The ‘where’ and the ‘when’ of the BOLD response to pain in the insular cortex. Discussion on amplitudes and latencies

Fractal Similarity of Pain Brain Networks

The conscious perception of pain is the result of dynamic interactions of neural activities from local brain regions to distributed brain networks. Mapping out the networks of functional connections between brain regions that form and disperse when an experimental participant received nociceptive stimulations allow to characterize the pattern of network connections related to the pain experience.Although the pattern of intra- and inter-areal connections across the brain are incredibly complex, they appear also largely scale free, with "fractal" connectivity properties reproducing at short and long-time scales. Our results combining intracranial recordings and functional imaging in humans during pain indicate striking similarities in the activity and topological representation of networks at different orders of temporality, with reproduction of patterns of activation from the millisecond to the multisecond range. The connectivity analyzed using graph theory on fMRI data was organized in four sets of brain regions matching those identified through iEEG (i.e., sensorimotor, default mode, central executive, and amygdalo-hippocampal).Here, we discuss similarities in brain network organization at different scales or "orders," in participants as they feel pain. Description of this fractal-like organization may provide clues about how our brain regions work together to create the perception of pain and how pain becomes chronic when its organization is altered.

Non-response to first-line hormonal treatment for symptomatic endometriosis: overcoming tunnel vision. A narrative review

One-fourth to one-third of women with endometriosis receiving first-line hormonal treatment lacks an adequate response in terms of resolution of painful symptoms. This phenomenon has been ascribed to "progesterone resistance", an entity that was theorized to explain the gap between the ubiquity of retrograde menstruation and the 10% prevalence of endometriosis among women of reproductive age.Nevertheless, the hypothesis of progesterone resistance is not free of controversies. As our understanding of endometriosis is increasing, authors are starting to set aside the traditionally accepted tunnel vision of endometriosis as a strictly pelvic disease, opening to a more comprehensive perspective of the condition. The question is: are patients not responding to first-line treatment because they have an altered signaling pathway for such treatment, or have we been overlooking a series of other pain contributors which may not be resolved by hormonal therapy?Finding an answer to this question is evermore impelling, for two reasons mainly. Firstly, because not recognizing the presence of further pain contributors adds a delay in treatment to the already existing delay in diagnosis of endometriosis. This may lead to chronicity of the untreated pain contributors as well as causing adverse consequences on quality of life and psychological health. Secondly, misinterpreting the consequences of untreated pain contributors as a non-response to standard first-line treatment may imply the adoption of second-line medical therapies or of surgery, which may entail non-negligible side effects and may not be free of physical, psychological and socioeconomic repercussions.The current narrative review aims at providing an overview of all the possible pain contributors in endometriosis, ranging from those strictly organic to those with a greater neuro-psychological component. Including these aspects in a broader psychobiological approach may provide useful suggestions for treating those patients who report persistent pain symptoms despite receiving first-line hormonal medical treatment.

Amygdala and anterior insula control the passage from nociception to pain

Activation of the spinothalamic system does not always result in a subjective pain perception. While the cerebral network processing nociception is relatively well known, the one underlying its transition to conscious pain remains poorly described. We used intracranial electroencephalography in epileptic patients to investigate whether the amplitudes and functional connectivity of posterior and anterior insulae (PI and AI) and amygdala differ according to the subjective reports to laser stimuli delivered at a constant intensity set at nociceptive threshold. Despite the constant intensity of stimuli, all patients reported variable subjective perceptions from one stimulus to the other. Responses in the sensory PI remained stable throughout the experiment, hence reflecting accurately the stability of the stimulus. In contrast, both AI and amygdala responses showed significant enhancements associated with painful relative to nonpainful reports, in a time window corresponding to the conscious integration of the stimulus. Functional connectivity in the gamma band between these two regions increased significantly, both before and after stimuli perceived as painful. While the PI appears to transmit faithfully the actual stimulus intensity received via the spinothalamic tract, the AI and the amygdala appear to play a major role in the transformation of nociceptive signals into a painful perception.

