Generalization of placebo and nocebo effects across pain modalities and from pain to itch.

In this study on healthy participants, we are investigating whether placebo and nocebo effects on pain can generalize to another type of pain and to itch. Placebo and nocebo effects will be induced by combining verbal suggestion (by telling the participant that the sensation will decrease/increase) with conditioning with heat pain (by actual changes in the stimulus intensity of induced heat pain). To test generalization, pressure pain stimuli will be applied during six trials; three with a control cue and three with a conditioned cue. Moreover, cowhage-evoked itch will be applied twice, once with a control cue and once with a conditioned cue. This study uses a within-subjects design, pain/itch ratings with the conditioned cue will be compared with the control cue in the placebo and nocebo group, respectively.

1.The primary objective of this study is to test whether placebo and nocebo effects generalize within pain stimulus modalities, i.e. from heat pain to pressure pain.
2.The secondary objective of this study is to test whether placebo and nocebo effects generalize across somatosensory modalities, i.e., from heat pain to cowhage-evoked itch.
3.The exploratory objective of this study is to explore the role of individual characteristics variables (i.e., anxiety and depression; stress; attention to pain and itch; optimism and pessimism; pain catastrophizing; itch catastrophizing) in the generalization of placebo and nocebo effects within pain stimulus modalities and from pain to itch.

The whole experiment will take around 2 hours and 30 minutes per participant in a single session.

Participants will learn the links between the changes of heat pain intensity with an Electrical Nerve Stimulation (ENS) device ‘ON/OFF’. This ENS device serves as a sham device and does not work in the main test. In the placebo group, participants will be told that ENS ‘ON’
means a decrease of heat pain (a conditioned cue) and ENS ‘OFF’ means no change of heat pain (a control cue). In fact, participants will receive low heat pain with ENS ‘ON’ and medium heat pain with ENS ‘OFF’ during the learning phase. In the nocebo group, participants will be told that ENS ‘ON’ means an increase of heat pain (a conditioned cue) and ENS ‘OFF’ means no change of heat pain (a control cue). In fact, participants will receive high heat pain with ENS ‘ON’ and medium heat pain with ENS ‘OFF’ during the learning phase.