An fMRI-based neurologic signature of physical pain

Assessment and Management of Chemical Coping in Patients With Cancer

Chemical coping is a working definition that describes patients' intake of opioids on a scale that spans the range between normal nonaddictive opioid use for pain all the way to opioid addiction. Most patients will fall somewhere between the two extremes in using opioid analgesics to cope with their psychological or spiritual distress. The degree to which patients use their medications in a maladaptive manner will determine their susceptibility to drug toxicity and harm. When there are no obvious cancer-related causes for increased pain intensity, chemical coping and other patient-related factors such as delirium, somatization, and depression should be considered. As part of the initial evaluation of patients with cancer-related pain, a brief screening tool such as the CAGE questionnaire should be used to identify patients who may be at risk for chemical coping. Identifying patients at risk will allow clinicians to avoid unnecessary opioid toxicity, control pain, and improve quality of life. A structured approach for managing opioid use should be adopted, including standardized documentation, opioid treatment agreements, urine drug screens, frequent visits, and restricted quantities of breakthrough opioids. All patients at risk should receive brief motivational interviewing with an objective, nonjudgmental, and empathic style that includes personalized feedback, particularly about markers of risk or harm. For chemical copers approaching the addiction end of the spectrum, with evidence of compulsive use and destructive behavior, referral should be made to substance abuse specialists.

Descending pain modulation and chronification of pain

PURPOSE OF REVIEW

Chronic pain is an important public health problem that negatively impacts quality of life of affected individuals and exacts an enormous socio-economic cost. Currently available therapeutics provide inadequate management of pain in many patients. Acute pain states generally resolve in most patients. However, for reasons that are poorly understood, in some individuals, acute pain can transform to a chronic state. Our understanding of the risk factors that underlie the development of chronic pain is limited. Recent studies have suggested an important contribution of dysfunction in descending pain modulatory circuits to pain 'chronification'. Human studies provide insights into possible endogenous and exogenous factors that may promote the conversion of pain into a chronic condition.

RECENT FINDINGS

Descending pain modulatory systems have been studied and characterized in animal models. Human brain imaging techniques, deep brain stimulation and the mechanisms of action of drugs that are effective in the treatment of pain confirm the clinical relevance of top-down pain modulatory circuits. Growing evidence supports the concept that chronic pain is associated with a dysregulation in descending pain modulation. Disruption of the balance of descending modulatory circuits to favour facilitation may promote and maintain chronic pain. Recent findings suggest that diminished descending inhibition is likely to be an important element in determining whether pain may become chronic. This view is consistent with the clinical success of drugs that enhance spinal noradrenergic activity, such as serotonin/norepinephrine reuptake inhibitors (SNRIs), in the treatment of chronic pain states. Consistent with this concept, a robust descending inhibitory system may be normally engaged to protect against the development of chronic pain. Imaging studies show that higher cortical and subcortical centres that govern emotional, motivational and cognitive processes communicate directly with descending pain modulatory circuits providing a mechanistic basis to explain how exogenous factors can influence the expression of chronic pain in a susceptible individual.

SUMMARY

Preclinical studies coupled with clinical pharmacologic and neuroimaging investigations have advanced our understanding of brain circuits that modulate pain. Descending pain facilitatory and inhibitory circuits arising ultimately in the brainstem provide mechanisms that can be engaged to promote or protect against pain 'chronification'. These systems interact with higher centres, thus providing a means through which exogenous factors can influence the risk of pain chronification. A greater understanding of the role of descending pain modulation can lead to novel therapeutic directions aimed at normalizing aberrant processes that can lead to chronic pain.

Corticofugal projections induce long-lasting effects on somatosensory responses in the trigeminal complex of the rat

The sensory information flow at subcortical relay stations is controlled by the action of topographic connections from the neocortex. To determinate the functional properties of the somatosensory corticofugal projections to the principal (Pr5) and caudal spinal (Sp5C) trigeminal nuclei, we performed unitary recordings in anesthetized rats. To examine the effect of these cortical projections we used tactile stimulation of the whisker and electrical stimulation of somatosensory cortices. Corticofugal anatomical projections to Pr5 and Sp5C nuclei were detected by using retrograde fluorescent tracers. Neurons projecting exclusively to Pr5 were located in the cingulate cortex while neurons projecting to both Sp5C and Pr5 nuclei were located in the somatosensory and insular cortices (>75% of neurons). Physiological results indicated that primary somatosensory cortex produced a short-lasting facilitating or inhibiting effects (<5 min) of tactile responses in Pr5 nucleus through activation of NMDA glutamatergic or GABA_A receptors since effects were blocked by iontophoretically application of APV and bicuculline, respectively. In contrast, stimulation of secondary somatosensory cortex did not affect most of the Pr5 neurons; however both cortices inhibited the nociceptive responses in the Sp5C nucleus through activation of glycinergic or GABA_A receptors because effects were blocked by iontophoretically application of strychnine and bicuculline, respectively. These and anatomical results demonstrated that the somatosensory cortices projects to Pr5 nucleus to modulate tactile responses by excitatory and inhibitory actions, while projections to the Sp5C nucleus control nociceptive sensory transmission by only inhibitory effects. Thus, somatosensory cortices may modulate innocuous and noxious inputs simultaneously, contributing to the perception of specifically tactile or painful sensations.

