Figure 1. Experimental design.

Subjects were pseudorandomly assigned to either Group 1 (A) or Group 2 (B). Note that subjects of Group 1 underwent one session of conditioning, whereas subjects of Group 2 received four sessions of conditioning. In both the groups, there were two sessions of testing. As shown in legend (bottom right) each session consisted of a nocebo, control and placebo condition. Each condition was repeated twice for each session. A colored cue informed subjects about the condition under investigation throughout the experiment: they were told that the green cue would signal the procedure (a sub-threshold electric shock at the ankle) that in turn, induced a reduction of either non-painful or painful perception, the red cue would signal the procedure for increasing either non-painful or painful perception, whereas the yellow cue would indicate the control intensity of stimulation in which the sub-threshold electric shock at the ankle was turned off and no modulation of non-painful and painful perception was expected. The intensity of stimulation was respectively lower in green trials and increased in the red trials during the conditioning sessions, so that subjects experienced respectively real decrease and increase of their non-painful and painful perception. The characteristics of each trial of learning are detailed (C). At the beginning of each trial, the subjects received a visual warning about what condition was tested, either non-painful or painful. Then a visual cue, lasting 60s was presented on the monitor in front of the subject and a total of five stimuli were delivered at the same intensity but with a different inter-stimulus duration. A chain-light appeared from 4 to 9s before each electrical shock. Before the end of the trial, there was 1s for resting and 5s for rating. The inter-trial interval (ITI) was 35s. Behavioral ratings were assessed in each trial by means of two Visual Analogue Scales (VASs) differently anchored for non-painful and painful stimuli.