Nonconscious Activation Of Placebo And Nocebo Pain Responses, Proceedings of the National Academy of Sciences, September 2012
"Significant placebo and nocebo effects were found in both experiment 1 (using clearly visible stimuli) and experiment 2 (using nonconscious stimuli), indicating that the mechanisms responsible for placebo and nocebo effects can operate without conscious awareness of the triggering cues. This is a unique experimental verification of the influence of nonconscious conditioned stimuli on placebo/nocebo effects and the results challenge the exclusive role of awareness and conscious cognitions in placebo responses."The National Center for Complementary and Alternative Medicine (NCCAM) described the study:
"In this study, researchers conducted two experiments on a total of 40 healthy participants. Both experiments began with a conditioning (or introductory) phase in which researchers administered a series of rapid high and low heat stimuli to participants’ forearms, while showing pictures of men’s facial expressions of either mild discomfort or severe pain that corresponded with the stimuli.The impact of this is bewildering. It's amazing enough that we can feel more or less pain by just looking at a picture of someone showing or not showing pain. But ... How many subliminal cues are we exposed to in a day that have the potential to effect our behavior and our health?
In the first experiment, the researchers again administered heat stimuli to the participants’ arms while showing them pictures of men’s faces – but this time, the stimuli were at a constant moderate temperature throughout the experiment. Participants were told that each picture is paired with a pain stimulus on their arms, and they were asked to rate the level of pain they felt using a scale of 0 to 100 (0 = no pain, and 100 = worst imaginable pain). Although the heat stimuli were constant, when participants were shown the facial expression of high pain, they rated their own pain levels at almost three times higher than when they were shown facial expressions of low pain.
In the second experiment, the researchers repeated the test on a separate group of participants, but showed the images for a much shorter time so that the participants were not able to consciously recognize the expressions. Despite this, the participants’ pain scores still correlated with the “high” and “low” pain images."