Ethics of neuroimaging after serious brain injury

BACKGROUND

Patient outcome after serious brain injury is highly variable. Following a period of coma, some patients recover while others progress into a vegetative state (unresponsive wakefulness syndrome) or minimally conscious state. In both cases, assessment is difficult and misdiagnosis may be as high as 43%. Recent advances in neuroimaging suggest a solution. Both functional magnetic resonance imaging and electroencephalography have been used to detect residual cognitive function in vegetative and minimally conscious patients. Neuroimaging may improve diagnosis and prognostication. These techniques are beginning to be applied to comatose patients soon after injury. Evidence of preserved cognitive function may predict recovery, and this information would help families and health providers. Complex ethical issues arise due to the vulnerability of patients and families, difficulties interpreting negative results, restriction of communication to "yes" or "no" answers, and cost. We seek to investigate ethical issues in the use of neuroimaging in behaviorally nonresponsive patients who have suffered serious brain injury. The objectives of this research are to: (1) create an approach to capacity assessment using neuroimaging; (2) develop an ethics of welfare framework to guide considerations of quality of life; (3) explore the impact of neuroimaging on families; and, (4) analyze the ethics of the use of neuroimaging in comatose patients.

METHODS/DESIGN

Our research program encompasses four projects and uses a mixed methods approach. Project 1 asks whether decision making capacity can be assessed in behaviorally nonresponsive patients. We will specify cognitive functions required for capacity and detail their assessment. Further, we will develop and pilot a series of scenarios and questions suitable for assessing capacity. Project 2 examines the ethics of welfare as a guide for neuroimaging. It grounds an obligation to explore patients' interests, and we explore conceptual issues in the development of a quality of life instrument adapted for neuroimaging. Project 3 will use grounded theory interviews to document families' understanding of the patient's condition, expectations of neuroimaging, and the impact of the results of neuroimaging. Project 4 will provide an ethical analysis of neuroimaging to investigate residual cognitive function in comatose patients within days of serious brain injury.

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.

Inferring affect from fMRI data

Neuroimaging findings are often interpreted in terms of affective experience, but researchers disagree about the advisability or even possibility of such inferences, and few frameworks explicitly link these levels of analysis. Here, we suggest that the spatial and temporal resolution of functional magnetic resonance imaging (fMRI) data could support inferences about affective states. Specifically, we propose that fMRI nucleus accumbens (NAcc) activity is associated with positive arousal, whereas a combination of anterior insula activity and NAcc activity is associated with negative arousal. This framework implies quantifiable and testable inferences about affect from fMRI data, which may ultimately inform predictions about approach and avoidance behavior. We consider potential limits on neurally inferred affect before highlighting theoretical and practical benefits.

The importance of psychological assessment in chronic pain

PURPOSE OF REVIEW

Much confusion has surrounded the purpose of the psychological assessment in the context of chronic pain. For many clinicians, the psychological assessment is used to rule out psychiatric illness and to identify the nonmedical causes for pain and disability. In essence, it is used to identify the causes of pain that fall outside of the biomedical model. Supported by over 30 years of evidence, the bio-psycho-social model acknowledges that psychosocial factors are inherent in chronic pain and require assessment if meaningful diagnostics and treatments are to occur.

RECENT FINDINGS

Five broad categories of psychosocial assessment are relevant to chronic pain. These categories have been shown to enhance the diagnosis of the underlying forms of pain, predict the transition from acute to chronic status, and help to phenotype individuals for the discovery of the underlying mechanisms responsible for pain.

SUMMARY

Informed assessment of chronic pain needs to include relevant biological, psychological, and social domains. This article describes those domains and offers suggestions of specific instruments to use in clinical or research settings.

The anterior cingulate cortex and pain processing

The neural network that contributes to the suffering which accompanies persistent pain states involves a number of brain regions. Of primary interest is the contribution of the cingulate cortex in processing the affective component of pain. The purpose of this review is to summarize recent data obtained using novel behavioral paradigms in animals based on measuring escape and/or avoidance of a noxious stimulus. These paradigms have successfully been used to study the nature of the neuroanatomical and neurochemical contributions of the anterior cingulate cortex (ACC) to higher order pain processing in rodents.

