How Pain Prediction Affects Perception

Post by Elisa Guma

What's the science?

Our past experiences often shape our expectations of the future, which in turn can influence how we perceive future events. Previous work has shown that our expectation of how painful a stimulus will be can influence our perception of that stimulus and our response to it. Moreover, expectations about pain (e.g., following placebo treatments) can be surprisingly resistant to change, acting much like self-fulfilling prophecies. The brain and behavioural mechanisms underlying these phenomena are largely unknown. This week in Nature: Human Behavior Jepma and colleagues used behavioural assessments and functional magnetic resonance imaging (fMRI) to investigate how expectations about pain affect pain experience, and why expectations of high or low pain sometimes persist despite evidence to the contrary.

How did they do it?

The authors designed two experiments in which they independently manipulated predictive pain cues (to investigate pain expectation), and the intensity of a pain stimulus (to investigate pain perception). In both studies, participants first went through a learning phase where they learned to associate abstract visual cues with either low or high temperatures (displayed on thermometers). In the subsequent test phase participants were presented with both sets of cues followed by a painful heat stimulus to the inner forearm in the first study, and to the lower leg in the second study. Unbeknownst to the participants, the cues were no longer predictive of heat intensity. Participants were instructed to rate how much pain they expected following each cue, and how much pain they experienced following each heat stimulus. In the second study, fMRI activity was also recorded. The authors tested responses in a measure called the Neurologic Pain Signature (NPS); a measure of activity across brain areas validated to be sensitive and specific to pain in tests performed to date. They used these brain areas to guide their investigation of brain signatures of pain perception and expectation in this study. Finally, the authors used computational models to quantify their findings, using both a reinforcement learning model  and a Bayesian model.

What did they find?

The authors found evidence that cue based expectations influence pain perception; higher pain expectation led to larger subjective pain rating, and to higher brain activity in the neurologic pain network. In addition, larger pain rating and higher activity in the NPS predicted higher pain expectation in subsequent trials with the same cue. The authors also observed a confirmation bias in learning about pain intensity, with stronger learning from new pain stimuli that confirmed one's initial pain expectation than from new pain stimuli that disconfirmed expectations. The computational models allowed the authors to determine that participants' pain expectations influenced both perceived pain and participants' learning rates. Finally, participants with stronger confirmation biases in their learning rate also showed a greater confirmation bias in the updating of pain-anticipatory brain activity in regions important for threat, anxiety, and value-based decision-making.

What's the impact?

This study is among the first to identify brain and behavioural processes underlying self-reinforcing expectations. This may have implications for chronic pain, as self-reinforcing expectations may be part of what makes people transition from acute to chronic pain. The findings may help in our understanding of how perceptual learning is applied to pain, and can be applied to individuals at risk for chronic pain. More broadly, it may help us understand how beliefs are sometimes so resistant to new evidence.

Jepma et al. Behavioural and neural evidence for self-reinforcing expectancy effects on pain. Nature: Human Behavior (2018). Access the original scientific publication here.