Every 15 seconds during the scans, they were asked to rate the intensity of their pain sensations on a scale of 0 to 100, and they gave more detailed first-person ratings after the experiment. The researchers correlated the participants' ratings with their PET scan images, which were made using a technique that reveals the activity of the brain's natural painkilling endorphin chemicals, also called endogenous opioids.
Endogenous opioids bind to brain cell receptors called mu-opioid receptors, and stop the transmission of pain signals from one nerve cell to the next. Besides the brain's own chemicals, drugs such as heroin, morphine, methadone and anesthetics also act on the mu opioid receptor system to reduce pain.
Because the endorphin system naturally tries to quell pain whenever it occurs, the researchers slowly increased the amount of concentrated salt water being injected in the muscle as the scans continued, in order to keep the participants' rating of their pain within the same point range throughout the experiment. The placebo, a small amount of hydrating solution, was then given intravenously every four minutes.
As the researchers alerted participants that the placebo was coming, and injected the placebo dose, the amount of additional concentrated salt water needed to maintain participants' pain over time increased -- indicating a reduction in pain sensitivity that the subjects were not aware of. In other words, thinking they were getting a pain drug actually allowed the participants to tolerate even more pain-inducing concentrated salt water than before.
After each scan, the researchers asked the participants more questions about the
Source:University of Michigan Health System