Pupil dilation evoked by painful electrical stimulation is abolished during pain inhibition by distraction.

The aim of the present study was to examine the contribution of spinal and supraspinal processes to pain modulation by attention. It is hypothesized that pain inhibition by distraction is accompanied by reduced pain-evoked pupil dilation and cerebral activity, but no inhibition of the nociceptive flexion reflex (NFR), while pain anticipation is expected to increase pain perception and pain-related responses. Twenty healthy volunteers received 90 painful stimuli in control, distraction (mental arithmetic), and anticipation (visual cue) conditions. Anticipation did not modulate pain (p = .7), while distraction decreased pain significantly (p < .001). Moreover, pupil diameter increased 500-1000 ms post-stimulus in the control condition (p < .05), but this response was abolished by distraction. Distraction also decreased pain-related brain activity (high-gamma oscillations) (p = .004), but not the NFR (p = .3). These results suggest that pain inhibition by distraction is produced, in part, by supraspinal inhibition of nociceptive processes.