Brain mediators of the effects of noxious heat on pain

Abstract

Recent human neuroimaging studies have investigated the neural correlates of either noxious stimulus intensity or reported pain. Although useful, analyzing brain relationships with stimulus intensity and behavior separately does not address how sensation and pain are linked in the central nervous system. In this study, we used multi-level mediation analysis to identify brain mediators of pain--regions in which trial-by-trial responses to heat explained variability in the relationship between noxious stimulus intensity (across 4 levels) and pain. This approach has the potential to identify multiple circuits with complementary roles in pain genesis. Brain mediators of noxious heat effects on pain included targets of ascending nociceptive pathways (anterior cingulate, insula, SII, and medial thalamus) and also prefrontal and subcortical regions not associated with nociceptive pathways per se. Cluster analysis revealed that mediators were grouped into several distinct functional networks, including the following: somatosensory, paralimbic, and striatal-cerebellar networks that increased with stimulus intensity; and 2 networks co-localized with "default mode" regions in which stimulus intensity-related decreases mediated increased pain. We also identified "thermosensory" regions that responded to increasing noxious heat but did not predict pain reports. Finally, several regions did not respond to noxious input, but their activity predicted pain; these included ventromedial prefrontal cortex, dorsolateral prefrontal cortex, cerebellar regions, and supplementary motor cortices. These regions likely underlie both nociceptive and non-nociceptive processes that contribute to pain, such as attention and decision-making processes. Overall, these results elucidate how multiple distinct brain systems jointly contribute to the central generation of pain.

Keywords: Connectivity; Human; Mediation; Neuroimaging; Nociception; Pain; fMRI.

Figure 1. Task and analysis framework

A) Task design. On each trial, participants saw a cue, followed by a 6-second anticipatory delay. They then experienced ten seconds of noxious thermal stimulation at temperatures calibrated to elicit ratings of non-painful warmth, low pain, moderate pain, or high pain. Following a delay, participants rated the pain they had experienced. Our analyses examine brain responses during noxious stimulation, using area under the curve (AUC) as a measure of the evoked response. B) Multi-level mediation analysis. Our mediation analyses use linear mixed models to examine the dynamic, trial-by-trial relationship between objective stimulus (Left, X; Temperature), voxelwise stimulus-evoked responses (Bottom, M; AUC estimates from single trial analysis), and pain experience (Right, Y; Pain report). The four components of multilevel mediation analysis address the current study's key questions. Top, Path c/c′: Does temperature affect perceived pain as measured by trial-by-trial pain reports? Path c reflects the total relationship between predictive cue and reported pain on medium trials, and Path c′ reflects the direct behavioral relationship, controlling for activity in the mediator—in this case a brain voxel or region. Left, Path a (“Temperature effects”) provides inferences on whether brain voxels are modulated by noxious input (Fig. 2A). Right, Path b (“report- related responses”) provides inferences on whether brain activity in each voxel predicts trial-by-trial pain reports, controlling for temperature (Fig. 2B). To supplement the standard linear model, we controlled for nonlinear effects of temperature when assessing Path b. Middle, The mediation effect (c – c′) provides inferences on whether brain voxels explain a significant amount of the covariance between temperature and perceived pain (Fig. 3, Fig. 4). C) Pain-processing brain network. We were most interested in testing for effects within the a priori pain-processing brain network. We created an independent localizer using a mega-analytic approach across four different studies of noxious thermal stimulation (n = 114; see Methods) to identify regions associated with pain and nociception.

Figure 2. Connectivity analysis

We used a three-step approach to define networks of mediators: A) Step 1: Dimension reduction with principal components analysis; B) Step 2: Parcellation to group voxels into regions; C) NMDS and clustering to identify 13 networks of parcels.

Figure 3. Variability in temperature effects on pain

The relationship between temperature and pain is reliable. However, there is also substantial variability in this relationship both within and between individuals, as demonstrated by plots of each participant's pain ratings as a function of temperature. Mediators help to explain the within-subject variability. A) Variability across subjects. Each participant is displayed in a different color. B) Variability within subjects. Green = non-painful warmth (calibrated level 1), Yellow = low pain (calibrated level 3), Orange = medium pain (calibrated level 5), Red = high pain (calibrated level 7).

