NEW YORK: Individual differences in the area of the brain linked with reward may help explain why some people report pain relief from inert treatments like sugar pills, according to a new study.

This phenomenon, known as the placebo effect, is well known to researchers. In studies that pit an actual treatment against a placebo, or inactive substance, some placebo recipients will inevitably report feeling better. But the question remains whether this represents a purely psychological reaction, or whether some people have an actual physical response to substances they believe will make them better.

In the new study, researchers found that people who expected to get pain relief from a treatment showed a greater release of dopamine in the part of the brain called the nucleus accumbens.

Dopamine is a chemical messenger and its activity in the nucleus accumbens is related to reward anticipation. In this study, people who expected a reward from their pain treatment showed a greater dopamine response.

In turn, they were more likely than other study participants to report pain relief - even though they all received a placebo. The findings, published in the journal Neuron, build on evidence that there is a physiological component to the placebo effect. In a previous study, the same research team found that, in true believers, a placebo "painkiller" triggered the brain to release natural pain-relieving substances called endorphins.

Dopamine itself is unlikely to have pain-relieving effects, said Dr Jon-Kar Zubieta, a neuroscientist at the University of Michigan in Ann Arbor who was the principal investigator in both studies. But, he said, its activation does appear necessary to spur the release of endorphins.

Understanding the processes that generate the placebo effect is important, Zubieta said, because it might be possible to affect these natural mechanisms for therapeutic purposes. In clinical trials, researchers try to minimise the placebo effect so they can clearly see if the drug or other intervention they are testing is effective. But in other instances, Zubieta explained, it would be better to enhance the placebo effect, so we can "take advantage of our own capacities to modulate our own bodily response."