Placebo effects in laser-evoked pain potentials

Placebo treatment may affect multiple components of pain, including inhibition of nociceptive input, automatic or deliberative appraisal of pain, or cognitive judgments involved in pain reporting. If placebo analgesia is due in part to an attenuation of early nociceptive processing, then pain-evoked event-related potentials (ERPs) should be reduced with placebo. In this study, we tested for placebo effects in P2 laser-evoked potentials at midline scalp electrodes. We found that placebo treatment produced significant decreases in P2 amplitude, and that P2 placebo responses were large enough to reflect a meaningful difference in nociceptive processing. However, we also found evidence that the very robust placebo-induced decreases in reported pain are not solely explained by early reductions in P2. N2 amplitude was affected by neither placebo nor reduction of laser intensity. These results suggest that placebo treatment affects early nociceptive processing, but that another component of placebo effects in reported pain occurs later, either in evaluation of pain or cognitive judgments about pain reports.