The influence of repeated pain stimulation on the emotional aspect of pain: a preliminary study in healthy volunteers.

Pain is a multidimensional experience with sensory-discriminative, cognitive-evaluative, and affective-motivational components. Emotional factors, such as unpleasantness or anxiety, are known to have influence on pain in humans. Repeated painful stimulation has been reported to reduce subjective pain intensity. Nevertheless, there is little evidence of the influence of such stimulation on the emotional factors of pain. The aim of this study was to evaluate the influence of repeated painful stimulation on the experience of unpleasantness and anxiety.  Eight subjects (six females, two males) volunteered to participate in this study. Subjects received repeated painful stimulation for 3 consecutive days each instance lasting 6 seconds, 60 times per day, on the medial side of the forearm of the nondominant hand. The following items were examined to evaluate changes of responses to painful stimulation: pain thresholds, pain tolerance levels, pain intensities, unpleasantness, and anxiety. Furthermore, pain thresholds and pain tolerance levels were compared between different sites on the ipsilateral and contralateral forearms.  There was no immediate or chronological changes in pain thresholds or pain tolerance levels were observed. Pain intensities were reduced significantly over the 3-day experimental period (P<0.05). On the other hand, there was no great change in unpleasantness during the 3-day period. Anxiety was increased significantly after the painful stimulation compared with that without the painful stimulation and before day 1 of the stimulation (P<0.05).

The authors conclude that these results suggest that repeated painful stimulation may result in habituation to pain intensities but not habituation to emotional factors.