Fig 1. Study design.

A total of 42 female volunteers were randomly assigned to three groups: placebo, nocebo, and control group. Each group consisted of 14 participants. The study consisted of three phases: calibration, conditioning, and testing. Calibration was conducted to determine nonpainful tactile sensation (t) and the pain threshold (T). In the conditioning phase, a total of 72 electrical stimuli were delivered–half of the stimuli were of moderate intensity (1.5 × T mA, control stimuli) and the remaining half were nonpainful ([t + 0.8 × (T—t)] mA in placebo group) or higher intensity ([2.2 × T—0.2 × t] mA in nocebo group). The intensity of the electrical stimuli was always set at 1.5 × T mA in the control group. The testing phase consisted of 24 control stimuli (1.5 × T mA) regardless of group assignment.