Abstract
Somatic sensations induced by placebos are a frequent phenomenon whose etiology and clinical relevance remains unknown. In this study, we have evaluated the quantitative, qualitative, spatial, and temporal characteristics of placebo-induced somatic sensations in response to three different placebo interventions: (1) placebo irritant solution, (2) placebo laser stimulation, and (3) imagined laser stimulation. The quality and intensity of evoked sensations were assessed using the McGill pain questionnaire and visual analogue scales (VAS), while subjects’ sensation drawings processed by a geographic information system (GIS) were used to measure their spatial characteristics. We found that all three interventions are capable of producing robust sensations most frequently described as “tingling” and “warm” that can reach consider-able spatial extent (≤ 205mm²) and intensity (≤ 80/100 VAS). Sensations from placebo stimulation were often referred to areas remote from the stimulation site and exhibit considerable similarity with referred pain. Interestingly, there was considerable similarity of qualitative features as well as spatial patterns across subjects and placebos. However, placebo laser stimulation elicited significantly stronger and more widespread sensations than placebo irritant solution. Finally, novelty seeking, a character trait assessed by the Temperament and Character Inventory and associated with basal dopaminergic activity, was less pronounced in subjects susceptible to report placebo-induced sensations. Our study has shown that placebo-induced sensations are frequent and can reach considerable intensity and extent. As multiple somatosensory subsystems are involved despite the lack of peripheral stimulus, we propose a central etiology for this phenomenon.
Highlights
Placebo effects have established their veracity both in clinical trials and experimental studies [1]
Shapiro’s definition from 1968 [14]), here we study sham procedures similar to those commonly used in placebo studies without explicitly referring to any therapeutic potential. Using this broader definition of placebos, we study a little-researched phenomenon that may complement current views, namely somatic sensations induced by placebo interventions
Experiment 3 aimed at minimizing possible influences from the experimenter or laboratory setting by replacing “actual” placebo stimulation with laser stimulation that was imagined by subjects at home
Summary
Placebo effects have established their veracity both in clinical trials and experimental studies [1]. While previous studies have predominantly seen placebos as a form of therapeutic intervention (cf Shapiro’s definition from 1968 [14]), here we study sham procedures similar to those commonly used in placebo studies (lasers, topical medication) without explicitly referring to any therapeutic potential. Using this broader definition of placebos, we study a little-researched phenomenon that may complement current views, namely somatic sensations induced by placebo interventions. They are frequently reported in the literature, little is known about their etiology
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