They showed the benefits of painkillers could be boosted or completely wiped out by manipulating expectations.
The study, published in Science Translational Medicine, also identifies the regions of the brain which are affected.
Experts said this could have important consequences for patient care and for testing new drugs.
Heat was applied to the legs of 22 patients, who were asked to report the level of pain on a scale of one to 100. They were also attached to an intravenous drip so drugs could be administered secretly.
The initial average pain rating was 66. Patients were then given a potent painkiller, remifentanil, without their knowledge and the pain score went down to 55.
They were then told they were being given a painkiller and the score went down to 39.
Then, without changing the dose, the patients were then told the painkiller had been withdrawn and to expect pain, and the score went up to 64.