Increases in Arousal Are Associated with Reductions in the Human Nociceptive Flexion Reflex Threshold and Pain Ratings

Abstract

The nociceptive flexion reflex has been regarded as an objective correlate of pain perception. This study examined whether the nociceptive flexion reflex is modulated by physiological arousal in the same manner as pain. Cardiovascular activity, nociceptive flexion reflex thresholds, and pain ratings were measured in 40 healthy adults under three conditions that demanded varying degrees of physiological arousal: Rest, number repetition, and mental arithmetic. Heart rates were faster and R-wave to pulse intervals shorter during mental arithmetic than number repetition, whereas heart rates were slower and R-wave to pulse intervals longer during rest compared to number repetition. Both mental arithmetic and number repetition were associated with reduced nociceptive flexion reflex thresholds compared to rest. Although the nociceptive flexion reflex thresholds were the same during mental arithmetic and number repetition, pain ratings were lower during mental arithmetic than number repetition and rest. The results indicate that nociception was facilitated but pain was inhibited by increased physiological arousal. This dissociation suggests that the nociceptive flexion reflex threshold is not a suitable correlate of pain during states of increased arousal.