1 Chapter
1.6 Supplementary Information
2.4.3 Neural Activity
2.4.3.1 Reference trials
As a first step, we examined pain-specific (i.e. elicited by pain but not by disgust) and disgust-specific (i.e. elicited by disgust but not by pain) brain activations during the reference trials (i.e. with valid cues), as tested with interaction effects in which highly unpleasant events of one modality were contrasted with both its tailored control, and the highly unpleasant event of the opposite modality (i.e, for pain [cHP_HP > cLP_LP] > [cHD_HD > cLD_LD], for disgust [cHD_HD > cLD_LD] > [cHP_HP >
cLP_LP]).
In line with previous studies on pain processing84,85,212,523, pain-specific areas included the bilateral posterior insula (PI) and middle portion of the cingulate cortex (MCC) – see Figure 20 (red blobs) and Table 3. In the case of disgust-specific areas (Figure 20, green blobs; Table 3), we found strong activations bilaterally in the amygdala (Amyg), extending to the piriform cortex (PirC). Furthermore, a bilateral portion of the ventral PCG was also activated, over and around somatosensory regions where
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the face (including the nose) is represented. These effects accord with previous studies on disgust processing 488,498,246,524,525, specifically in the case of olfaction 526,527. We then run a conjunction analysis searching for regions sensitive to both pain and disgust pain ([cHP_HP > cLP_LP] ∩ [cHD_HD > cLD_LD], for each contrast contributing to the conjunction we used a height-threshold corresponding to p <
0.01, thus leading to a conjoint probability of p < 0.01*0.01, i.e. p < 0.001). The conjunction analysis revealed shared activations in the ventral portion of the insular cortex bilaterally (VI), extending in the left hemisphere to the middle insula (MI) but also to the dorsal portions of the amygdala – see Figure 20 (yellow blobs) and Table 3.
Overall, this analysis of reference trials replicates previous studies highlighting brain networks associated with the experience of pain and disgust. In particular, as anticipated, neural responses triggered by pain and disgust overlapped in some parts of the anterior and ventral insular cortex (VI), whereas other brain regions (PI, PirC) were engaged by one modality only.
Figure 20 - Neural structures associated with modality-specific and modality-shared activations during the reference trials.
The figure displays whole brain maps of neural structures that are pain-specific (red blobs), disgust-specific (green blobs), and those that are shared between the two (yellow blobs).
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Table 3 - Clusters of modality-specific and modality-shared activations associated with stimulations of reference trials.
Reference trials: Pain-specific (cHP_HP > cLP_LP) > (cHD_HD > cLD_LD)
Region Side x y z T Cluster
Reference trials: Disgust-specific (cHD_HD > cLD_LD) > (cHP_HP > cLP_LP)
Region Side x y z T Cluster
Amygdala / PirC R 22 -4 -18 4.88 163 <0.05
Postcentral Gyrus (PCG) R 56 -4 26 4.67 437 <0.001
Amygdala / PirC L -26 0 -20 5.62 134 <0.05
Postcentral Gyrus (PCG) L -52 -10 28 4.53 354 <0.001
Reference trials: Shared (cHP_HP > cLP_LP) ∩ (cHD_HD > cLD_LD)
Region Side x y z T Cluster
L, Left hemisphere; R, right hemisphere; M, medial activations. Activations clusters are corrected for multiple comparisons for the whole brain.
2.4.3.2 Medium trials
Our critical experimental conditions concerned the medium trials. These trials allowed us to test whether the neural response evoked by physically identical thermal or olfactory stimuli (i.e., with fixed intensity and similar unpleasantness level at baseline) were modulated by expectancy of a high (relative to low) unpleasant event, in either the same or different sensory modality.