Evaluation of eight registration algorithms applied to the insula and insular gyri

BACKGROUND AND PURPOSE

Spatial registration is crucial in establishing correspondence between anatomic brain regions for research and clinical purposes. The insular cortex (IC) and gyri (IG) are implicated in various functions and pathologies including epilepsy. Optimizing registration of the insula to a common atlas can improve the accuracy of group-level analyses. Here, we compared six nonlinear, one linear, and one semiautomated registration algorithms (RAs) for registering the IC and IG to the Montreal Neurologic Institute standard space (MNI152).

METHODS

3T images acquired from 20 controls and 20 temporal lobe epilepsy patients with mesial temporal sclerosis underwent automated segmentation of the insula. This was followed by manual segmentation of the entire IC and six individual IGs. Consensus segmentations were created at 75% agreement for IC and IG before undergoing registration to MNI152 space with eight RAs. Dice similarity coefficients (DSCs) were calculated between segmentations after registration and the IC and IG in MNI152 space. Statistical analysis involved the Kruskal-Wallace test with Dunn's test for IC and two-way analysis of variance with Tukey's honest significant difference test for IG.

RESULTS

DSCs were significantly different between RAs. Based on multiple pairwise comparisons, we report that certain RAs performed better than others across population groups. Additionally, registration performance differed according to specific IG.

CONCLUSION

We compared different methods for registering the IC and IG to MNI152 space. We found differences in performance between RAs, which suggests that algorithm choice is important factor in analyses involving the insula.

Processing of sensory, painful and vestibular stimuli in the thalamus

OBJECTIVES

The thalamus plays an important role in the mediation and integration of various stimuli (e.g., somatosensory, pain, and vestibular). Whether a stimulus-specific and topographic organization of the thalamic nuclei exists is still unknown. The aim of our study was to define a functional, in vivo map of multimodal sensory processing within the human thalamus.

METHODS

Twenty healthy individuals (10 women, 21-34 years old) participated. Defined sensory stimuli were applied to both hands (innocuous touch, mechanical pain, and heat pain) and the vestibular organ (galvanic stimulation) during 3 T functional MRI.

RESULTS

Bilateral thalamic activations could be detected for touch, mechanical pain, and vestibular stimulation within the left medio-dorsal and right anterior thalamus. Heat pain did not lead to thalamic activation at all. Stimuli applied to the left body side resulted in stronger activation patterns. Comparing an early with a late stimulation interval, the mentioned activation patterns were far more pronounced within the early stimulation interval.

CONCLUSIONS

The right anterior and ventral-anterior nucleus and the left medio-dorsal nucleus appear to be important for the processing of multimodal sensory information. In addition, galvanic stimulation is processed more laterally compared to mechanical pain. The observed changes in activity within the thalamic nuclei depending on the stimulation interval suggest that the stimuli are processed in a thalamic network rather than a distinct nucleus. In particular, the vestibular network within the thalamus recruits bilateral nuclei, rendering the thalamus an important integrative structure for vestibular function.

Cerebral structural alterations in the patients undergoing postherpetic neuralgia: A VBM-MRI study

This study aimed to investigate the changes in gray matter (GM) volume and density in patients with postherpetic neuralgia (PHN). Using voxel-based morphometry (VBM), the differences in cerebral GM volume and concentration between 25 PHN patients and 25 healthy controls with similar gender ratios, ages, and education were compared. Meanwhile, correlation analysis was performed between the value of GM volume/concentration in the brain areas with discrepancy and the visual analog scale (VAS) score/lesion duration. The global GM volume in PHN patients was lower than that of healthy controls, while the total volume of cerebrospinal fluid in PHN patients was higher than that of healthy controls. In PHN patients, the GM volume decreased in the striatum, cerebellum, precentral gyrus, middle frontal gyrus, parahippocampal gyrus, postcentral gyrus, and so forth; the GM concentration decreased in the striatum, insula, middle and posterior cingulate, and superior temporal gyrus. There was a negative correlation between GM concentration in the right parahippocampal gyrus and the VAS in patients with PHN. In PHN patients, GM volume and density in the brain regions involved in nociceptive sensation, pain perception, and integration decreased significantly. The interaction between chronic pain of PHN and alteration of the cerebral structure may contribute to the occurrence and development of PHN.

