Conditioned stress-induced analgesia in humans

The purpose of this experiment was the demonstration of conditioned stress-induced analgesia in humans. Following a baseline day (day 1), two groups of 10 subjects were each exposed to green light as the conditioned stimulus (CS) and mental arithmetic plus noise as the unconditioned stimulus (US) for 5 days (days 2–6), a third group of 10 subjects was only exposed to the US. On the test day (day 7), pain threshold and pain tolerance were tested, while groups 1 and 3 received the green light CS and group 2 received no CS. Group 1 showed a significantly higher pain threshold and higher pain tolerance on the test day, whereas the two control groups did not have altered pain perception. Heart rate data confirmed successful stress induction. Heart rate and blood pressure were unaltered on the test day, thus making cardiovascular mediation of the conditioning effect unlikely. Conditioned stress analgesia might be useful in the understanding of chronic pain-syndromes.