In this study, we found that after invoking nocebo effects through the creation of negative expectancy to a sham treatment, subjective pain ratings (post- minus pre-) increased significantly more on nocebo sites of the arm as compared to control sites. fMRI analysis showed brain regions involved in hyperalgesic nocebo effect during pain administration to include bilateral dorsal ACC, insula, superior temporal gyrus; left frontal and parietal operculum, medial frontal gyrus, orbital prefrontal cortex, superior parietal lobule and hippocampus; right claustrum / putamen, lateral prefrontal gyrus and middle temporal gyrus. Further analysis of spontaneous fMRI data collected before pain application showed functional connections among left frontal operculum and hippocampus and the brain regions belonging to the pain network, including bilateral insula, operculum, ACC and left M1.
We found the brain regions preferentially activated during nocebo hyperalgesic pain administration, including bilateral ACC, insula, left orbital frontal cortex, and right lateral prefrontal cortex, to reside primarily in the medial system of the pain matrix. This result is consistent with our hypothesis that nocebo hyperalgesia is predominantly produced though the affective-cognitive pain pathway. Previous studies have characterized the behavioral response to nocebo hyperalgesia and described the important role of anxiety in this process (Benedetti et al., 2006
). We believe our work is the first to elucidate the brain network underlying nocebo hyperalgesia.
Previous brain imaging studies have indicated that expectation can significantly modulate subsequent noxious stimuli perception (Sawamoto et al., 2000
; Koyama et al., 2005
; Keltner et al., 2006
). For instance, Sawamoto and colleagues (Sawamoto et al., 2000
) found that uncertain expectation regarding impending painful stimuli could enhance brain responses to non-painful stimuli, increasing the intensity and range of activation in the ACC and parietal operculum and posterior insula respectively. In a later study, Keltner and colleagues (Keltner et al., 2006
) found that expectancy to higher levels of pain could significantly increase reported pain intensity ratings and enhance activation of afferent pain circuits in the ipsilateral ACC, caudate, cerebellum and contralateral nucleus cuneiformis. They further hypothesized that facilitation of the descending pain modulation pathway may be involved in this process. Although not completely the same, these studies are partly consistent with our finding that during nocebo hyperalgesia, brain activity in bilateral ACC, insula and operculum on nocebo sites increases significantly more than on control sites.
Recently, spontaneous brain activity has been used to investigate functional connectivity among different brain regions (Raichle and Mintun, 2006
; Buckner and Vincent, 2007
; Fox and Raichle, 2007
). In an early study, Biswal and colleagues (Biswal et al., 1995
) found that while subjects were at rest, spontaneous fMRI BOLD signal fluctuations observed in left sensory motor areas showed a high degree of temporal correlation with right sensory motor-related brain areas and medial motor areas. This study has been replicated and findings extended to many other brain systems (Fox and Raichle, 2007
). Consistent with these findings, we found highly symmetric left and right side connectivity correlations for both seed regions. Furthermore, in addition to the brain regions surrounding each seed, many regions observed in this analysis belonged to the pain network, suggesting that connectivity persists in the absence of pain stimuli as well.
In this experiment, we observed left frontal operculum activation in both group analysis and regression analysis, indicating this region’s role in nocebo hyperalgesia. Previous studies implicate operculum / insula as the most reliable region in brain imaging studies on pain (Peyron et al., 2000
), and report its direct association with S1, SII, prefrontal areas, superior temporal gyrus, amygdaloid, and perirhinal cortex which is an important source of hippocampal and ACC afferents (Augustine, 1996
; Cipolloni and Pandya, 1999
). The above connections link brain regions to the somatosensory, limic / paralimbic, and working memory systems, providing an anatomic basis for the multiple functions and extensive functional connectivity observed in spontaneous fMRI data.
Our study also showed nocebo-induced fMRI signal changes in left hippocampus, a region known to play an important role in encoding relations between various learning context cues (Olsson and Phelps, 2007
) and mediating aversive drive and the affective characteristics of pain (Melzack and Casey, 1968
). The left hippocampus has been previously reported in fMRI studies on pain and anxiety. In one such study, Ploghaus and colleagues (Ploghaus et al., 2001
) investigated brain response to identical pain at varying anxiety levels, observing a relationship between greater anxiety and higher pain intensity ratings. They also found left hippocampus to be uniquely involved in this process and reported that, during anxiety-induced emotional pain modulation, hippocampal responses can predict activity in closely connected, affective (perigenual cingulate), and intensity coding (mid-insula) areas. This study (Ploghaus et al., 2001
) indicates that, during states of heightened anxiety, the hippocampus can amplify aversive events so as to prime behavioral responses that are adaptive for dealing with the worst possible outcome. We speculate similar mechanism may also underlie nocebo hyperalgesia.
