Check out our new paper: http://www.jneurosci.org/content/33/13/5638.full.pdf - free online at the Journal of Neuroscience.
Placebo and nocebo effects are well known - and describe how predictions bias experience of pain. In a nutshell, we show that if those predictions are more uncertain, this uncertainty increases pain independently of the bias induced by the basic placebo / nocebo effect. This effect is correlated with fMRI BOLD activity in the PAG.
Uncertainty increases pain: evidence for a novel mechanism of pain modulation involving the periaqueductal gray. Yoshida W, Seymour B, Koltzenburg M, Dolan RJ. J Neurosci. 2013 Mar 27;33(13):5638-46. doi: 10.1523/JNEUROSCI.4984-12.2013. PMID: 23536078