Prostate cancer detection using combined auto-fluorescence and light reflectance spectroscopy: ex vivo study of human prostates

This study was conducted to evaluate the capability of detecting prostate cancer (PCa) using auto-fluorescence lifetime spectroscopy (AFLS) and light reflectance spectroscopy (LRS). AFLS used excitation at 447 nm with four emission wavelengths (532, 562, 632, and 684 nm), where their lifetimes and weights were analyzed using a double exponent model. LRS was measured between 500 and 840 nm and analyzed by a quantitative model to determine hemoglobin concentrations and light scattering. Both AFLS and LRS were taken on n = 724 distinct locations from both prostate capsular (nc = 185) and parenchymal (np = 539) tissues, including PCa tissue, benign peripheral zone tissue and benign prostatic hyperplasia (BPH), of fresh ex vivo radical prostatectomy specimens from 37 patients with high volume, intermediate-to-high-grade PCa (Gleason score, GS ≥7). AFLS and LRS parameters from parenchymal tissues were analyzed for statistical testing and classification. A feature selection algorithm based on multinomial logistic regression was implemented to identify critical parameters in order to classify high-grade PCa tissue. The regression model was in turn used to classify PCa tissue at the individual aggressive level of GS = 7,8,9. Receiver operating characteristic curves were generated and used to determine classification accuracy for each tissue type. We show that our dual-modal technique resulted in accuracies of 87.9%, 90.1%, and 85.1% for PCa classification at GS = 7, 8, 9 within parenchymal tissues, and up to 91.1%, 91.9%, and 94.3% if capsular tissues were included for detection. Possible biochemical and physiological mechanisms causing signal differences in AFLS and LRS between PCa and benign tissues were also discussed.

Atypical age-related cortical thinning in episodic migraine

Background

Prior studies demonstrate reduced cortical thickness and volume in migraineurs. However, the effect of age on cortical thickness has not been assessed in migraineurs. In this study we investigated whether the process of aging on cortical thickness affects migraineurs differently compared to age-matched healthy controls, i.e. whether aging exacerbates cortical thinning in migraineurs.

Methods

Cortical thickness was estimated using a general linear model vertex-by-vertex approach for 32 healthy controls (mean age = 35.3 years; SD = 11.6) and 27 episodic migraine patients (mean age = 33.6 years; SD = 12.3). Results were modeled using a main effect analysis to estimate the effect of age on cortical thickness for each group separately, and an age-by-group analysis to estimate differences in age-related cortical thinning between migraine patients and normal controls.

Results

Although migraineurs and normal controls both have expected age-related thinning in many regions along the cortical mantle, migraineurs have age-related thinning of regions that do not thin in healthy controls, including: bilateral postcentral, right fusiform, and right temporal pole areas. Cortical thinning of these regions is more prominent with advancing age.

Conclusion

Results suggest that migraine is associated with atypical cortical aging, suggesting that the migraine disease process interacts with aging to affect cortical integrity.

Brain signature of chronic orofacial pain: a systematic review and meta-analysis on neuroimaging research of trigeminal neuropathic pain and temporomandibular joint disorders

Brain neuroimaging has been widely used to investigate the bran signature of chronic orofacial pain, including trigeminal neuropathic pain (TNP) and pain related to temporomandibular joint disorders (TMD). We here systematically reviewed the neuroimaging literature regarding the functional and structural changes in the brain of TNP and TMD pain patients, using a computerized search of journal articles via PubMed. Ten TNP studies and 14 TMD studies were reviewed. Study quality and risk of bias were assessed based on the criteria of patient selection, the history of medication, the use of standardized pain/psychological assessments, and the model and statistics of imaging analyses. Qualitative meta-analysis was performed by examining the brain regions which showed significant changes in either brain functions (including the blood-oxygen-level dependent signal, cerebral blood flow and the magnetic resonance spectroscopy signal) or brain structure (including gray matter and white matter anatomy). We hypothesized that the neuroimaging findings would display a common pattern as well as distinct patterns of brain signature in the disorders. This major hypothesis was supported by the following findings: (1) TNP and TMD patients showed consistent functional/structural changes in the thalamus and the primary somatosensory cortex, indicating the thalamocortical pathway as the major site of plasticity. (2) The TNP patients showed more alterations at the thalamocortical pathway, and the two disorders showed distinct patterns of thalamic and insular connectivity. Additionally, functional and structural changes were frequently reported in the prefrontal cortex and the basal ganglia, suggesting the role of cognitive modulation and reward processing in chronic orofacial pain. The findings highlight the potential for brain neuroimaging as an investigating tool for understanding chronic orofacial pain.

