Conditioned pain modulation (CPM) is a natural process where feeling pain in one part of the body (e.g., a conditioning stimulus to the wrist) can reduce pain felt in another part of the body (a test stimulus to the foot). There is considerable evidence from both animal and human studies that the painful conditioning stimulus can stimulate the brain to activate neurochemical pain-control pathways (e.g., endogenous opioids), which are believed to take 30-120 seconds to fully engage. The current CPM study measures whether attentional processes can also make a measurable contribution to pain inhibition alongside neurochemical mechanisms.
Primary measures: Graphic ratings scale "worst pain" ratings.
The primary measure of the current study graphic ratings scales (worst pain ratings) will measure how much pain participants feel on their foot during foot only (test stimulus), vs. how much pain participants feel on their foot (test stimulus during simultaneous foot (test stimulus) + wrist (conditioning stimulus).
The current investigators predict that pain (e.g., the conditioning stimulus) naturally demands attention and can pull attention away from the original pain source (the test stimulus), resulting in CPM-like reductions in pain of the test stimulus.
Secondary measure #1. Cognitive measures are introduced in the current study, to quantify how much (if at all) the conditioning stimulus reduces pain of the target stimulus via attentional mechanisms. Participants estimate what percentage of their attention was directed to their foot (the test stimulus) during foot only vs. during foot + wrist (test + conditioning stimulus). This will measure whether the conditiioning stimulus to the wrist reduces how much attention participants focus on their foot. Analyses will also measure whether the amount of attention shift away from the foot (via participants; subjective ratings) is correlated with CPM robustness (reduction in pain of foot during duel stimulation).
Secondary measure #2. As another secondary measure of attention during one vs. two simultaneous pain stimuli, this study involves a divided attention auditory listening task that becomes harder when pain demands more attention (Craik et al., odd number divided attention task during single test pain stimuli vs. simultaneous test + conditioning stimuli). The more errors the participant makes on the divided attention task, the more attention diverted away from the auditory task by pain. Analyses will measure whether the number of errors on the divided attention task is correlated with CPM robustness (reduction in pain of foot, during duel thermal stimulation).
Exploratory measures. As exploratory measures, participants will rate how distracted they were and how difficult it was to concentrate (on 0-10 rating scales during test stimulus alone verses test + conditioning noxious thermal heat stimuli).
In summary, embedding these cognitive measures into the CPM protocol may enhance mechanistic interpretation accuracy by quantifying attentional contributions to pain inhibition in specific CPM protocols, thereby complementing existing approaches to pain phenotyping and reducing unexplained variance in CPM outcomes.
