Understanding the Influence of the Experimental Setting on Pain Ratings

The experimental setting influences the ways that study participants rate painful stimuli report researchers from the National Center for Complementary and Integrative Health (NCCIH) in a study published in the Scandinavian Journal of Pain. These findings may help researchers design studies to better understand the mechanisms involved in chronic pain. NCCIH is a component of the National Institutes of Health (NIH).

In the study, 31 people with fibromyalgia (a chronic disorder that causes muscle pain and fatigue) and 44 healthy volunteers who had recently rated painful heat stimuli in an experimental setting were interviewed about their experiences. Researchers asked participants the initial question of “Thinking back to when you were getting the heat pain on your leg, what were you thinking about when deciding on your pain score?” For participants who found it difficult to select a pain score number, researchers asked them to “Describe what made it difficult to choose a number.”

There were some notable differences found between the two populations:

• People with fibromyalgia used current daily pain as a benchmark to judge the experimental heat pain more often than healthy volunteers;
• People with fibromyalgia became mentally fatigued more easily than healthy volunteers;
• Healthy volunteers were more likely to want to appear strong in front of the investigators than people with fibromyalgia.

The researchers also noted six overarching themes regarding the thought processes of participants as they determined their pain scores:

• Making comparisons with previous or current pain
• Self-monitoring of the ability to endure pain
• Focusing on the pain’s physical aspects
• Accepting the safety of the experimental setting
• Focusing on the pain scale as an anchor
• Desiring to appear strong.

Participants reported that the following factors made the rating task difficult:

• Falling asleep
• Mental fatigue
• Guessing/vacillating
• Having to make a quick decision
• Difficulty remaining consistent.

These factors are unique to the experimental setting. The NCCIH scientists advise other pain researchers that they “should be cognizant that lengthy and complex studies may exacerbate mental fatigue in certain patient groups such as fibromyalgia patients.” Based on their findings, the NCCIH scientists recommend that future experimental pain studies reduce the number of pain stimuli, provide more breaks to reduce study participant fatigue, tailor instructions to reassure participants who might want to “save face” in front of researchers, and clarify instructions regarding a focus on physical or mental aspects of the pain experience.