Visually induced analgesia during face or limb stimulation in healthy and migraine subjects

BACKGROUND: Visually induced analgesia (VIA) defines a phenomenon in which viewing one’s own body part during its painful stimulation decreases the perception of pain. VIA occurs during direct vision of the stimulated body part and also when seeing it reflected in a mirror. To the best of our knowledge, VIA has not been studied in the trigeminal area, where it could be relevant for the control of headache. SUBJECTS AND METHODS: We used heat stimuli (53°C) to induce pain in the right forehead or wrist in 11 healthy subjects (HSs) and 14 female migraine without aura (MO) patients between attacks. The subjects rated pain on a visual analog scale (VAS) and underwent contact heat-evoked potential (CHEP) recordings (five sequential blocks of four responses) with or without observation of their face/wrist in a mirror. RESULTS: During wrist stimulation, amplitude of the first block of P1–P2 components of CHEPs decreased compared to that in the control recording when HSs were seeing their wrist reflected in the mirror (p = 0.036; Z = 2.08); however, this was not found in MO patients. In the latter, the VAS pain score increased viewing the reflected wrist (p = 0.049; Z = 1.96). Seeing their forehead reflected in the mirror induced a significant increase in N2 latency of CHEPs in HSs, as well as an amplitude reduction in the first block of P1–P2 components of CHEPs both in HSs (p = 0.007; Z = 2.69) and MO patients (p = 0.035; Z = 2.10). Visualizing the body part did not modify habituation of CHEP amplitudes over the five blocks of averaged responses, neither during wrist nor during forehead stimulation. CONCLUSION: This study adds to the available knowledge on VIA and demonstrates this phenomenon for painful stimuli in the trigeminal area, as long as CHEPs are used as indices of central pain processing. In migraine patients during interictal periods, VIA assessed with CHEPs is within normal limits in the face but absent at the wrist, possibly reflecting dysfunctioning of extracephalic pain control.