Interventions

tDCS was applied using Starstim 8 equipment (Neuroelectrics, Barcelona, Spain). Cathodal and anodal electrodes featured an area of 25 cm2 each placed inside a round sponge soaked in saline solution. Electrodes were placed at spatial positions F3-(Anodal) and F4-(Cathodal) according to the international 10–20-system for electroencephalogram electrode placement,36 commonly considered surface locations above (mid-) left and right DLPFC, respectively.37 Stimulation was delivered at an intensity of 2 mA for 20 min, including a 30s ramp up to 2 mA at the start and a 30s ramp down to 0 mA at the end. In the sham group, current was initially ramped to 2 mA over 30 s and then immediately ramped down to 0 mA. In the final 30 s, the current ramped up to 2mA and back to 0 mA. During stimulation, participants were asked to relax their bodies while sitting and supported in a comfortable position.

The techniques of hypnosis developed for this study are found on the classical approach developed by the American clinician and Ph.D. Mark P Jensen.38 To guarantee that the hypnotic analgesia suggestion would be conducted equally in all subjects, only one psychologist with more than ten years of experience in practicing hypnotherapy conducted all HS sessions. The standard hypnotic protocol begins with an induction to the subjects to focus their attention on a single stimulus and associate this with breathing and relaxation. Through the 8 final minutes of the induction, suggestions were used to reduce the pain of the participants and increase control over their own sensations.39 We used the protocol of HS previously published by Jansen,28 which follows standardized steps (see the hypnotic analgesia suggestion protocol in the Supplemental material (Appendix I)). The duration of experimental manipulation (induction + suggestions) is 20 min.