Functional magnetic resonance imaging shows that the expectation of pain relief associated with use of a placebo results in decreased activity in brain regions associated with perception of pain, according to an article in the February 20th issue of Science.

In two related studies, American researchers used functional magnetic resonance imaging to map changes in blood flow in the brains of healthy volunteers. The volunteers were subjected to harmless but occasionally painful electric shocks or heat. When they believed an anti-pain cream had been applied to their arms, they rated the pain as less intense--and the pain circuits in their brain showed less activity.

Doctors have long recognized the power of placebo to make patients feel better. However, the new studies provide the first scans documenting the changes induced by placebo in the brain’s pain pathways.

“We’ve shown what the old family doctor knew very well?that his interaction with the patient made a great difference in the effectiveness of whatever treatment he was giving,” said one of the researchers, Kenneth L. Casey, MD, who has studied pain for three decades.

The two pain studies, each involving about two dozen volunteers, show the prefrontal cortex as the area of the brain active in the placebo response. Scientists have developed intriguing models of how this region guides thought and action based on internal goals and expectations. In support of those theories, the new research provides the first images of how the prefrontal cortex is activated by the expectation of pain relief, and how this in turn triggers a reduction of activity in pain-sensing areas of the brain: the thalamus, somatosensory cortex, and other parts of the cerebral cortex.

According to Casey, this clearer knowledge of the brain’s pain pathways may lead to new therapies for those with chronic or acute pain. “One could imagine compounds that would activate these control systems specifically,” he said.

Casey also said the research sheds new light on the tangible benefits of the placebo effect in medicine. “If you’re providing a treatment to a patient, it’s important that you realistically provide them with the expectation that it would work, so you enhance the effect. If you gave them a drug or any kind of treatment with the attitude, either explicit or implicit, that this might not be effective, it would be much less likely to be effective.”