Graphical scheme of the experimental settings applied in either group 1 (top bars) or group 2 (bottom bars) subjects.

<p>The paradigm applied to either group is shown in each two lines of differently colored small bars of which each denotes a CO₂ stimulus and the associated cues. Specifically, in the upper lines, the CO₂ stimuli are shown in their chronological succession differently colored for 25% v/v CO₂ stimuli (green), CO₂ concentration at the individual pain threshold level (grey) and 75% v/v CO₂ (red). Below each stimulus, in the second line the cues given before stimulus application are denoted as “No pain” (green), “Stimulus”, i.e., a neutral cue (grey) and “Pain” (red). In the first group, the far-from-thresholds stimuli were cued randomly, i.e., green and red bars in lines 1 and 2 are mixed, whereas in the second group, the 25 and 75% stimuli were always preceded by correct cues, which can be seen by the consistent agreement between green or red bars in lines 1 and 2. The inequality of the experimental settings between the groups is commented on in the discussion of this paper.