July 20 (HealthDay News) -- A new study reveals a brain region behind the placebo effect -- the phenomenon in which a person's belief in the effectiveness of a treatment boosts its effect.

With the placebo effect, your belief that a treatment will work actually causes you to feel the beneficial effects, even when you are only given a placebo (e.g., sugar pill).

In a study published in the July 19 issue of Neuron, researchers at the University of Michigan, Ann Arbor, studied the nucleus accubens (NAC), a region deep in the brain that is known to play a role in reward expectation.

Previous research has suggested that the NAC may be involved in the placebo effect.

The researchers told study participants that they were testing a new pain-killing drug and that the participants would receive either the drug or a placebo. But the researchers gave all of the participants a placebo injection of salt solution.

The participants rated their expectation of the pain-killing effects of the "drug," as well as the level of pain relief they felt with and without the "drug" from a moderately painful injection of salt solution in their jaw muscle.

For the first experiment, the researchers used positron emission spectroscopy to measure release from the NAC of the neurotransmitter dopamine -- a chemical trigger of the brain's reward response.

The greater the participants' expectation of the pain-killing benefit of the "drug," the greater the dopamine release from the NAC. In addition, the participants who reported more relief from the "drug" when they did experience pain showed greater NAC activity when they received the placebo before the pain.

In another experiment, the researchers told the subjects to expect monetary rewards of different amounts, while scanning their brains with functional magnetic resonance imaging. The participants who showed greater activation of the NAC during this reward processing also showed greater anticipation of the effectiveness of a placebo.

According to the study authors, the findings support the theory that the NAC system may need to be activated to allow the placebo effect to occur. This information may be used in the future to develop new therapies for a variety of conditions.