Placebo analgesia: a PET study.

Placebo analgesia involves complex mechanisms and sometimes has a marked effect on patients in pain. In this study we examined changes in regional cerebral blood flow (rCBF) under three different conditions (resting, hot, painful) before and after placebo administration using H(2)(15)O and positron emission tomography in ten healthy subjects. In five subjects, placebo administration significantly decreased pain-intensity score (placebo responders), and rCBF in the medial prefrontal cortex (MPFC), posterior parietal cortex (PPC), and inferior parietal lobe (IPL) increased after placebo administration compared with before placebo administration under the painful condition. Furthermore, in the placebo responders, rCBF in the MPFC, PPC and IPL also increased under the resting condition (without sensory stimulation) after placebo administration compared with before placebo administration. However, there was no rCBF change under the rest condition in the placebo nonresponders after placebo administration. These results suggest that placebo analgesia has its effect under the resting condition and MPFC, IPL and PPC may have an important role in placebo analgesia.