How the brain decides when to work and when to rest: dissociation of implicit-reactive from explicit-predictive computational processes

A pervasive case of cost-benefit problem is how to allocate effort over time, i.e. deciding when to work and when to rest. An economic decision perspective would suggest that duration of effort is determined beforehand, depending on expected costs and benefits. However, the literature on exercise performance emphasizes that decisions are made on the fly, depending on physiological variables. Here, we propose and validate a general model of effort allocation that integrates these two views. In this model, a single variable, termed cost evidence, accumulates during effort and dissipates during rest, triggering effort cessation and resumption when reaching bounds. We assumed that such a basic mechanism could explain implicit adaptation, whereas the latent parameters (slopes and bounds) could be amenable to explicit anticipation. A series of behavioral experiments manipulating effort duration and difficulty was conducted in a total of 121 healthy humans to dissociate implicit-reactive from explicit-predictive computations. Results show 1) that effort and rest durations are adapted on the fly to variations in cost-evidence level, 2) that the cost-evidence fluctuations driving the behavior do not match explicit ratings of exhaustion, and 3) that actual difficulty impacts effort duration whereas expected difficulty impacts rest duration. Taken together, our findings suggest that cost evidence is implicitly monitored online, with an accumulation rate proportional to actual task difficulty. In contrast, cost-evidence bounds and dissipation rate might be adjusted in anticipation, depending on explicit task difficulty.

Imagine that ahead of you is a long time of work: when will you take a break? This sort of issue – how to allocate effort over time – has been addressed by distinct theoretical fields, with different emphasis on reactive and predictive processes. An intuitive view is that you start working, stop when you are tired, and start again when fatigue goes away. Biologically, this means that decisions are taken when some physiological variable reaches a given bound on the risk of homeostatic failure. In a more economic perspective, fatigue translates into effort cost, which must be anticipated and compared to expected benefit before engaging an action. We proposed a computational model that bridges these perspectives from sport physiology and decision theory. Decisions are made in reaction to bounds being reached by an implicit cost variable that accumulates during effort, at a rate proportional to task difficulty, and dissipates during rest. However, some latent parameters (bounds and dissipation rate) are adjusted in anticipation, depending on explicit costs and benefits. This model was supported by behavioral data obtained using a paradigm where participants squeeze a handgrip to win a monetary payoff proportional to effort duration.

Functional MRI and outcome in traumatic coma

Advances in task-based functional MRI (fMRI), resting-state fMRI (rs-fMRI), and arterial spin labeling (ASL) perfusion MRI have occurred at a rapid pace in recent years. These techniques for measuring brain function have great potential to improve the accuracy of prognostication for civilian and military patients with traumatic coma. In addition, fMRI, rs-fMRI, and ASL perfusion MRI have provided novel insights into the pathophysiology of traumatic disorders of consciousness, as well as the mechanisms of recovery from coma. However, functional neuroimaging techniques have yet to achieve widespread clinical use as prognostic tests for patients with traumatic coma. Rather, a broad spectrum of methodological hurdles currently limits the feasibility of clinical implementation. In this review, we discuss the basic principles of fMRI, rs-fMRI, and ASL perfusion MRI and their potential applications as prognostic tools for patients with traumatic coma. We also discuss future strategies for overcoming the current barriers to clinical implementation.

No pain, no social gains: A social-signaling perspective of human pain behaviors

In this review article, we describe a social-signaling perspective of human pain and pain empathizing behaviors which is based on the premise that pain percepts evolved to serve both intrapersonal as well as interpersonal, communicative functions. This perspective offers a generative framework for understanding the natural origin and proximate expression of felt pain and pain empathizing behaviors. The basic thesis is that humans evolved sensory-behavioral heuristics for perceiving and inhibiting exogenous and endogenous pain sensations as part of more general expressive styles characterized by the demonstration of vulnerability gestures (i.e., trustworthiness cues) versus empowerment gestures (i.e., capacity cues), and these styles ultimately facilitate broader self-protection and social novelty-seeking life-history behavior strategies, respectively. We review the extant literature on how social contextual factors (e.g., audience characteristics) and how structural and functional components of individual’s social network appear to influence the expression of pain behaviors in ways that support basic predictions from the social-signaling perspective. We also show how the perspective can be used to interpret conventional findings of sex differences in pain percepts and pain empathizing behaviors and for predicting how the situational context and individual’s peer networks modulate these differences in vitro and in vitro. We conclude the article by describing how pain researchers may better understand how varying levels and divergent directions of changes in affect tend to co-occur with systematic changes in internal vs external pain sensitivities, and thus why, from an evolutionary perspective, pain may occur in the presence and absence of physical tissue damage.