We first focused on the Stimulus Modality x Cue Modality interaction, testing for specific increases of neural activity when the modality of the stimulus was consistent vs. inconsistent with the modality of the cue (but regardless of whether this was predictive of high/low unpleasantness). In consistent trials (Table 4; Figure 21, blue areas), significant increase was observed in the ventral portion of the medial
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prefrontal cortex (VMPFC), extending to the orbitofrontal cortex (OFC). The opposite contrast showed that inconsistent trials activated the bilateral temporo-parietal junction (TPJ), the bilateral inferior frontal sulcus, and the precuneus (Table 4; Figure 21, yellow areas), overlapping with networks typically implicated in attention shifts528,354,529.
Figure 21 - Neural structures that are modulated by consistent and inconsistent expectations (Stimulus Modality x Cue Modality interaction) during medium trials.
(Upper part) Whole brain maps of consistent and inconsistent medium trials. Blue blobs refer to regions with increased activity when modality of the cue was consistent with modality of the subsequent stimulus (regardless of cue unpleasantness). Yellow blobs refer to regions with increased activity when modality of the cue was inconsistent with the stimulus. (Lower part) Average parameter estimates are plotted for two representative regions (ventromedial prefrontal cortex, VMPFC, and temporo-parietal junction, TPJ), with S.E.M. error bars. As in Figure 18, left subplots refer to activity associated with thermal pain, whereas right subplots refer to activity associated with olfactory disgust. Red bars refer to trials preceded by pain cues, whereas green bars refer to trials preceded by disgust cues.
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Table 4 - Clusters of activation associated with interaction terms of medium trials.
2-way (Cue Modality x Stimulus Modality) interaction, and with 3-way (Cue Modality x Stimulus Modality x Cue Unpleasantness) interaction, during medium (consistent or inconsistent) trials.
Cue Modality x Stimulus Modality: Consistent trials
Region Side x y z T Cluster Cue Modality x Stimulus Modality: Inconsistent trials
Region Side x y z T Cluster
L, Left hemisphere; R, right hemisphere; M, medial activations. Activations clusters are corrected for multiple comparisons, for the whole brain.
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Next, we tested for the main effect of Cue Unpleasantness (high vs. low) to determine whether aversive signals triggered by the cue may influence the neural response to subsequent stimuli (regardless of whether its modality matches with the cue or not). This analysis revealed increased activity in the cerebellum only (Supplementary Information [SI] Table 6). We then restricted our analysis to those regions which were sensitive to both pain and disgust in the reference trials as tested by the conjunction analysis (Figure 20, yellow blobs) as these regions are the most likely to process pain and disgust in supramodal fashion. Even when applying small volume correction for these regions, we found no main effect of Cue unpleasantness.
Most importantly, and in line with our behavioral results, we tested for the 3-way interaction (Stimulus Modality x Cue Modality x Cue Unpleasantness) investigating whether the unpleasant value of the cue (high vs. low) influenced neural responses to subsequent stimuli only in modality-consistent (but not modality-inconsistent) trials. This contrast revealed significant increases in the right dorsal anterior insula (dAI) and MCC (Table 4; Figure 22, purple areas). No effect (at least under stringent correction for multiple comparisons for the whole brain) was observed in the left hemisphere, although a weaker effect in the left dAI was also observed with a more liberal cluster threshold (x = -34, y = 14, z = 0, t = 3.60, 32 consecutive voxels). Activity parameters (beta coefficients) extracted from these regions show that, for both thermal and olfactory stimuli, dAI and MCC exhibited greater responses when participants expected a high (vs. low) unpleasant event of the same modality. In addition, however, activity in dAI and MCC showed an negative modulation when participants expected a high (vs. low) unpleasant event of a different modality. This pattern of responses suggests that these areas kept track of the modality of both predictions and sensory events, with differential responses according to whether the latter match with prior expectations. No region, at least under our strict threshold, exhibited stronger unpleasantness effects when the modality of the stimulus was opposite of that of the preceding cue.
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Figure 22 Neural structures identified in the 3-way (Stimulus Modality x Cue Modality x Cue Unpleasantness) interaction during medium trials.
