We hear a lot about the placebo effect, that remarkable ability we have to make ourselves feel better through the belief that something beneficial is happening to us. Occasionally it's sold to us as the power of positive thinking, or just looking on the bright side, being optimistic, expecting good things to happen. But how does it work? How can we kid ourselves so effectively? What is really going on?
The placebo effect is certainly very well supported by evidence. People report that they feel better when told they are being given treatment for an ailment, even when the treatment is fake. As long as they believe they are getting something, they behave as if they were actually getting it. They will report that pains have lessened, that conditions have improved, that they feel different, better.
Two questions arise from this. First, has anything changed to make them feel differently, perhaps in their brain chemistry, or in their perceptions? And secondly, has anything actually changed in the body, in the condition of the ailment itself? The first is about the brain's perception of the state of the body, and the second is about the state of the body itself. They are very different questions and both are important.
Our brains construct representations of reality that allow us to negotiate the world very effectively. Not only do we perceive the world through our senses, but being part of the world, we also perceive sensations from our own bodies. Normally, the brain suppresses many of these sensations which we would otherwise feel, because we would be inundated with signals from all parts of our bodies. That would render us unable to discriminate between the important and the mundane.
Our brain discriminates and lets us ignore the unimportant signals from our body in favour of the significant ones that help us move around in the world. That filtering of our body sensations is one of the reasons you can't tickle yourself. Your brain desensitises your perceptions to help you focus on the important things in the external world.
But the brain also constructs our mental world. We learn to empathise by mimicking the expressions of people we see, and this in turn creates similar emotional changes to those we are observing, and hence similar changes in brain chemistry. Just the slight action of copying some facial expression that we see in others, triggers changes in our brain chemistry which produce similar feelings. This is the key to understanding the placebo effect.