Supraspinal characterization of the thermal grill illusion with fMRI

Background

Simultaneous presentation of non-noxious warm (40°C) and cold (20°C) stimuli in an interlacing fashion results in a transient hot burning noxious sensation (matched at 46°C) known as the thermal grill (TG) illusion. Functional magnetic resonance imaging and psychophysical assessments were utilized to compare the supraspinal events related to the spatial summation effect of three TG presentations: 20°C/20°C (G2020), 20°C/40°C (G2040) and 40°C/40°C (G4040) with corresponding matched thermode stimuli: 20°C (P20), 46°C (P46) and 40°C (P40) and hot pain (HP) stimuli.

Results

For G2040, the hot burning sensation was only noted during the initial off-line assessment. In comparison to P40, G4040 resulted in an equally enhanced response from all supraspinal regions associated with both pain sensory/discriminatory and noxious modulatory response. In comparison to P20, G2020 presentation resulted in a much earlier diminished/sedative response leading to a statistically significantly (P < 0.01) higher degree of deactivation in modulatory supraspinal areas activated by G4040. Granger Causality Analysis showed that while thalamic activation in HP may cast activation inference in all hot pain related somatosensory, affective and modulatory areas, similar activation in G2040 and G2020 resulted in deactivation inference in the corresponding areas.

Conclusions

In short, the transient TG sensation is caused by a dissociated state derived from non-noxious warm and cold spatial summation interaction. The observed central dissociated state may share some parallels in certain chronic neuropathic pain states.

Specifying the non-specific factors underlying opioid analgesia: expectancy, attention, and affect

RATIONALE

Psychological processes such as expectancy, attention, and affect directly influence clinical outcomes. These factors are grouped together as "nonspecific" factors, or placebo effects, in the medical literature, and their individual contributions are rarely considered. The pain-reducing effects of analgesic treatments may reflect changes in these psychological factors, rather than pure drug effects on pain. Furthermore, drug effects may not be isolated by drug vs. placebo comparisons if drugs interact with relevant psychological processes.

OBJECTIVES

We sought to determine whether the analgesic effects of opioid and placebo treatment are mediated by changes in attention, expectancy, or affect.

METHODS

We crossed intravenous administration of a potent opioid analgesic, remifentanil, with information about drug delivery (treatment expectancy or placebo) using a balanced placebo design. We measured drug and treatment expectancy effects on pain, attention, and responses to emotional images. We also examined interactions with cue-based expectations about noxious stimulation or stimulus expectancy.

RESULTS

Pain was additively influenced by treatment expectancy, stimulus expectancy, and drug concentration. Attention performance showed a small but significant interaction between drug and treatment expectancy. Finally, remifentanil enhanced responses to both positive and negative emotional images.

CONCLUSIONS

The pain-relieving effects of opioid drugs are unlikely to be mediated by changes in threat or affective processing. Standard open-label opioid administration influences multiple clinically relevant cognitive and emotional processes. Psychological factors can combine with drug effects to influence multiple outcomes in distinct ways. The influence of specific psychological factors should be considered when developing and testing pharmacological treatments.

Neuroimaging Evidence of Motor Control and Pain Processing in the Human Midcingulate Cortex

Human neuroimaging and virus-tracing studies in monkey predict that motor control and pain processes should overlap in anterior midcingulate cortex (aMCC), but there is currently no direct evidence that this is the case. We used a novel functional magnetic resonance imaging paradigm to examine brain activity while subjects performed a motor control task, experienced a pain-eliciting stimulus on their hand, and performed the motor control task while also experiencing the pain-eliciting stimulus. Our experiment produced 3 novel results. First, group-level analyses showed that when separate trials of motor control and pain processing were performed, overlapping functional activity was found in the same regions of aMCC, supplementary motor area (SMA), anterior insula, and putamen. Secondly, increased activity was found in the aMCC and SMA when motor control and pain processing occurred simultaneously. Thirdly, individual-level analyses showed that 93% of subjects engaged the same region of aMCC during separate trials of motor control and pain processing irrespective of differences in the sulcal/gyral morphology of the cingulate cortex across individuals. These observations provide direct evidence in humans that the same region of aMCC is engaged for motor control and pain processing.