Figure 4. Mediation analysis results

A) Path a identifies brain regions that show linear effects of temperature. B) Path b identifies brain regions that correlate with pain reports, controlling for temperature. All identified regions were significantly related to trial-by-trial variations in pain using both linear and nonlinear models of temperature-related changes.

Figure 5. Brain mediators of the relationship between stimulus and response

A) Mediators of the relationship between temperature and pain. Brain regions whose responses to noxious stimulation formally mediate the relationship between temperature and pain include those that were activated consistently across the group, as well as those that show covariance between paths. B) Consistent mediators (significant Path a and Path b effects across subjects). Regions that were activated consistently across the group include both regions that showed temperature- and pain-related increases, illustrated in red, as well as those that showed temperature- and pain-related decreases, illustrated in blue.

Figure 6. Relationship to stimulus versus response

Regions illustrated in yellow and green were affected by temperature (p < 0.001), but were unrelated to pain reports, controlling for temperature (p > 0.05) and showed no evidence of mediation (p >0 .05). Blue and purple regions were associated with trial-by-trial pain reports, controlling for temperature (p < 0.001), but showed neither temperature-related changes nor mediation (p's > 0.05), and each was significantly related to variations in pain controlling for nonlinear effects of temperature and for activation in PPBN mediators. Mediators that showed consistent effects within subjects, with significant temperature and pain-related effects, are depicted in orange (all paths: p < .001). A) We observed anterior-to-posterior gradients in several regions. In the anterior cingulate cortex (ACC; left inset) and left insula (right inset), anterior portions were stimulus-related, middle regions showed activation that was correlated with both increases in stimulus intensity and increases in pain, and posterior regions were related to pain reports. The opposite pattern was observed in left parahippocampus (middle inset): more posterior portions were related to the stimulus, whereas anterior portions were related to pain reports. B) Regions related to temperature but not pain included bilateral medial prefrontal cortex, right dorsomedial prefrontal cortex (DMPFC), bilateral primary motor cortex, left S1, precuneus, left hippocampus, bilateral posterior hippocampus/fusiform gyrus, bilateral occipital lobe, right VMPFC, bilateral DLPFC (BA46), right SMA, right pre-SMA, left posterior insula, left posterior parahippocampus, and bilateral cerebellum. C) Regions related to pain but not temperature included right mOFC, bilateral DLPFC (BA46), right DMPFC, right SMA, right pre-SMA, left posterior insula, left posterior parahippocampus, and bilateral cerebellum. D) We extracted responses from the regions that showed unique relationships to temperature or pain and averaged across regions and subjects. Loess curves depict responses as a function of applied temperature (top row), and pain as a function of brain response to heat (AUC estimate), controlling for temperature (bottom row). Shaded regions reflect standard error across subjects and therefore should not be used to make inference on random effects.

Figure 7. Classes of mediators

We performed nonmetric multi-dimensional scaling to identify networks of mediators, based on functional coactivation within-subjects across trials. Eleven functional networks were identified (see Table 4). Top: Three networks consistently showed temperature- and pain-related increases, including a network in bilateral secondary somatosensory cortex (orange), a second network that consisted of bilateral anterior and middle insula, dorsal anterior cingulate, right latPFC, right premotor cortex, and right VLPFC (red), and a third network that included bilateral thalamus, left ventral striatum, midbrain/PAG, and bilateral cerebellum (yellow). Plots of average activation reveal that Networks 2 and 3 were largely linear in relationship to noxious input, whereas Network 1 showed nonlinearities that mirror the behavioral effects of temperature on pain. Bottom: Two networks were inversely related to both noxious stimulus and pain reports: one included bilateral retrosplenial cortex / posterior cingulate and inferior parietal lobule (green) and the second included bilateral parahippocampal cortex, right hippocampus, bilateral middle temporal gyrus / temporoparietal junction (TPJ), bilateral temporal pole, and right posterior hippocampus (cyan).