Seeing facial expressions enhances placebo analgesia

The strength of the placebo effect is influenced by social contexts and individual personality. Although facial expressions provide important contextual cues, no study of their influence on the placebo response has been performed hitherto. Here we tested (1) whether the observation of facial expressions with different emotional content (Neutral, Pain, and Happy) affects the magnitude of placebo analgesia, and (2) whether interindividual differences in personality traits interact with any modulation of placebo response induced by facial expression. Twenty-seven healthy participants underwent classical placebo conditioning, and subsequently rated the intensity and unpleasantness of their pain experience associated with nociceptive-specific laser pulses delivered to the right hand dorsum. On each trial, different visual cues signalled the occurrence of a laser stimulus alone or of a laser stimulus accompanied by a sham analgesic treatment. In the conditioning period, cues signalling the sham treatment were followed by laser stimuli whose intensity was surreptitiously lowered. In the test period, either cue was followed by laser stimuli of the same intensity. The observation of facial expressions with different emotional content enhanced significantly the placebo analgesia. In particular, a significantly greater analgesic effect was observed when facial expressions with emotional content were presented concomitantly to the nociceptive stimulation. The enhancement of placebo analgesia during the observation of facial expressions was not correlated with personality traits like empathy and behavioural activation/inhibition. These findings quantify for the first time the effect of facial expressions on the magnitude of placebo analgesia.