A thermal nociceptive patch in the S2 cortex of nonhuman primates: a combined functional magnetic resonance imaging and electrophysiology study

ABSTRACT

Human functional magnetic resonance imaging (fMRI) and behavioral studies have established the roles of cortical areas along the Sylvian fissure in sensing subjective pain. Yet, little is known about how sensory aspects of painful information are represented and processed by neurons in these regions and how their electrophysiological activities are related to fMRI signals. The current study aims to partially address this critical knowledge gap by performing fMRI-guided microelectrode mapping and recording studies in the homologous region of the parietal operculum in squirrel monkeys under light anesthesia. In each animal studied (n = 8), we detected mesoscale mini-networks for heat nociception in cortical regions around the lateral sulcus. Within the network, we discovered a ∼1.5 × 1.5-mm2-sized cortical patch that solely contained heat nociceptive neurons that aligned with the heat fMRI activation locus. These neurons responded slowly to thermal (heat and cold) nociceptive stimuli exclusively, continued firing for several seconds after the succession of stimulation, and exhibited multidigit receptive fields and high spontaneous firing rates. Similar to the fMRI responses, increasing temperatures in the nociceptive range led to a nonlinear increase in firing rates. The finding of a clustering of heat nociceptive neurons provides novel insights into the unique functional organization of thermal nociception in the S2 subregion of the primate brain. With fMRI, it supports the existence of a modality-preferred heat nociceptive patch that is spatially separated and intermingled with touch patches containing neurons with comparable receptive fields and the presence of functionally distinct mini-networks in primate opercular cortex.

The parietal operculum preferentially encodes heat pain and not salience

Substantial controversy exists as to which part of brain activity is genuinely attributable to pain-related percepts and which activity is due to general aspects of sensory stimulation, such as its salience, or the accompanying arousal. The challenge posed by this question rests largely in the fact that pain per se exhibits highly intense but unspecific characteristics. These therefore should be matched by potential control conditions. Here, we used a unique combination of functional magnetic resonance imaging (fMRI) and behavioral and autonomic measures to address this longstanding debate in pain research. Subjects rated perceived intensity in a sequence alternating between heat and sound stimuli. Neuronal activity was monitored using fMRI. Either modality was presented in 6 different intensities, 3 of which lay above the pain threshold (for heat) or the unpleasantness threshold (for sound). We performed our analysis on 26 volunteers in which psychophysiological responses (as per skin conductance responses [SCRs]) did not differ between the 2 stimulus modalities. Having thus ascertained a comparable amount of stimulation-related but unspecific activation, we analyzed stimulus-response functions (SRFs) after painful stimulation and contrasted them with those of the matched acoustic control condition. Furthermore, analysis of fMRI data was performed on the brain surface to circumvent blurring issues stemming from the close proximity of several regions of interest located in heavily folded cortical areas. We focused our analyses on insular and peri-insular regions that are strongly involved in processing of painful stimuli. We employed an axiomatic approach to determine areas showing higher activation in painful compared to nonpainful heat and, at the same time, showing a steeper SRF for painful heat compared to unpleasant sound. Intriguingly, an area in the posterior parietal operculum emerged, whose response showed a pain preference after satisfying all axiomatic constraints. This result has important implications for the interpretation of functional imaging findings in pain research, because it clearly demonstrates that there are areas where activity following painful stimulation is not due to general attributes or results of sensory stimulation, such as salience or arousal. Conversely, several areas did not conform to the formulated axioms to rule out general factors as explanations.

The brain activity detected during pain could be due merely to the fact that pain is arousing and attention-grabbing, rather than being directly attributable to the pain itself. This study identifies an area of the brain — the parietal operculum — whose activity can only be explained by the painfulness of pain.

