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.

Figure 1.

Perceptual results: subjective ratings of pain intensity as a function of stimulation intensity and visual condition. Error bars indicate the SEM difference between the hand and object conditions at each laser intensity.

Figure 2.

Left, The wide network of areas displaying BOLD fMRI responses to pain-related laser stimulation, including SI, SII, insula, and ACC. Right, fMRI activations to laser stimulation when looking at the hand compared with when looking at the object. An extensive posterior network was activated when laser stimuli were delivered while seeing the body.

Figure 3.

Regions of interest identified from previous studies as coding for experienced pain levels, and here showing reduced activation to laser stimulation when viewing the body in contrast to the object. Left, Right SI. Right, Left operculoinsular cortex.

Figure 4.

Regions whose functional coupling with PPC nodes in the posterior visual body network increased when viewing the body compared with the object. Vision of the body increased functional coupling between these PPC nodes and regions in the pain network evoked by laser stimulation. Areas overlapping with the pain matrix (i.e., [laser stimulation > rest]) are highlighted, via inclusive masking.