or treatment outcomes may in some situations interfere with optimal recovery, both by enhancing perceived pain and by preventing people from noticing that they are getting better," said Jepma. "Just realizing that things may not be as bad as you think may help you to revise your expectation and, in doing so, alter your experience.”