Brain activity sustaining the modulation of pain by empathetic comments

Empathetic verbal feedback from others has been shown to alleviate the intensity of experimental pain. To investigate the brain changes associated with this effect, we conducted 3T-fMRI measurements in 30 healthy subjects who received painful thermal stimuli on their left hand while overhearing empathetic, neutral or unempathetic comments, supposedly made by experimenters, via headsets. Only the empathetic comments significantly reduced pain intensity ratings. A whole-brain BOLD analysis revealed that both Empathetic and Unempathetic conditions significantly increased the activation of the right anterior insular and posterior parietal cortices to pain stimuli, while activations in the posterior cingulate cortex and precuneus (PCC/Prec) were significantly stronger during Empathetic compared to Unempathetic condition. BOLD activity increased in the DLPFC in the Empathetic condition and decreased in the PCC/Prec and vmPFC in the Unempathetic condition. In the Empathetic condition only, functional connectivity increased significantly between the vmPFC and the insular cortex. These results suggest that modulation of pain perception by empathetic feedback involves a set of high-order brain regions associated with autobiographical memories and self-awareness, and relies on interactions between such supra-modal structures and key nodes of the pain system.

Strategy-dependent modulation of cortical pain circuits for the attenuation of pain

The effectiveness of cognitive strategies to attenuate pain has been reported in various behavioural studies, however the underlying neuronal mechanisms are only now beginning to be understood. Using a 7 T fMRI, we investigated three different pain attenuation strategies in 20 healthy subjects via: (a) non-imaginal distraction by counting backwards in steps of seven; (b) imaginal distraction by imagining a safe place; and (c) reinterpretation of the pain valence (reappraisal). Although we found considerable variability in the performances, all strategies exhibited a significant relief of pain compared to an unmodulated pain condition. Our finding argues against a subject's potential predisposition for a certain attenuation approach, as some of the subjects performed well on all attenuation tasks yet others performed low on all attenuation tasks. We further investigated the variability of performance within-subjects and explored the cortical regions that contribute to successful single attempts of pain attenuation at trial level. For each of the three tasks, we found a different pattern of brain activity that reflects the performance of pain attenuation. The more successful trials are related to reduced activity of different parts of the insular cortex. Behavioural data suggest that distraction is the preferable cognitive strategy to modulate pain perception. For three different cognitive strategies we revealed brain regions that are suggested to reliably modulate the perception of pain. The findings could be of utmost benefit for future attempts to integrate neuroscientific techniques into the treatment of pain. Further studies are necessary to investigate whether the present results are transferable to patients as an essential part of the multimodal therapy for chronic pain. These patients may also benefit from additional neurofeedback techniques by combining the strategies with the cortical feedback in order to modulate pain-related brain activity.

Functional brain imaging: what has it brought to our understanding of neuropathic pain? A special focus on allodynic pain mechanisms

Brain responses to nociception are well identified. The same is not true for allodynic pain, a strong painful sensation in response to touch or innocuous cold stimuli that may be experienced by patients with neuropathic pain. Brain (or spinal cord) reorganization that may explain this paradoxical perception still remains largely unknown. Allodynic pain is associated with abnormally increased activity in SII and in the anterior insular cortex, contralateral and/or ipsilateral to allodynia. Because a bilateral increase in activity has been repeatedly reported in these areas in nociceptive conditions, the observed activation during allodynia can explain that a physiologically nonpainful stimulus could be perceived by the damaged nervous system as a painful one. Both secondary somatosensory and insular cortices receive input from the thalamus, which is a major relay of sensory and spinothalamic pathways, the involvement of which is known to be crucial for the development of neuropathic pain. Both thalamic function and structure have been reported to be abnormal or impaired in neuropathic pain conditions including in the basal state, possibly explaining the spontaneous component of neuropathic pain. A further indication as to how the brain can create neuropathic pain response in SII and insular cortices stems from examples of diseases, including single-case reports in whom a focal brain lesion leads to central pain disappearance. Additional studies are required to certify the contribution of these areas to the disease processes, to disentangle abnormalities respectively related to pain and to deafferentation, and, in the future, to guide targeting of stimulation studies.

Insular Cortex is Critical for the Perception, Modulation, and Chronification of Pain

An increasing body of neuroimaging and electrophysiological studies of the brain suggest that the insular cortex (IC) integrates multimodal salient information ranging from sensation to cognitive-affective events to create conscious interoception. Especially with regard to pain experience, the IC has been supposed to participate in both sensory-discriminative and affective-motivational aspects of pain. In this review, we discuss the latest data proposing that subregions of the IC are involved in isolated pain networks: the posterior sensory circuit and the anterior emotional network. Due to abundant connections with other brain areas, the IC is likely to serve as an interface where cross-modal shaping of pain occurs. In chronic pain, however, this mode of emotional awareness and the modulation of pain are disrupted. We highlight some of the molecular mechanisms underlying the changes of the pain modulation system that contribute to the transition from acute to chronic pain in the IC.