To further evaluate the hippocampus’s role in hyperalgesic nocebo, we performed a correlation analysis between subjects’ cluster beta values for left hippocampus and other brain regions as shown in . We found that left orbital prefrontal gyrus (p = 0.003, r = 0.76) and right dACC (p = 0.024, r = 0.62) were significantly correlated with left hippocampus brain activity. The lateral orbital prefrontal cortex and dACC are known to play key roles in cognitively modulating the emotional components of pain (Petrovic and Ingvar, 2002
) and processing affective aspects of pain (Price, 2000
Interestingly, although the brain imaging literature for placebo analgesia is quite robust (Petrovic et al., 2002
; Lieberman et al., 2004
; Wagner et al., 2005
; Zubieta et al., 2005
; Bingel et al., 2006
; Kong et al., 2006b
; Craggs et al., 2007
; Price et al., 2007
; Scott et al., 2007
; Wager et al., 2007
; Scott et al., 2008
), no placebo analgesia study to date has ever reported involvement of the hippocampus. Functional connectivity analyses using the hippocampus as a seed region demonstrate its widespread connection to pain matrix brain regions, including bilateral insula / operculum, ACC, superior parietal lobule, left M1, and pre-motor areas. This result provides further support for a possible unique role of the hippocampus in mediating nocebo hyperalgesia compared with placebo analgesia.
In this experiment, we found a significant fMRI signal increase to pain in bilateral ACC, a key region involved in processing the affective components of pain (Rainville et al., 1997
; Price, 2000
; Rainville, 2002
). Interestingly, when we preformed the functional connectivity analysis using left frontal operculum and hippocampus as seed regions, bilateral ACC activity was observed in both cases. Signal patterns were adjacent or overlapped with fMRI group analysis results ( and ). These results further support the role of affective processing in nocebo hyperalgesia.
Our study found activity in left orbital prefrontal cortex (PFC) and right DLPFC for fMRI group analysis. A negative correlation between fMRI signal change and subjective ratings was also observed in bilateral DLPFC and left OPFC. We speculate that activity changes in PFC and parietal lobule may imply multiple functions, including memory retrieval of previous experience, expectation generation, modulation of pain perception and pain ratings, as well as attention and emotion modulation (Benedetti et al., 2006
; Kong et al., 2007
In a recent Positron Emission Tomography study, Scott and colleagues (Scott et al., 2008
) asked subjects to undergo a 20 minute pain challenge and found placebo-enhanced opioid neurotransmission in the anterior cingulate, orbitofrontal and insular cortex, nucleus accumbens, amygdala and periaqueductal gray, as well as dopamine activation (DA) in the ventral basal ganglia, including the nucleus accumbens. In the same study, five subjects responding negatively to the pain challenge showed opposite changes in brain activity, a deactivation of DA and decreased opioid release in brain regions mentioned above. According to this finding, nocebo-related brain regions, including anterior cingulate, orbitofrontal and insular cortex, nucleus accumbens, and amygdale are all important limbic regions related to the interaction of emotion and pain (Vogt, 2005
). Although the nature of this study may differ from our own (please note that in Scott et al.’s study, which we believed was originally designed to test placebo effect, subjects were told they would receive either an active analgesic drug or placebo, and thus the relation of these findings to nocebo effects may be limited), we similarly found nocebo hyperalgesia to exert its effects through the affective component of pain network. In addition, our work also further indicates a more extensive network of brain regions involved in nocebo hyperalgesia, including the lateral prefrontal cortex, parietal lobule and left hippocampus.
In conclusion, we found evidence that the nocebo hyperalgesic effect may be produced through the medial system of the central pain matrix responsible for affective / emotional and cognitive aspects of pain perception. Analysis of spontaneous fMRI data, collected in the absence of and preceding any pain stimuli, showed a functional connection among the brain regions observed in the subsequent nocebo scans. The left hippocampus may play an important role in nocebo hyperalgesia.