Ranking brain areas encoding the perceived level of pain from fMRI data

Pain perception is thought to emerge from the integrated activity of a distributed brain system, but the relative contribution of the different network nodes is still incompletely understood. In the present functional magnetic resonance imaging (fMRI) study, we aimed to identify the more relevant brain regions to explain the time profile of the perceived pain intensity in healthy volunteers, during noxious chemical stimulation (ascorbic acid injection) of the left hand. To this end, we performed multi-way partial least squares regression of fMRI data from twenty-two a-priori defined brain regions of interest (ROI) in each hemisphere, to build a model that could efficiently reproduce the psychophysical pain profiles in the same individuals; moreover, we applied a novel three-way extension of the variable importance in projection (VIP) method to summarize each ROI contribution to the model. Brain regions showing the highest VIP scores included the bilateral mid-cingulate, anterior and posterior insular, and parietal operculum cortices, the contralateral paracentral lobule, bilateral putamen and ipsilateral medial thalamus. Most of these regions, with the exception of medial thalamus, were also identified by a statistical analysis on mean ROI beta values estimated using the time course of the psychophysical rating as a regressor at the voxel level. Our results provide the first rank-ordering of brain regions involved in coding the perceived level of pain. These findings in a model of acute prolonged pain confirm and extend previous data, suggesting that a bilateral array of cortical areas and subcortical structures is involved in pain perception.

Cluster-extent based thresholding in fMRI analyses: pitfalls and recommendations

Cluster-extent based thresholding is currently the most popular method for multiple comparisons correction of statistical maps in neuroimaging studies, due to its high sensitivity to weak and diffuse signals. However, cluster-extent based thresholding provides low spatial specificity; researchers can only infer that there is signal somewhere within a significant cluster and cannot make inferences about the statistical significance of specific locations within the cluster. This poses a particular problem when one uses a liberal cluster-defining primary threshold (i.e., higher p-values), which often produces large clusters spanning multiple anatomical regions. In such cases, it is impossible to reliably infer which anatomical regions show true effects. From a survey of 814 functional magnetic resonance imaging (fMRI) studies published in 2010 and 2011, we show that the use of liberal primary thresholds (e.g., p<.01) is endemic, and that the largest determinant of the primary threshold level is the default option in the software used. We illustrate the problems with liberal primary thresholds using an fMRI dataset from our laboratory (N=33), and present simulations demonstrating the detrimental effects of liberal primary thresholds on false positives, localization, and interpretation of fMRI findings. To avoid these pitfalls, we recommend several analysis and reporting procedures, including 1) setting primary p<.001 as a default lower limit; 2) using more stringent primary thresholds or voxel-wise correction methods for highly powered studies; and 3) adopting reporting practices that make the level of spatial precision transparent to readers. We also suggest alternative and supplementary analysis methods.

A meta-analysis of brain mechanisms of placebo analgesia: consistent findings and unanswered questions

Placebo treatments reliably reduce pain in the clinic and in the lab. Because pain is a subjective experience, it has been difficult to determine whether placebo analgesia is clinically relevant. Neuroimaging studies of placebo analgesia provide objective evidence of placebo-induced changes in brain processing and allow researchers to isolate the mechanisms underlying placebo-based pain reduction. We conducted formal meta-analyses of 25 neuroimaging studies of placebo analgesia and expectancy-based pain modulation. Results revealed that placebo effects and expectations for reduced pain elicit reliable reductions in activation during noxious stimulation in regions often associated with pain processing, including the dorsal anterior cingulate, thalamus, and insula. In addition, we observed consistent reductions during painful stimulation in the amygdala and striatum, regions implicated widely in studies of affect and valuation. This suggests that placebo effects are strongest on brain regions traditionally associated with not only pain, but also emotion and value more generally. Other brain regions showed reliable increases in activation with expectations for reduced pain. These included the prefrontal cortex (including dorsolateral, ventromedial, and orbitofrontal cortices), the midbrain surrounding the periaqueductal gray, and the rostral anterior cingulate. We discuss implications of these findings as well as how future studies can expand our understanding of the precise functional contributions of the brain systems identified here.

The influence of visual perspective on the somatosensory steady-state response during pain observation

The observation and evaluation of other’s pain activate part of the neuronal network involved in the actual experience of pain, including those regions subserving the sensori-discriminative dimension of pain. This was largely interpreted as evidence showing that part of the painful experience can be shared vicariously. Here, we investigated the effect of the visual perspective from which other people’s pain is seen on the cortical response to continuous 25 Hz non-painful somatosensory stimulation (somatosensory steady-state response: SSSR). Based on the shared representation framework, we expected first-person visual perspective (1PP) to yield more changes in cortical activity than third-person visual perspective (3PP) during pain observation. Twenty healthy adults were instructed to rate a series of pseudo-dynamic pictures depicting hands in either painful or non-painful scenarios, presented either in 1PP (0–45° angle) or 3PP (180° angle), while changes in brain activity was measured with a 128-electode EEG system. The ratings demonstrated that the same scenarios were rated on average as more painful when observed from the 1PP than from the 3PP. As expected from previous works, the SSSR response was decreased after stimulus onset over the left caudal part of the parieto-central cortex, contralateral to the stimulation side. Moreover, the difference between the SSSR was of greater amplitude when the painful situations were presented from the 1PP compared to the 3PP. Together, these results suggest that a visuospatial congruence between the viewer and the observed scenarios is associated with both a higher subjective evaluation of pain and an increased modulation in the somatosensory representation of observed pain. These findings are discussed with regards to the potential role of visual perspective in pain communication and empathy.

