Linking Pain and the Body: Neural Correlates of Visually Induced Analgesia

Abstract

The visual context of seeing the body can reduce the experience of acute pain, producing a multisensory analgesia. Here we investigated the neural correlates of this "visually induced analgesia" using fMRI. We induced acute pain with an infrared laser while human participants looked either at their stimulated right hand or at another object. Behavioral results confirmed the expected analgesic effect of seeing the body, while fMRI results revealed an associated reduction of laser-induced activity in ipsilateral primary somatosensory cortex (SI) and contralateral operculoinsular cortex during the visual context of seeing the body. We further identified two known cortical networks activated by sensory stimulation: (1) a set of brain areas consistently activated by painful stimuli (the so-called "pain matrix"), and (2) an extensive set of posterior brain areas activated by the visual perception of the body ("visual body network"). Connectivity analyses via psychophysiological interactions revealed that the visual context of seeing the body increased effective connectivity (i.e., functional coupling) between posterior parietal nodes of the visual body network and the purported pain matrix. Increased connectivity with these posterior parietal nodes was seen for several pain-related regions, including somatosensory area SII, anterior and posterior insula, and anterior cingulate cortex. These findings suggest that visually induced analgesia does not involve an overall reduction of the cortical response elicited by laser stimulation, but is consequent to the interplay between the brain's pain network and a posterior network for body perception, resulting in modulation of the experience of pain.

Illusory self-identification with an avatar reduces arousal responses to painful stimuli

“…However whereas these pain thresholds data have recently been extended with explicit ratings of pain experience [22], we did not observe similar effects in explicit pain ratings in experiment 1 and 2. Consistently with our results, a recent study (conducted in two different laboratories) showed that explicit pain ratings for stimuli delivered to the biological hand did not change during the RHI [54].…”

“…We hypothesized that the increased self-identification, when seeing the virtual body stroked in a congruent fashion, would be reflected in changes in the processing of painful stimuli akin to those described during the direct observation of one's own body typically consisting of a reduced response to painful stimuli [22,20,44]. Questionnaire data showed that higher selfidentification was recorded with the avatar only when presented with an anatomically correct body configuration and when stroked congruently, according to the literature [4,53,28,9,8].…”

“…Looking at one's own body but not looking at a neutral object or at another person's body has been reported to induce analgesia for acute painful stimulation [22,20]. Here, we sought for a similar modulation of pain responses induced by the vision of a virtual body that also depended upon the level of illusory self-identification with that body during a full-body illusion [9] induced by controlled robotic stimulations [23][24][25].…”

“…It has been argued that such responses are mainly based on the cognitive evaluation of the approaching stimulus which would produce an automatic anticipatory response [29,50] that has been hypothesized to be mediated by emphatic sharing of the affective component of pain [51,52]. Furthermore, previous works reported that the vision of one's own stimulated body parts can modulate pain thresholds [20,44] or the rating of pain intensity [22], suggesting an analgesic effect when looking at one's own body. The present study goes beyond such previous results by linking the visual response to an incoming threat directed towards another person's body, to illusory self-identification experienced towards an observed virtual body.…”

“…Previous work has shown that looking at one's own body but not to an object or at another person's body, while receiving a painful stimulus, produces analgesic effects [22,20]. Starting from this observation, we aimed at investigating the relationship between pain processing and body ownership; here, we sought for evidence that reduced responses to nociceptive stimuli can be obtained not only by looking at one's own body [20] but also when looking at another person's body or avatar, especially under conditions of self-identification with the virtual body.…”