Topical anesthetics interact with treatment expectation on reported pain in a balanced placebo design and are associated with insular, cingulate and striatal fMRI signal changes.
We investigated the effect of a possible interaction between topical analgesic treatment and treatment expectation on pain at the behavioral and neuronal level by combining topical lidocaine/prilocaine treatment with an expectancy manipulation in a 2 by 2 within-subject design (open treatment, hidden treatment, placebo, control). Thirty-two healthy subjects received heat pain stimuli on capsaicin-pretreated skin and rated their experienced pain during functional magnetic resonance imaging. This allowed us to separate drug- and expectancy-related effects at the behavioral and neuronal levels and to test whether they interact during the processing of painful stimuli. Pain ratings were reduced during active treatment and were associated with reduced activity in the anterior insular cortex. Pain ratings were lower in open treatment compared with hidden treatment and were related to reduced activity in the anterior insular cortex, the anterior cingulate cortex, the secondary somatosensory cortex, and the thalamus. Testing for an interaction revealed that the expectation effect was significantly larger in the active treatment conditions compared with the no-treatment conditions and was associated with signal changes in the anterior insular cortex, the anterior cingulate cortex, and the ventral striatum. In conclusion, this study shows that even in the case of a topical analgesic, expectation interacts with treatment at the level of pain ratings and neuronal responses in placebo-related brain regions. Our results are highly relevant in the clinical context as they show (i) that expectation can boost treatment and (ii) that expectation and treatment are not necessarily additive as assumed in placebo-controlled clinical trials.