A subconscious placebo effect?

Los Angeles Times

To most people, the story of the placebo effect is simple: Because we believe that medicine makes us healthier, a pill—even if it is just sugar—causes us to feel better when we take it. As a result, the placebo effect has generally been considered a conscious process, the result of seeing the pill and the doctor in the white coat who gives it to us.

But a new study, published this week in the Proceedings of the National Academy of Sciences, suggests that the placebo effect doesn’t require conscious perception at all. Instead, we may constantly be registering subconscious cues that directly impact the likelihood of a treatment working.

The researchers at Massachusetts General Hospital who performed the study didn’t give their subjects pills. Instead, they put them through an experiment in which the subjects saw one human face while they were exposed to a very hot stimulus, and another human face while they were exposed to a cooler stimulus. They then showed the subjects each face while exposing them to the same stimulus of medium heat. In this case, the faces were taking the place of the placebo pill, and the heat of the stimulus was taking the place of the physical response to that pill.

As expected, the subjects thought the medium-heat stimulus was very hot when they saw the face that had previously been attached to the hotter, more painful stimulus, and cooler when it was connected to the face that had been shown with the low heat stimulus.


Those findings were nothing new. But then the researchers repeated the experiment while showing the faces so rapidly that they could not be perceived by the subjects’ conscious visual systems. Even though the subjects said they could not differentiate between the faces, they still thought the stimuli were hotter or cooler than they really were—and it again depended on the face rapidly flashed upon the screen.

The researchers argue that the results show the placebo effect can occur below the level of conscious perception or thought. This means that subconscious environmental cues like a physician’s subtle body language or the color of a hospital room may contribute to a patient’s recovery—or diminish it.

You can read a summary of the article here.

Return to the Science Now blog.