The important role of CNS facilitation and inhibition for chronic pain

Multiple studies have demonstrated that the pain experience among individuals is highly variable. Even under circumstances where the tissue injuries are similar, individual pain experiences may vary drastically. However, this individual difference in pain sensitivity is not only related to sensitivity of peripheral pain receptors, but also to variability in CNS pain processing. Peripheral impulses derived from tissue receptors undergo modification in dorsal horn neurons that can either result in inhibition or facilitation of pain. Such influences are particularly apparent in inflammation where not only peripheral, but also central, pain modulatory mechanisms can significantly increase nociceptive pain. Emotional state, level of anxiety, attention and distraction, memories, stress, fatigue and many other factors can either increase or reduce the pain experience. Increasing evidence suggests that 'bottom-up' and 'top-down' modulatory circuits within the spinal cord and brain play an important role in pain processing, which can profoundly affect the experience of pain.

Pain and depression: an integrative review of neurobiological and psychological factors

The comorbidity of pain and depression has been well established in the literature and is associated with a greater burden to the individual and society than either condition alone. The relationship between pain and depression is quite complex and multiple factors must be considered when trying to disentangle the pain-depression link including shared neurobiology, precipitating environmental factors and cognitive influences. This article aims to provide an overview of the leading neurobiological and psychosocial theories that have advanced our understanding of the link between pain and depression. To this end we describe the shared neurobiological mechanisms in the brain thought to explain the overlap and consider psychological processes and how they inform a cognitive behavioral model. The article also provides an overview of the evidence based treatment for comorbid pain and depression.

Shape shifting pain: chronification of back pain shifts brain representation from nociceptive to emotional circuits

Chronic pain conditions are associated with abnormalities in brain structure and function. Moreover, some studies indicate that brain activity related to the subjective perception of chronic pain may be distinct from activity for acute pain. However, the latter are based on observations from cross-sectional studies. How brain activity reorganizes with transition from acute to chronic pain has remained unexplored. Here we study this transition by examining brain activity for rating fluctuations of back pain magnitude. First we compared back pain-related brain activity between subjects who have had the condition for ∼2 months with no prior history of back pain for 1 year (early, acute/subacute back pain group, n = 94), to subjects who have lived with back pain for >10 years (chronic back pain group, n = 59). In a subset of subacute back pain patients, we followed brain activity for back pain longitudinally over a 1-year period, and compared brain activity between those who recover (recovered acute/sub-acute back pain group, n = 19) and those in which the back pain persists (persistent acute/sub-acute back pain group, n = 20; based on a 20% decrease in intensity of back pain in 1 year). We report results in relation to meta-analytic probabilistic maps related to the terms pain, emotion, and reward (each map is based on >200 brain imaging studies, derived from neurosynth.org). We observed that brain activity for back pain in the early, acute/subacute back pain group is limited to regions involved in acute pain, whereas in the chronic back pain group, activity is confined to emotion-related circuitry. Reward circuitry was equally represented in both groups. In the recovered acute/subacute back pain group, brain activity diminished in time, whereas in the persistent acute/subacute back pain group, activity diminished in acute pain regions, increased in emotion-related circuitry, and remained unchanged in reward circuitry. The results demonstrate that brain representation for a constant percept, back pain, can undergo large-scale shifts in brain activity with the transition to chronic pain. These observations challenge long-standing theoretical concepts regarding brain and mind relationships, as well as provide important novel insights regarding definitions and mechanisms of chronic pain.

TLR 2 and 4 responsiveness from isolated peripheral blood mononuclear cells from rats and humans as potential chronic pain biomarkers

Background

Chronic pain patients have increased peripheral blood mononuclear cell Interkeukin-1β production following TLR2 and TLR4 simulation. Here we have used a human-to-rat and rat-to-human approach to further investigate whether peripheral blood immune responses to TLR agonists might be suitable for development as possible systems biomarkers of chronic pain in humans.

Methods and Results

Study 1: using a graded model of chronic constriction injury in rats, behavioral allodynia was assessed followed by in vitro quantification of TLR2 and TLR4 agonist-induced stimulation of IL-1β release by PBMCs and spinal cord tissues (n = 42; 6 rats per group). Statistical models were subsequently developed using the IL-1β responses, which distinguished the pain/no pain states and predicted the degree of allodynia. Study 2: the rat-derived statistical models were tested to assess their predictive utility in determining the pain status of a published human cohort that consists of a heterogeneous clinical pain population (n = 19) and a pain-free population (n = 11). The predictive ability of one of the rat models was able to distinguish pain patients from controls with a ROC AUC of 0.94. The rat model was used to predict the presence of pain in a new chronic pain cohort and was able to accurately predict the presence of pain in 28 out of the 34 chronic pain participants.

