Lesser evil paradigm: evaluating pain unpleasantness by a novel cross-modality matching technique

Psychophysical evaluations of evoked pain sensations target two dimensions - intensity or unpleasantness. This study developed alternative, behavioral measures of unpleasantness by directly matching two stimulus modalities. In each of two paradigms, subjects receive both an aversive non-nociceptive stimulus (loud sound) and painful blunt pressure applied to the thumb. For each pair, subjects determine the least aversive stimulus (the lesser evil) and, based on subject choice, the intensity of the pressure stimulus is adjusted by a tracking algorithm to match two (low and high) specified levels of unpleasantness of auditory stimulation. The two paradigms differ in the temporal presentation of the stimulus pair. In the “Escape” method, an auditory and a pressure stimulus are delivered simultaneously, while in the “Choice” method, these stimuli are delivered separately in a random counterbalanced order. Eighteen women with fibromyalgia (FM) and 17 healthy control women (HC) completed both methods. The results of each method did not differ for either FM (p=0.43) or HC (p=0.57). The effect of stimulus level (low, high) was significant for both methods (p<0.001) with no interaction between stimulus level and method (p=0.19). Correlational analyses revealed significant associations between stimulus levels for each method and group (r=0.65 to 0.93, p<0.001 to p=0.004). Interestingly, the Escape measure was the only measure associated with clinical pain magnitude, both with current pain (r=-0.74, p<0.001) and pain over the last two weeks (r=-0.56, p=0.024). In the Escape method, greater clinical pain in FM was associated with decreased pressure matches to a constant auditory sound, suggesting a facilitatory modulation of low pressure pain sensitivity by concomitant clinical pain and aversive sound. Our results show that behavioral paradigms can discriminate levels of unpleasantness of evoked painful and non-painful sensations and may provide a sensitive indirect measure of pain modulation in patients with fibromyalgia or other pain conditions.