Mapping multidimensional pain experience onto electrophysiological responses to noxious laser heat stimuli

The origin of the conscious experience of pain in the brain is a continuing enigma in neuroscience. To shed light on the brain representation of a multifaceted pain experience in humans, we combined multivariate analysis of subjective aspects of pain sensations with detailed, single-trial analysis of electrophysiological brain responses. Participants were asked to fully focus on any painful or non-painful sensations occurring in their left hand during an interval surrounding the onset of noxious laser heat stimuli, and to rate their sensations using a set of visual analogue scales. Statistical parametric mapping was used to compute a multivariate regression analysis of subjective responses and single-trial laser evoked potentials (LEPs) at subject and group levels. Standardized Low Resolution Electromagnetic Tomography method was used to reconstruct sources of LEPs. Factor analysis of subjective responses yielded five factors. Factor 1, representing pain, mapped firstly as a negative potential at the vertex and a positive potential at the fronto-temporal region during the 208-260ms interval, and secondly as a strong negative potential in the right lateral frontal and prefrontal scalp regions during the 1292-1340ms interval. Three other factors, labelled "anticipated pain", "stimulus onset time", and "body sensations", represented non-specific aspects of the pain experience, and explained portions of LEPs in the latency range from 200ms to 700ms. The subjective space of pain during noxious laser stimulation is represented by one large factor featuring pain intensity, and by other factors accounting for non-specific parts of the sensory experience. Pain is encoded in two separate latency components with different scalp and brain representations.

The primary somatosensory cortex and the insula contribute differently to the processing of transient and sustained nociceptive and non-nociceptive somatosensory inputs

Transient nociceptive stimuli elicit consistent brain responses in the primary and secondary somatosensory cortices (S1, S2), the insula and the anterior and mid-cingulate cortex (ACC/MCC). However, the functional significance of these responses, especially their relationship with sustained pain perception, remains largely unknown. Here, using functional magnetic resonance imaging, we characterize the differential involvement of these brain regions in the processing of sustained nociceptive and non-nociceptive somatosensory input. By comparing the spatial patterns of activity elicited by transient (0.5 ms) and long-lasting (15 and 30 s) stimuli selectively activating nociceptive or non-nociceptive afferents, we found that the contralateral S1 responded more strongly to the onset of non-nociceptive stimulation as compared to the onset of nociceptive stimulation and the sustained phases of nociceptive and non-nociceptive stimulation. Similarly, the anterior insula responded more strongly to the onset of nociceptive stimulation as compared to the onset of non-nociceptive stimulation and the sustained phases of nociceptive and non-nociceptive stimulation. This suggests that S1 is specifically sensitive to changes in incoming non-nociceptive input, whereas the anterior insula is specifically sensitive to changes in incoming nociceptive input. Second, we found that the MCC responded more strongly to the onsets as compared to the sustained phases of both nociceptive and non-nociceptive stimulation, suggesting that it could be involved in the detection of change regardless of sensory modality. Finally, the posterior insula and S2 responded maximally during the sustained phase of non-nociceptive stimulation but not nociceptive stimulation, suggesting that these regions are preferentially involved in processing non-nociceptive somatosensory input.

Pain Processing after Social Exclusion and Its Relation to Rejection Sensitivity in Borderline Personality Disorder

OBJECTIVE

There is a general agreement that physical pain serves as an alarm signal for the prevention of and reaction to physical harm. It has recently been hypothesized that "social pain," as induced by social rejection or abandonment, may rely on comparable, phylogenetically old brain structures. As plausible as this theory may sound, scientific evidence for this idea is sparse. This study therefore attempts to link both types of pain directly. We studied patients with borderline personality disorder (BPD) because BPD is characterized by opposing alterations in physical and social pain; hyposensitivity to physical pain is associated with hypersensitivity to social pain, as indicated by an enhanced rejection sensitivity.

