The Role of Prefrontal Inhibition in Regulating Facial Expressions of Pain: A Repetitive Transcranial Magnetic Stimulation Study

Although research on facial expressions of pain has a long history, little is known about the cerebral mechanisms regulating these expressions. It has been suggested that the medial prefrontal cortex (mPFC) might be involved in regulating/inhibiting the degree to which-pain is facially displayed. To test whether such a prefrontal regulation does indeed take place, we reduced medial prefrontal excitability via repetitive transcranial magnetic stimulation (rTMS) and assessed its effect on facial expressions. In a within-subject design, facial and subjective responses to experimental pain as well as "situational" pain, catastrophizing were assessed in 35 healthy participants; once after receiving low-frequency rTMS over the mPFC (1 Hz) and once after sham stimulation. Compared with sham stimulation, rTMS over the mPFC resulted in enhanced facial expressions of pain, whereas self-report and pain catastrophizing did not change. The current data show that reducing medial prefrontal excitability (via low-frequency rTMS) makes individuals facially more expressive to pain. This finding indicates that the mPFC is crucially involved in the inhibition of facial expressions of pain. Because this effect was independent of changes in self-report and pain catastrophizing suggests that this inhibitory mechanism is mainly governing the facial expression and not the underlying experience of pain. Perspective: Using rTMS, it was shown that the mPFC is causally involved in the downregulation or silencing of one's facial expression of pain. This might explain why individuals with low mPFC functioning (eg, patients with dementia) are facially more expressive in response to pain. (C) 2016 by the American Pain Society