The role of learning in nocebo and placebo effects.

The nocebo effect consists in delivering verbal suggestions of negative outcomes so that the subject expects clinical worsening. Here we show that nocebo suggestions, in which expectation of pain increase is induced, are capable of producing both hyperalgesic and allodynic responses. By extending previous findings on the placebo effect, we investigated the role of learning in the nocebo effect by means of a conditioning procedure. To do this, verbal suggestions of pain increase were given to healthy volunteers before administration of either tactile or low-intensity painful electrical stimuli. This nocebo procedure was also carried out after a pre-conditioning session in which two different conditioned visual stimuli were associated to either pain or no-pain. Pain perception was assessed by means of a Numerical Rating Scale raging from 0=tactile to 10=maximum imaginable pain. We found that verbal suggestions alone, without prior conditioning, turned tactile stimuli into pain as well as low-intensity painful stimuli into high-intensity pain. A conditioning procedure produced similar effects, without significant differences. Therefore, in contrast to placebo analgesia, whereby a conditioning procedure elicits larger effects compared to verbal suggestions alone, learning seems to be less important in nocebo hyperalgesia. Overall, these findings indicate that, by defining hyperalgesia as an increase in pain sensitivity and allodynia as the perception of pain in response to innocuous stimulation, nocebos can indeed produce both hyperalgesic and allodynic effects. These results also suggest that learning is not important in nocebo hyperalgesia compared to placebo analgesia.