METHOD

Twenty unmedicated female BPD patients and 20 healthy participants (HC, matched for age and education) played a virtual ball-tossing game (cyberball), with the conditions for exclusion, inclusion, and a control condition with predefined game rules. Each cyberball block was followed by a temperature stimulus (with a subjective pain intensity of 60% in half the cases). The cerebral responses were measured by functional magnetic resonance imaging. The Adult Rejection Sensitivity Questionnaire was used to assess rejection sensitivity.

RESULTS

Higher temperature heat stimuli had to be applied to BPD patients relative to HCs to reach a comparable subjective experience of painfulness in both groups, which suggested a general hyposensitivity to pain in BPD patients. Social exclusion led to a subjectively reported hypersensitivity to physical pain in both groups that was accompanied by an enhanced activation in the anterior insula and the thalamus. In BPD, physical pain processing after exclusion was additionally linked to enhanced posterior insula activation. After inclusion, BPD patients showed reduced amygdala activation during pain in comparison with HC. In BPD patients, higher rejection sensitivity was associated with lower activation differences during pain processing following social exclusion and inclusion in the insula and in the amygdala.

DISCUSSION

Despite the similar behavioral effects in both groups, BPD patients differed from HC in their neural processing of physical pain depending on the preceding social situation. Rejection sensitivity further modulated the impact of social exclusion on neural pain processing in BPD, but not in healthy controls.

The neurobiology of pain perception in normal and persistent pain

SUMMARY 

Pain is a significant national burden in terms of patient suffering, expenditure and lost productivity. Understanding pain is fundamental to improving evaluation, treatment and innovation in the management of acute and persistent pain syndromes. Pain perception begins in the periphery, and then ascends in several tracts, relaying at different levels. Pain signals arrive in the thalamus and midbrain structures which form the pain neuromatrix, a constantly shifting set of networks and connections that determine conscious perception. Several cortical regions become active simultaneously during pain perception; activity in the cortical pain matrix evolves over time to produce a complex pain perception network. Dysfunction at any level has the potential to produce unregulated, persistent pain.

Pain modulation in waking and hypnosis in women: event-related potentials and sources of cortical activity

Using a strict subject selection procedure, we tested in High and Low Hypnotizable subjects (HHs and LHs) whether treatments of hypoalgesia and hyperalgesia, as compared to a relaxation-control, differentially affected subjective pain ratings and somatosensory event-related potentials (SERPs) during painful electric stimulation. Treatments were administered in waking and hypnosis conditions. LHs showed little differentiation in pain and distress ratings between hypoalgesia and hyperalgesia treatments, whereas HHs showed a greater spread in the instructed direction. HHs had larger prefrontal N140 and P200 waves of the SERPs during hypnotic hyperalgesia as compared to relaxation-control treatment. Importantly, HHs showed significant smaller frontocentral N140 and frontotemporal P200 waves during hypnotic hypoalgesia. LHs did not show significant differences for these SERP waves among treatments in both waking and hypnosis conditions. Source localization (sLORETA) method revealed significant activations of the bilateral primary somatosensory (BA3), middle frontal gyrus (BA6) and anterior cingulate cortices (BA24). Activity of these contralateral regions significantly correlated with subjective numerical pain scores for control treatment in waking condition. Moreover, multivariate regression analyses distinguished the contralateral BA3 as the only region reflecting a stable pattern of pain coding changes across all treatments in waking and hypnosis conditions. More direct testing showed that hypnosis reduced the strength of the association of pain modulation and brain activity changes at BA3. sLORETA in HHs revealed, for the N140 wave, that during hypnotic hyperalgesia, there was an increased activity within medial, supramarginal and superior frontal gyri, and cingulated gyrus (BA32), while for the P200 wave, activity was increased in the superior (BA22), middle (BA37), inferior temporal (BA19) gyri and superior parietal lobule (BA7). Hypnotic hypoalgesia in HHs, for N140 wave, showed reduced activity within medial and superior frontal gyri (BA9,8), paraippocampal gyrus (BA34), and postcentral gyrus (BA1), while for the P200, activity was reduced within middle and superior frontal gyri (BA9 and BA10), anterior cingulate (BA33), cuneus (BA19) and sub-lobar insula (BA13). These findings demonstrate that hypnotic suggestions can exert a top-down modulatory effect on attention/preconscious brain processes involved in pain perception.

