Here's what placebos can heal -- and what they can't

Parkinson’s is the perfect disease to talk about placebos. It is a chronic deficiency of dopamine, which is one of those brain chemicals that does a lot of jobs in our bodies. One of [dopamine’s] important roles is in reward processing: how we think about good things we might get in the future.

Expectation drives placebos. And dopamine is a chemical that’s very responsive to our expectations. Parkinson’s happens to be a deficiency in the very chemical that’s very important in placebo effects and rewards.

If you look at Alzheimer’s, which does not have a high placebo response, you start to see that there are rules at play when it comes to placebos. It’s not your brain magically doing all these crazy things. There are certain chemicals we have access to and others we don’t.