Expectations of pain intensity can become a self-fulfilling prophecy, according to a new brain imaging study published in the journal Nature Human Behaviour. In fact, false expectations of pain levels can persist even when reality repeatedly demonstrates otherwise.
“We discovered that there is a positive feedback loop between expectation and pain,” said senior author Dr. Tor Wager, 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.”
Researchers have long been intrigued with the idea of self-fulfilling prophecy, with studies showing that expectations can influence everything from how one performs on a test to how one responds to a medication.
The new study is the first to directly model the dynamics of the feedback loop between expectation and pain as well as the underlying neural mechanisms.
The research was inspired when Dr. Marieke Jepma, then a postdoctoral researcher in Wager’s lab, noticed how when participants were shown time and again that something wouldn’t hurt badly, some still expected it to.
“We wanted to get a better understanding of why pain expectations are so resistant to change,” said Jepma, lead author and now a researcher at the University of Amsterdam.
For the study, the researchers recruited 34 participants and taught them to associate one symbol with low heat and another with high, painful heat.
Then, the volunteers were placed in a functional magnetic resonance imaging (fMRI) machine, which measures blood flow in the brain as a signal of neural activity. For 60 minutes, the participants were shown low or high pain cues (the symbols, the words Low or High, or the letters L and W), then asked to rate how much pain they expected.
Next, varying degrees of painful but non-damaging heat were applied to their forearm or leg, with the hottest reaching “about what it feels like to hold a hot cup of coffee,” Wager said.
Then the volunteers were asked to rate their pain. Unbeknownst to them, heat intensity was not actually related to the preceding cue.
The findings show that when the volunteers expected more heat, brain regions associated with threat and fear were more activated during the anticipation period. Regions involved in the generation of pain were more active when they received the stimulus. Individuals 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 Jepma.
Surprisingly, the subjects’ expectations also significantly influenced their ability to learn from experience. Many individuals demonstrated high “confirmation bias,” the tendency to learn from things that reinforce our beliefs and discount those that don’t.
For example, 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.
This phenomenon could have notable effects on recovery from painful conditions, suggests Jepma.
“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,” she said. “Positive expectations, on the other hand, could have the opposite effects.”
The study also may shed light on why, for some people, chronic pain can linger long after damaged tissues have healed.