Predicting value of pain and analgesia: nucleus accumbens response to noxious stimuli changes in the presence of chronic pain

Abstract

We compared brain activations in response to acute noxious thermal stimuli in controls and chronic back pain (CBP) patients. Pain perception and related cortical activation patterns were similar in the two groups. However, nucleus accumbens (NAc) activity differentiated the groups at a very high accuracy, exhibiting phasic and tonic responses with distinct properties. Positive phasic NAc activations at stimulus onset and offset tracked stimulus salience and, in normal subjects, predicted reward (pain relief) magnitude at stimulus offset. In CBP, NAc activity correlated with different cortical circuitry from that of normals and phasic activity at stimulus offset was negative in polarity, suggesting that the acute pain relieves the ongoing back pain. The relieving effect was confirmed in a separate psychophysical study in CBP. Therefore, in contrast to somatosensory pathways, which reflect sensory properties of acute noxious stimuli, NAc activity in humans encodes its predicted value and anticipates its analgesic potential on chronic pain.

Copyright 2010 Elsevier Inc. All rights reserved.

Figures

Figure 1. Brain activity maps for perception of thermal pain in healthy and CBP subjects

(a) Top panel shows average pain ratings for painful heat in healthy (black trace) and CBP (gray trace), data presented as mean +/− S.E.M. Bottom panel shows the time course of the thermal stimulus applied to the lower back. (b) Random-effects analysis for pain rating tasks in healthy controls and CBP patients. Many cortical areas were commonly activated including bilateral thalamus, insula, S2. The conjunction is shown in the top row and represents the brain regions that were commonly significantly activated for both groups. The contrast map shows regions with higher activity in healthy in contrast to CBP. Only bilateral nucleus accumbens survived this contrast. Activity maps were generated using random effects contrasts with z-score > 3.0 and cluster threshold p<0.01 corrected for multiple comparisons.

Figure 2. Differences in time course of NAc BOLD signal between CBP patients and healthy controls

(a) Group-averaged BOLD signals from NAc in healthy (blue) and CBP patients (red) are shown superimposed on the respective group-averaged pain ratings (grey area). The black trace represents the derivative of the stimulus (stimulus and pain ratings are convolved with hemodynamic function). The BOLD signal closely follows the derivative or rectified derivative of the stimulus, in CBP and healthy controls, respectively. Moreover, the baseline activity in the interval between thermal stimuli tends to be more negative in the healthy subjects. (b) Top panels show the average time course of the stimulus (black trace) and pain rating (convolved with hemodynamic function) for healthy (blue) and CBP (red) during start (left) and end (right panel) of thermal stimulus. The time courses were averaged across all stimulation epochs where subjects reported pain (> 5 on a scale of 0–100). Bottom panels show the absolute value of the derivative, |d/dt|, for the stimulus and pain ratings. (c) Top panel shows the time course of average BOLD responses for NAc in healthy (blue) and CBP (red) for the same time periods depicted in (b). (d) shows the same data as in (b–c) for time periods where subjects did not report any significant pain in response to the thermal stimulus. (e) shows NAc activity when both groups rated the length of a visual bar. Top panels are averaged stimulus and ratings, lower panels are averaged NAc BOLD signal. (c–e thin lines are +/− S.E.M.)

Figure 3. Phasic and tonic NAC activity distinguish between the groups and depend on stimulus and chronic pain parameters

(a) Average NAc signal differences between the two groups for four time windows: p1, phasic response when thermal pain is increasing; s, tonic response during thermal pain; p2, phasic response when thermal pain is decreasing; b, tonic baseline activity between stimuli. Healthy and CBP exhibited similar activity for p1 and s. Healthy subjects exhibited higher activity for p2 and lower activity for b, when compared to CBP. Similar results are seen for scan 1 and scan 2. (b) Panel shows individual mean BOLD responses for p2 and b, in CBP (red) and healthy controls (blue) for scan1 (left column) and scan2 (right column). The ordinate is the BOLD signal for each subject for p2 and b, averaged across all stimuli. (c) Activity for p2 is significantly correlated with ratings of thermal pain in healthy subjects, for both scan 1 and scan 2 (blue). This relationship is reversed in CBP (red). (d) Activity for b shows a positive correlation with individual scores for magnitude of back pain (visual analogue scale, VAS) in CBP in scan 1 and scan 2. (In a, error bars are +/− S.E.M. In b–d, each symbol is a subject’s value)

Figure 4. Functional connectivity of NAC and its differential dependence on specific cortical regions

(a) Group average connectivity maps when NAc activity is used as seed, in healthy subjects (top panel) and CBP (lower panel). The NAc exhibited significant connectivity to bilateral amygdala, caudate, putamen, medial thalamus, PAG, ventral striatum and ACC in both groups. However, NAc connectivity to mPFc was stronger in CBP (unpaired t-test, random effects z-score > 3.0 and cluster threshold p<0.01 corrected for multiple comparisons, see Supplementary Figure 3). (b) Scatter plot shows a strong correlation between the strength of NAc-mPFc connectivity (z-score is standardized correlation coefficient for each subject) and intensity of back pain (VAS) at the day of the scan in CBP. (c) The relationship between activity in NAc at p2 with magINS (portion of insula related to magnitude perception) and mPFc (computed as % change in BOLD signal) in healthy (blue) and CBP (red), shows a double dissociation between the two groups. In healthy subjects the NAc p2 exhibits a strong positive correlation with magINS activity and no correlation with mPFc, this relationship was reversed in CBP. Similar results are seen for data derived from scan 1 and scan 2.

Figure 5. Reduction in perceived magnitude of back pain by thermal painful stimuli

(a) Top panel shows average ratings (n = 8 CBP patients) of the magnitude of the stimulus (red trace) and spontaneous fluctuations of back pain (blue trace) during the application of a thermal stimulus to the lower back. Bottom panel shows the time course of the thermal stimulus. (b) Average time course of the stimulus (black trace), rating the stimulus pain (red) and spontaneous back pain (blue) during start (left) and end (right panel) of thermal stimuli. The time curves were averaged across all stimulation epochs and all 8 CBP patients. (c) Bar graph shows the group averaged pleasantness (on a +100 to −100 pleasantness to unpleasantness scale) evaluation of the experience of rating the thermal pain (red) and the spontaneous pain (blue), indicating that attending to the back pain during the thermal painful stimulus reveals that the back pain is reduced and this is accompanied by increased pleasantness. (b – c error bars and thin lines are +/− S.E.M.).