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. 2017 Jun;153:109–121. doi: 10.1016/j.neuroimage.2017.03.041

Fig. 1.

Fig. 1

A) Experimental task. Top-down attentional set was manipulated at the block level by presenting a verbal instruction to attend away from pain and towards the visual stimulation on the screen (i.e., cue=“CROSS”; unattended pain) or attend to the painful stimuli perceived on the dorsum of the hands (i.e., cue=“HAND”; attended pain). In each block, a total of 25 trains of painful stimuli were delivered to the dorsum of one hand at the time, using an oddball roving sequence. Each train included 3–7 stimulus repetitions at a constant inter-stimulus interval of 1 s. Each painful stimulus consisted of two rapidly square-wave pulses of 50 μs duration, with an inter-pulse interval of 5 ms. The same stimulus intensity was used for both left and right stimuli. We considered a deviant (d) the first stimulus in each train (i.e., change in stimulus location). To match the number of trials of deviant and standard stimuli, we only modeled the last repetition before a change as a standard (s). While painful stimuli were delivered, a fixation cross on the screen changed in color from black to white or vice versa every 2–5 s. The visual change never occurred at the same time as a change in the painful stimulus location. When instructed to pay attention to the visual stimulation, participants had to silently count the number of time the cross changed in color from white to black or vice versa (Block A). Instead, when instructed to pay attention to the painful stimuli, participants had to silently count the number of times the stimulation switched from the left to the right hand or vice versa (Block B). Block order was counterbalanced across participants. At the end of each block, participants were required to report the number of switches, as well as to rate the average pain intensity experienced for each hand. B) The probability of repetitions between 3–7 times was 5%, 15%, 60%, 15%, 5%, respectively. C) Mean and standard error of pain ratings for left and right hand and attended (white) and unattended (grey) pain, separately. Participants reported less intense pain when the painful somatosensory stimuli were unattended.