You may (not always) experience what you expect: in search for the limits of the placebo and nocebo effect.

A wealth of research shows that the expectancy to improve, either by verbal suggestion or by learning experiences, results in less pain and suffering [8]. There are also intriguing studies showing the relevance and usefulness of placebo in clinical settings [4]. Far less research is available on the reverse, the nocebo effect. Overall, results of these studies are concordant with the placebo research: expectation of adverse somatosensory events may induce their experience. It is acknowledged that the effects of placebo and nocebo are genuine, not artefacts of methodological shortcomings. It is also clear that the mechanisms underlying these phenomena are not unitary, but vary as a function of contextual and individual specificities. Although the key mediator is expectancy, different physiological and psychological mechanisms likely contribute under different circumstances. In the current issue of Pain, Van Laarhoven and colleagues [10] investigated effects of nocebo and placebo instructions on pain and itch. Healthy volunteers received verbal instructions that only slightly varied as to the somatosensory events to be experienced. The overall results are very clear. We tend to experience what we expect. More pain is experienced when participants were informed that most subjects experienced the stimuli as painful. More itch is experienced when participants were informed that most subjects experienced the stimuli as itch. Likewise, suggestions that the intensity of the sensations will decrease results in a decrease of the experience. This elegant study adds to the vast database on placebo and nocebo effects, however, the paper also reveals some remarkable novel findings. First, the paper focuses on the effects of verbal suggestions on pain and itch. Intriguingly, itch seemed to be more malleable to verbal suggestion than pain. Second, the authors used a wide range of somatosensory stimuli including mechanical stimuli (monofilaments), a series of short electrocutaneous stimulus at fixed intensity, an electrocuteanous stimulus at tolerance level, and the application of histamine. Apparently not all stimuli are equally potent in inducing a placebo or nocebo effect. All but mechanical stimuli successfully induced pain following nocebo instructions. These findings are subtle at first examination, but they raise a fundamental question that deserves scrutiny: Where are the limits of the placebo and nocebo effect? Taken together, the literature on placebo and nocebo seems to indicate that we experience what we expect. Although true to some extent in some situations, we do not live in a hallucinatory world, where expectations are self-fulfilling prophecies and …