The influence of conditioned fear on human pain thresholds: does preparedness play a role?

Emotionally charged facial expressions (happy, fear) served as conditioned stimuli in a differential fear conditioning procedure. Expressions were presented in pseudo-random order on a computer monitor. For half of the participants, the fear expression was paired with an aversive electric stimulation (UCS), whereas the happy expression was unpaired. The other participants had the opposite pairing. To assess the influence of conditioned fear on pain, expressions were shown again in the absence of the UCS and pain threshold was assessed during each expression. The latency of finger withdrawal from a radiant heat device was used to index pain threshold. Skin conductance response (SCR) and self-reported emotion were measured to assess fear conditioning. Consistent with preparedness theory, differential fear conditioning was only present when the fear expression was paired with the UCS. Moreover, pain threshold was only influenced by fear conditioning in persons for whom the fear expression was paired with the UCS. Specifically, finger withdrawal latencies were lower (suggesting hyperalgesia) during the fear expression than during the happy expression; an effect that was not present before CS-UCS pairing. This work suggests that some stimuli are more readily associated with an aversive event and can lead to pain enhancement.

Although preliminary, these results suggest that fear-relevant environmental stimuli (including facial expressions) may provide important environmental cues during aversive events that influence the level of pain experienced.