Our past experiences shape our expectations about the future, and these expectations affect how we experience new events. Understanding the role of expectations in pain is particularly important as expectations based on previous pain experiences could affect future pain responses.
A study in healthy volunteers, published late last year in the journal Nature Human Behaviour, shows that expectations can turn pain into a self-fulfilling prophecy.
Using visual cues to manipulate expectations of pain, Tor Wager, a pain researcher at the University of Colorado Boulder, US, and colleagues found that greater expectations about pain predicted larger subjective responses to pain—the more that people expected to have pain, the more pain they felt. Those larger pain responses predicted higher subsequent pain expectations.
The research also showed that study participants tended to change their expectations about pain only when new experiences were in line with what they initially believed would happen; their expectations about pain persisted even when the pain they actually experienced did not match those initial expectations. This suggests the presence of a confirmation bias—interpreting new information in a way that confirms what we already believe—in pain expectations.
“This is one of the best papers that I have read in a while,” said Javeria Hashmi, a researcher at Dalhousie University, Halifax, Canada, who was not involved in the study. The research “does a great job of setting the stage for a discussion of the role of bias in pain,” she said.
Over time, expectations that are not confirmed by actual experiences will eventually go away—a phenomenon known as extinction. However, in some cases, particularly with regard to pain, self-reinforcing expectations resistant to extinction can develop.
Marieke Jepma, lead author of the study at the University of Amsterdam in the Netherlands, had previously completed experiments where she manipulated expectations and subjected study participants to something painful that didn’t match their expectations.
“What fascinated me about those expectations was that even though the participants realized that their expectations were not in line with what they experienced, they didn’t really change their expectations. I wanted to try and find out why these expectations were so resistant to change,” said Jepma.
For the new research, 37 participants took part in a study that included a learning phase and a test phase. For the learning phase, participants were shown visual cues, in this case geometric shapes. The cues were paired with pictures of thermometers symbolically indicating either low heat (low-pain cues) or high heat (high-pain cues). This created a learned association between the visual cues and heat intensity.
During the test phase, participants were shown either a low- or high-pain cue and asked how much pain they expected to experience, using a scale from “no-pain” to “worst pain imaginable.” After making their predictions, the participants received a painful heat stimulus on the forearm or lower leg and were then asked to rate how much pain they felt.
However, what the researchers didn’t tell the participants was that the intensity of the heat stimulus was identical for both high-pain and low-pain cues.
“The only difference was that they expected either high or low pain, which allowed us to investigate how expectations can change the pain experience,” according to Jepma.
Pain ratings were higher for high-pain cues compared to low-pain cues. When the researchers examined the pain ratings more closely, they found that the pain reported following high-pain cues was consistently less than expected, while the pain reported following low-pain cues was always worse than expected. These discrepancies are called prediction errors and they persisted despite numerous trials.
Participants were also placed in a brain scanner during the test phase. These brain imaging experiments showed that manipulating expectations using the visual cues produced changes in brain activity in regions associated with pain processing.
A stubborn bias
Notably, the participants did not change their expectations to conform with the reality of what they experienced. That is, in the case of high pain cues, if the pain was lower than expected, participants didn’t alter their expectations following subsequent high-pain cues—they still expected higher pain. If the pain was worse than expected, the participants changed their expectations substantially and expected even higher pain after being presented with subsequent high-pain cues.
The opposite was true for low-pain cues followed by lower than expected pain. Here, the participants learned to change their expectations downward after being presented with subsequent low-pain cues, now expecting even less pain.
The results suggest the presence of a strong confirmation bias in the context of pain, which may help to explain, as Jepma observed in her previous work, why certain expectations of pain are so resistant to change. People put more weight on evidence that confirms their initial belief, compared to evidence that doesn’t do so.
Jepma believes that the findings in healthy participants may have relevance to patients with chronic pain.
“If a person was to have very negative beliefs or expectations regarding their recovery from acute [short-term] pain, coupled with a strong confirmation bias, they may be less likely to attribute a decrease in pain experienced to recovery and more likely to consider it a fluke or an aberration—all of which may facilitate the transition from acute to chronic pain.”
This story first appeared on the Pain Research Forum and has been adapted for RELIEF.
Dara Bree is a postdoctoral research fellow at Beth Israel Deaconess Medical Center and Harvard Medical School