Cognitively decreasing the perceived conditioning stimulus (CS) pain attenuates conditioned pain modulation (CPM) magnitude, although a ceiling effect may limit CPM enhancement after a cognitively increased CS pain.
Although painfulness of the conditioning stimulus (CS) is required for the activation of conditioned pain modulation (CPM), it is still unclear whether CPM expression depends on the objective physical intensity of the CS or the subjective perception of its pain. Accordingly, we cognitively manipulated the perceived CS pain, rendering the physical aspects of the CPM paradigm untouched. Baseline CPM was measured among 48 young healthy male subjects using the parallel paradigm with contact heat as test pain and hand immersion in hot water as CS. Subjects were then randomized into 4 groups, all of which were cognitively manipulated as to the CS-induced pain: group 1, placebo
(CS less painful); group 2, nocebo
(CS more painful); and groups 3 and 4, the informed control groups for groups 1 and 2, respectively. CPM was reassessed after the manipulation. Comparing the groups by MANCOVA (multivariate analysis of covariance) revealed that placebo
exerted decreased CS pain and consequent attenuation of CPM magnitudes, while nocebo
elicited increased CS pain, but without CPM elevation (P
< .0001). Within the placebo
group, the reduction in CS pain was associated with diminished CPM responses (r
= 0.767; P
= .001); however, no such relationship characterized the nocebo
group. Pain inhibition under CPM seems to depend on the perceived level of the CS pain rather than solely its physical intensity. Cognitively decreasing the perceived CS pain attenuates CPM magnitude, although a ceiling effect may limit CPM enhancement after cognitively increased CS pain. These findings emphasize the relevance of cognitive mechanisms in determining endogenous analgesia processes in humans.