In their experiments, the researchers tested the effect of placebo treatment on volunteer subjects' ratings of unpleasant pictures, such as images of mutilated bodies. Other researchers had shown that people's expectation of pain relief plays a major role in the effectiveness of placebos. So, in their experiments, the researchers first induced such an expectation in the subjects by administering an antianxiety drug to reduce the subjects' unpleasant perception of the pictures. They then administered an antidote to the drug, telling the subjects that it would restore the unpleasant perception.
In tests the following day, the subjects were told they would receive the same drugs. But instead, they received a saline solution as a placebo when they saw the pictures. The subjects' brains were also scanned during these experiments using functional magnetic resonance imaging (fMRI). This scanning technique uses harmless magnetic fields and radio waves to measure blood flow in brain regions, which reflects brain activity.
The researchers found that the placebo reduced the subjects' ratings of the unpleasant pictures by about 29%. Importantly, the fMRI scans showed that the placebo reduced activity in the brain's emotional centers, and this reduction correlated with a subject's amount of reduction in the rated unpleasantness of the pictures. That is, the subjects who reported the largest placebo response also showed the largest decrease in activity in the emotional centers.
Also importantly, the placebo increased brain activity in the same "modulatory network" whose activity is increased when placebos have been used to relieve pain.
The researchers found that, as in placebo treatment for pain, expectation played a major role in effectiveness. Those subjects who expected the largest effect--as measured by the effect they reported from the real antianxiety drug the previous day--showed the largest changes in activity in the emotional and modulatory areas.
"The present data demonstrate that emotional experience may be modulated through a placebo treatment in a similar manner as has been previously reported for pain perception," concluded Petrovic and his colleagues. They wrote that the study "demonstrated that very similar mechanisms are involved in the placebo response of emotional stimuli and in placebo analgesia, thereby generalizing the concept of placebo and its associated underlying neural processes." They also concluded that the placebo effect can be thought of "as a general process of modulation induced by the subjects' expectations, possibly using specific modulating systems."
The researchers include Predrag Petrovic, Thomas Dietrich, Peter Fransson, Jesper Andersson, Katrina Carlsson, and Martin Ingvar of the Cognitive Neurophysiology Research Group R2-01 of the Department of Clinical Neuroscience at Karolinska Institutet in Stockholm, Sweden. This study was supported by grants from the Swedish Medical Research Council, Förenade Liv, the Karolinska Institute, the Swedish Medical Association, the Knut and Alice Wallenberg foundation, and the Petrus and Augusta Hedlunds foundation.
Petrovic, P, Dietrich, T., Fransson, P., Andersson, J., Carlsson, K., and Ingvar, M. (2005). Placebo in Emotional Processing--Induced Expectations of Anxiety Relief Activate a Generalized Modulatory Network. DOI 10.1016/j.neuron.2005.05.023 Publishing in Neuron, Vol. 46, June 16, 2005, pages 957-969. www.neuron.org