Central noradrenergic mechanisms and the acute stress response during painful stimulation

Events that threaten tissue integrity including noxious stimulation activate central noradrenergic circuits, particularly locus coeruleus and its projections. Recent advances in theory hold that an adaptive, defensive shift in brain activity takes place in response to threat. In principle, this shift may accentuate the autonomic and central biomarkers of the perception of painful events and the experience of pain itself. We have examined the effects of an alpha-2 agonist on pupil dilation responses, skin conductance responses, near field somatosensory evoked potentials and pain reports in normal volunteers undergoing repeated trials of painful fingertip stimulation delivered at low, medium and high intensities. In a double-blinded study, 114 healthy male and female volunteers underwent repeated noxious stimulation under baseline, placebo and active drug conditions where the active drug was the alpha-2 agonist tizanidine 4 mg. In contrast to baseline and placebo conditions, tizanidine 4 mg significantly reduced the magnitudes of the mean pupil dilation response, the mean skin conductance response, the mean near field somatosensory evoked potential peak-to-peak amplitude and the mean pain intensity rating. Stimulus intensity significantly altered all three biomarkers and the pain report in a graded fashion. There were no sex differences. These findings support the hypotheses that painful events activate central noradrenergic circuits, and that these circuits play a role in the autonomic and central arousal associated with pain.

Cerebral responses and role of the prefrontal cortex in conditioned pain modulation: an fMRI study in healthy subjects

The mechanisms underlying conditioned pain modulation (CPM) are multifaceted. We searched for a link between individual differences in prefrontal cortex activity during multi-trial heterotopic noxious cold conditioning and modulation of the cerebral response to phasic heat pain. In 24 healthy female subjects, we conditioned laser heat stimuli to the left hand by applying alternatively ice-cold or lukewarm compresses to the right foot. We compared pain ratings with cerebral fMRI BOLD responses. We also analyzed the relation between CPM and BOLD changes produced by the heterotopic cold conditioning itself, as well as the impact of anxiety and habituation of cold-pain ratings. Specific cerebral activation was identified in precuneus and left posterior insula/SII, respectively, during early and sustained phases of cold application. During cold conditioning, laser pain decreased (n=7), increased (n=10) or stayed unchanged (n=7). At the individual level, the psychophysical effect was directly proportional to the cold-induced modulation of the laser-induced BOLD response in left posterior insula/SII. The latter correlated with the BOLD response recorded 80s earlier during the initial 10-s phase of cold application in anterior cingulate, orbitofrontal and lateral prefrontal cortices. High anxiety and habituation of cold pain were associated with greater laser heat-induced pain during heterotopic cold stimulation. The habituation was also linked to the early cold-induced orbitofrontal responses. We conclude that individual differences in conditioned pain modulation are related to different levels of prefrontal cortical activation by the early part of the conditioning stimulus, possibly due to different levels in trait anxiety.

Human models of pain for the prediction of clinical analgesia

Human experimental pain models are widely used to study drug effects under controlled conditions. However, efforts to improve both animal and human experimental model selection, on the basis of increased understanding of the underlying pathophysiological pain mechanisms, have been disappointing, with poor translation of results to clinical analgesia. We have developed an alternative approach to the selection of suitable pain models that can correctly predict drug efficacy in particular clinical settings. This is based on the analysis of successful or unsuccessful empirical prediction of clinical analgesia using experimental pain models. We analyzed statistically the distribution of published mutual agreements or disagreements between drug efficacy in experimental and clinical pain settings. Significance limits were derived by random permutations of agreements. We found that a limited subset of pain models predicts a large number of clinically relevant pain settings, including efficacy against neuropathic pain for which novel analgesics are particularly needed. Thus, based on empirical evidence of agreement between drugs for their efficacy in experimental and clinical pain settings, it is possible to identify pain models that reliably predict clinical analgesic drug efficacy in cost-effective experimental settings.