(Left side) Whole brain maps of the 3-way interaction testing for significant expectancy effect (cue predictive of high > low unpleasantness) in trials in which the modality of the cue is consistent (vs. inconsistent) with the subsequent stimulus.
Sagittal sections of the cingulate and insular cortex are displayed to highlight effects in these regions. (Right side) For each region, the average parameter estimates are displayed with S.E.M. error bars. As in Figure 18, left subplots refer to activity associated with thermal pain, whereas right subplots refer to activity associated with olfactory disgust. Red bars refer to trials preceded by pain cues, whereas green bars refer to trials preceded by disgust cues.
The three-way interaction described above revealed regions who are modulated for both pain and disgust in modality-specific fashion. Yet it is possible that modality-specific expectation of pain might be partially mediated by different neural structures than modality-specific expectation of disgust. To test this, we tested for Consistent – Inconsistent expectancy effects within each modality separately.
When testing for modality-specific effects of pain [(cHP_MP – cLP_MP) – (cHD_MP – cLD_MP)] we found no suprathreshold activations, neither when focusing on the whole brain, nor when focusing on pain-specific networks as mapped in the analysis of the reference trials (Figure 20, red blobs). When testing for modality-specific effects of disgust [(cHD_MD – cLD_MD) – (cHP_MD – cLP_MD)], we found activations in the right dorsal anterior insula, extending to the IFG (over and around the regions highlighted when testing the 3-way interaction – see SI Table 6). No other region was found, neither when inspecting the whole brain, nor when focusing on disgust-specific networks as mapped in the analysis of the reference trials (Figure 20, green blobs).
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2.5 Interim Discussion
We investigated how pain and disgust expectancy are encoded in the brain, and whether they modulate perceptual experience in a modality-specific or modality-independent fashion. Our participants saw cues highly predictive of either painful or disgusting events that were followed by stimuli of the same or different modality. Behaviorally, we found that the subjective evaluation of a stimulus is modulated by the aversive nature of the preceding cue (high vs. low unpleasantness), but only when the cue and stimulus share the same modality. No expectancy effect was observed when the modality of the stimulus is inconsistent with the cues, suggesting that the expectancy of pain or disgust engaged representations that are modality-specific. In agreement, our neuroimaging results revealed that the stimulus-induced activity in the right dorsal anterior insula (dAI) and middle cingulate cortex (MCC) is positively modulated by the expected unpleasantness of an event of the same modality of the stimulus. Concurrently, these regions are negatively modulated by expecting unpleasant events in a different modality. Taken together, the data suggest that, consistently with previous studies127, the right anterior insula and mid cingulate cortex are sensitive to both pain and disgust. However, this functional heterogeneity does not go at the expense of sensory-specificity, as each of these regions keep track of the sensory modality of unpleasant events and their match (or mismatch) with prior expectations.
2.5.1 Limitations of the study
Although our study converges with Experiment 1 providing evidence modality-specific expectation effects for pain and disgust, it revealed no significant cross-modal effect neither at the behavioral nor at the neural level. This is in partial conflict with Experiment 1, where a similar paradigm was employed and revealed reliable cross-modal expectation effect on subjective pleasantness ratings. Nevertheless, the latter effects were much smaller than the within-modality expectation effects, consistent with the current study. This difference between two studies employing a similar paradigm may be explained by a few changes in the experimental protocol due to MRI constraints (longer/jittered interval between the cue and the stimulus, reduced number of repetitions, subject’s posture, etc; see530), which might have further weakened the modality-independent expectancy effect.
Interestingly, in Experiment 1 we found that modality-specific effects were weaker in participants who reported stronger conscious reliance on the cue during anticipation prior to stimulation. It is possible that individuals who admittedly considered the cues unreliable might have focused more on the unpleasant aspects of the events (unpleasantness was predictable at 100%) rather than on their modality-specific aspects (modality was predictable at 75%). Interestingly, in Experiment 2, almost all participants reported a strong reliance on the cue during the experimental session, which might
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explain why in this specific dataset we observed only modality-specific effects, at the expense of cross-modal modulations.