Pain perception can vary greatly. Sometimes, we feel pain more intensely than expected due to an injury or physical ailment but may feel less intense pain in other similar instances. This variability indicates that our perception of pain is highly dependent on our expectations and uncertainty.

Two hypotheses have been proposed to explain how the brain perceives pain. One is the Estimate Hypothesis, where the brain estimates the intensity of pain based on predictions. The other is the Surprise Hypothesis, where the brain perceives pain as the difference between prediction and reality, otherwise known as the prediction error.

In this study, the mechanism underlying the perception of pain was investigated. In the experiment, healthy participants received painful thermal stimuli and reported feeling pain intensity while observing painful or non-painful visual stimuli in virtual reality. The researchers found that the participants strongly perceived pain when the prediction error was large, demonstrating that the Surprise Hypothesis more adequately explains the pain perception mechanism in the brain. The study further confirmed that pain was amplified when unexpected events occurred.

People with chronic pain often experience vague pain-related fears and anxieties. Possibly, this uncertain gap between expectation and reality further increases the perceived intensity of pain. Therefore, reducing the gap between pain expectation and reality or "surprise" is important in reducing pain. A better understanding of pain perception would facilitate the development of new treatments that would enhance recovery from chronic pain and trauma.

This work was supported by JSPS KAKENHI (grant numbers 19H05729 and 23KJ0261).