The Effect of the Type and Colour of Placebo Stimuli on Placebo Effects Induced by Observational Learning

Abstract

Research shows that placebo analgesia and nocebo hyperalgesia can be induced through observational learning. Our aim was to replicate and extend these results by studying the influence of the type and colour of stimuli used as placebos on the placebo effects induced by observational learning. Three experimental and two control groups were tested. All participants received pain stimuli of the same intensity preceded by colour lights (green and red) or geometric shapes (circles and squares). Before receiving pain stimuli, participants in the experimental groups, but not in the control groups, observed a model who rated pain stimuli that were preceded by either green lights (green placebo group), red lights (red placebo group), or circles (circle placebo group) as being less painful than those preceded by either red lights (green placebo group), green lights (red placebo group), or squares (circle placebo group). As a result participants in the experimental groups rated pain stimuli preceded by either green lights (green placebo group), red lights (red placebo group), or circles (circle placebo group) as being less painful than the participants in the control groups did, indicating that placebo effect was induced. No statistically significant differences were found in the magnitudes of the placebo effects between the three experimental groups (green placebo, red placebo, and circle placebo groups), indicating that neither the type nor the colour of placebo stimuli affected the placebo effects induced by observational learning. The placebo effects induced by observational learning were found to be unrelated to the individual differences in pain anxiety, fear of pain, and empathy.

Fig 1. Study design.

Participants in experimental groups observed a model who rated pain stimuli preceded by green lights (green placebo group), red lights (red placebo group) or circles (circle placebo group), as being less painful than those preceded by red lights (green placebo group), green lights (red placebo group), or squares (circle placebo group). All participants received 16 pain stimuli preceded by eight green lights and eight red lights (green placebo and red placebo groups) or eight circles and eight squares (circle placebo group). Participants in the control colours and control shapes groups did not observe a model before they received the 16 pain stimuli preceded by eight green lights and eight red lights (control colours group) or eight circles and eight squares (control shapes group).

Fig 2. Differences between placebo and nonplacebo pain ratings in the three experimental groups (model condition; green placebo, red placebo, and circle placebo groups) and two control groups (no-model condition; control colours and control shapes groups).

The difference between placebo and nonplacebo pain ratings in the model condition was significantly higher than in the no-model condition. However, there were no statistically significant differences between the three experimental groups as well as between the two control groups.

Fig 3. Trial by trial placebo versus nonplacebo mean pain ratings of the three experimental groups (model condition; green placebo, red placebo, and circle placebo groups) and two control groups (no-model condition; control colours and control shapes groups).

In the model condition, as opposed to the no-model condition, pain stimuli preceded by placebo stimuli (i.e., green lights in green placebo group, red lights in red placebo group, and circles in circle placebo group) were rated as less painful than pain stimuli that were preceded by nonplacebo stimuli (i.e., red lights in green placebo group, green lights in red placebo group, and squares in circle placebo group).