Evaluation of a Bayesian model of pain modulation and placebo effect

Pain is a highly subjective experience and several cognitive factors can modulate pain perception. Research in this field is mainly focused on neuronal circuits and neurobiological processes involved in pain perception and few studies have proposed theoretical models for better understanding the influence of these factors. The aim of this study was to evaluate a Bayesian theoretical model of pain perception. In particular, the model we propose can describe the changes in subject’s pain scoring after a conditioning session in a placebo experimental paradigm. To validate our model, two groups of volunteers underwent two different experiments in which analgesic placebo effect were induced by a conditioning training with high- and low-intensity painful stimuli, paired with two visual cues. Mechanical stimulation was used in the first experiment, electrical stimuli was applied in the second one. Pain perception was assessed by means of a Visual Analog Scale (VAS). We found that conditioning could modulate pain sensation as demonstrated by the difference in subjects’ pain scoring in the testing session, in which the same stimulus intensity was applied with both cues. Moreover, the results demonstrated that the stimulus-VAS curve was modified after the conditioning even in the absence of any visual cue as predicted by our model. These findings support the validity of our Bayesian model as a possible new theoretical framework for understanding the role of cognitive factors in pain modulation.