Observing pain related responses in others can impact our own pain perceptions

Although placebo analgesia is popularly believed to be prompted by taking a "sugar pill" (an inert substance that have no pharmacological effects) while being led to believe that it is an effective painkiller, there are actually many different ways of prompting the placebo response. These include verbal suggestions that induce expectations of efficacy (do these exercises and you'll feel better), conditioning (repeated exposure to a treatment and its effectiveness), etc.

But Luana Colloca and Fabrizio Benedetti, two very prominent researchers in the area placebo effects research recently published an intriguing study investigating social influences on placebo analgesia.

What they did was randomly assign participants to one of 3 groups: social observation, conditioning, and verbal suggestion.

In all groups, participants experience a series of electric shocks preceded by either a red or green light. The placebo part of the study is a sham electrode affixed to the middle finger, which participants are told  delivers a nonpainful stimulation that has an analgesic effect; that is it can reduce the pain of the shocks. The green light is supposed to indicate that this analgesic stimulus has been turned on and that reduced pain should be anticipated.

Now, in the social observation group, participants learn from watching a "demonstrator" go through the series of trials first, that the green light precedes diminished pain relative to red light trials. They can see this because they can see the demonstrator rate the painfulness each pain shock, and they can see that he always rates green trials as less painful than red trials. Subsequent to this "simulation" phase, participants receive a series of red and green-associated pain stimuli in the same way as the demonstrator. In fact green light stimuli were set to be equally painful to red light stimuli.

In the conditioning group, participants are informed that the green and red lights indicate the activation and deactivation, respectively, of the electrode on the middle finger and then are given a series of shocks. To condition the analgesic effect associated with the green light, the shock intensity was surreptitiously lowered. Thus, while participants were led to believe that the electrode affixed to their middle finger was being activated during green light trials and that it was this activation that had an analgesic effect, the electrode was not activated at all and instead, the shock intensity was lowered. After this conditioning phase, a testing period begun whereby the stimulus intensity of green light trials was raised to the same level as red light trials.

In the verbal suggestion group, participants were simply informed that the green light would signal the activation of the analgesic electrode while the red light would indicate its inactivation. In fact, all stimuli were equally painful as in the testing phase of the other conditions.

So what did they find? Well, Colloca and Benedetti were not surprised to find that the conditioning group exhibited a placebo effect as they had shown this effect in a previous study. The unique and truly cool finding here is that participants did not need to experience this conditioning first hand. It was sufficient to merely observe another person undergoing a beneficial effect of a treatment (the "analgesic" electrode). Indeed the magnitude of the placebo responses induced through observational learning was similar to the placebo responses induced through the direct experience of a conditioning procedure.

But that's not all. Another intriguing finding was that the amount of pain reduction exhibited in response to green trials compared to red trials was significantly correlated with how high participants scored on a measure of empathy. So the stronger one's empathic ability, the stronger was the placebo effect following social observation. Why do I find this so fascinating? I'll tell you about it in an upcoming blog post.

In the meantime, do check out this fascinating piece of research for yourself.

Colloca, L & Benedetti, F (2009). Placebo analgesia induced by social observational learning. Pain, 144 (1-2), pp. 28-34.