Brain mediators of the effects of noxious heat on pain

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.

Temporal profile of fronto-striatal-limbic activity during implicit decisions in drug dependence

BACKGROUND

Substance dependence is associated with impaired decision-making and altered fronto-striatal-limbic activity. Both greater and lesser brain activity have been reported in drug users compared to controls during decision-making. Inconsistent results might be explained by group differences in the temporal profile of the functional magnetic resonance imaging (fMRI) response. While most previous studies model a canonical hemodynamic response, a finite impulse response (FIR) model measures fMRI signal at discrete time points without assuming a temporal profile. We compared brain activity during decision-making and feedback in substance users and controls using two models: a canonical hemodynamic response function (HRF) and a FIR model.

METHODS

37 substance-dependent individuals (SDI) and 43 controls performed event-related decision-making during fMRI scanning. Brain activity was compared across group using canonical HRF and FIR models.

RESULTS

Compared to controls, SDI were impaired at decision-making. The canonical HRF model showed that SDI had significantly greater fronto-striatal-limbic activity during decisions and less activity during feedback than controls. The FIR model confirmed greater activity in SDI during decisions. However, lower activity in SDI during feedback corresponded to a lower post-stimulus undershoot of the hemodynamic response.

CONCLUSIONS

Greater activity in fronto-striatal-limbic pathways in SDI compared to controls is consistent with prior work, further supporting the hypothesis that abnormalities in these circuits underlie impaired decision-making. We demonstrate for the first time using FIR analysis that lower activity during feedback may simply reflect the tail end of the hemodynamic response to decision, the post-stimulus undershoot, rather than an actual difference in feedback response.

Massive modulation of brain areas after mechanical pain stimulation: a time-resolved FMRI study

To date, relatively little is known about the spatiotemporal aspects of whole-brain blood oxygenation level-dependent (BOLD) responses to brief nociceptive stimuli. It is known that the majority of brain areas show a stimulus-locked response, whereas only some are characterized by a canonical hemodynamic response function. Here, we investigated the time course of brain activations in response to mechanical pain stimulation applied to participants' hands while they were undergoing functional magnetic resonance imaging (fMRI) scanning. To avoid any assumption about the shape of BOLD response, we used an unsupervised data-driven method to group voxels sharing a time course similar to the BOLD response to the stimulus and found that whole-brain BOLD responses to painful mechanical stimuli elicit massive activation of stimulus-locked brain areas. This pattern of activations can be segregated into 5 clusters, each with a typical temporal profile. In conclusion, we show that an extensive activity of multiple networks is engaged at different time latencies after presentation of a noxious stimulus. These findings aim to motivate research on a controversial topic, such as the temporal profile of BOLD responses, the variability of these response profiles, and the interaction between the stimulus-related BOLD response and ongoing fluctuations in large-scale brain networks.

Interoceptive and multimodal functions of the operculo-insular cortex: tactile, nociceptive and vestibular representations

The operculo-insular cortex has been termed the 'homeostatic control center' or 'general magnitude estimator' of the human mind. In this study, somatosensory, nociceptive and caloric vestibular stimuli were applied to reveal, whether there are mainly common, or possibly specific regions activated by one modality alone and whether lateralization effects, time pattern differences or influences of the aversive nature of the stimuli could be observed. Activation of the dorsal posterior insula was caused by all stimuli alike thus terming this area multimodal. Early phases of the noxious heat and caloric vestibular stimulation led to responses in the anterior insula. Using conjunction analyses we found that left- and right-sided tactile stimulation, but not nociceptive stimulation, caused a joint activation of the cytoarchitectonic area OP1 and nociceptive but not tactile stimulation of the anterior insula bilaterally. Tactile activation in the parietal operculum (SII, OP1) was distinct from nociceptive activation (OP3 and frontal operculum). The joint activation by all three stimuli located in the dorsal posterior insula argues for the presence of multisensory structures. The distinct activation of the anterior insula by aversive stimuli and the posterior insula by multisensory signals supports the concept of a partitioned insular cortex recently introduced based on connectivity studies and meta-analyses.