Conclusions

These clinical findings confirm our previous discoveries of the involvement of the peripheral immune system in chronic pain. Given that these findings are reflected in the prospective graded rat data, it suggests that the TLR response from peripheral blood and spinal cord were related to pain and these clinical findings do indeed act as system biomarkers for the chronic pain state. Hence, they provide additional impetus to the neuroimmune interaction to be a drug target for chronic pain.

Completing the Pain Circuit: Recent Advances in Imaging Pain and Inflammation beyond the Central Nervous System

This review describes some of the recent developments in imaging aspects of pain in the periphery. It is now possible to image nerves in the cornea non-invasively, to image receptor level expression and inflammatory processes in injured tissue, to image nerves and alterations in nerve properties, to image astrocyte and glial roles in neuroinflammatory processes, and to image pain conduction functionally in the trigeminal ganglion. These advances will ultimately allow us to describe the pain pathway, from injury site to behavioral consequence, in a quantitative manner. Such a development could lead to diagnostics determining the source of pain (peripheral or central), objective monitoring of treatment progression, and, hopefully, objective biomarkers of pain.

Stress and visceral pain: focusing on irritable bowel syndrome

Recent advances in brain science have shown that the brain function encoding emotion depends on interoceptive signals such as visceral pain. Visceral pain arose early in our evolutionary history. Bottom-up processing from gut-to-brain and top-down autonomic/neuroendocrine mechanisms in brain-to-gut signaling constitute a circuit. Brain imaging techniques have enabled us to depict the visceral pain pathway as well as the related emotional circuit. Irritable bowel syndrome (IBS) is characterized by chronic recurrent abdominal pain or abdominal discomfort associated with bowel dysfunction. It is also thought to be a disorder of the brain-gut link associated with an exaggerated response to stress. Corticotropin-releasing hormone (CRH), a major mediator of the stress response in the brain-gut axis, is an obvious candidate in the pathophysiology of IBS. Indeed, administration of CRH has been shown to aggravate the visceral sensorimotor response in IBS patients, and the administration of peptidergic CRH antagonists seems to alleviate IBS pathophysiology. Serotonin (5-HT) is another likely candidate associated with brain-gut function in IBS, as 5-HT3 antagonists, 5-HT4 agonists, and antidepressants were demonstrated to regulate 5-HT neurotransmission in IBS patients. Autonomic nervous system function, the neuroimmune axis, and the brain-gut-microbiota axis show specific profiles in IBS patients. Further studies on stress and visceral pain neuropathways in IBS patients are warranted.

Beyond metaphor: contrasting mechanisms of social and physical pain

Physical pain can be clearly distinguished from other states of distress. In recent years, however, the notion that social distress is experienced as physically painful has permeated the scientific literature and popular media. This conclusion is based on the overlap of brain regions that respond to nociceptive input and sociocultural distress. Here we challenge the assumption that underlies this conclusion - that physical pain can be easily inferred from a particular pattern of activated brain regions - by showing that patterns of activation commonly presumed to constitute the 'pain matrix' are largely unspecific to pain. We then examine recent analytical advances that may improve the specificity of imaging for parsing pain from a broad range of perceptually unique human experiences.

Pain sensitivity and observer perception of pain in individuals with autistic spectrum disorder

The peer-reviewed literature investigating the relationship between pain expression and perception of pain in individuals with ASD is sparse. The aim of the present systematic PRIMSA review was twofold: first, to see what evidence there is for the widely held belief that individuals with ASD are insensitive to pain or have a high pain threshold in the peer-reviewed literature and, second, to examine whether individuals with ASD react or express pain differently. Fifteen studies investigating pain in individuals with ASD were identified. The case studies all reported pain insensitivity in individuals with ASD. However, the majority of the ten experimental studies reviewed indicate that the idea that individuals with ASD are pain insensitive needs to be challenged. The findings also highlight the strong possibility that not all children with ASD express their physical discomfort in the same way as a neurotypical child would (i.e., cry, moan, seek comfort, etc.) which may lead caregivers and the medical profession to interpret this as pain insensitivity or incorrectly lead them to believe that the child is in no pain. These results have important implications for the assessment and management of pain in children with ASD.

A brain signature for acute pain

Wager and colleagues developed a functional MRI-based spatial and magnitude pattern for perception of acute pain, which seems to generalize across many task conditions and subjects. This is a strong demonstration of the existence of a pain signature and raises important questions regarding what pain and perception are.