Pain does not follow the boxcar model: temporal dynamics of the BOLD fMRI signal during constant current painful electric nerve stimulation

Resting-state neural mechanisms of capability for suicide and their interaction with pain - A CAN-BIND-05 Study

BACKGROUND

Suicidal ideation is highly prevalent in Major Depressive Disorder (MDD). However, the factors determining who will transition from ideation to attempt are not established. Emerging research points to suicide capability (SC), which reflects fearlessness of death and increased pain tolerance, as a construct mediating this transition. This Canadian Biomarker Integration Network in Depression study (CANBIND-5) aimed to identify the neural basis of SC and its interaction with pain as a marker of suicide attempt.

METHODS

MDD patients (n = 20) with suicide risk and healthy controls (n = 21) completed a self-report SC scale and a cold pressor task measuring pain threshold, tolerance, endurance, and intensity at threshold and tolerance. All participants underwent a resting-state brain scan and functional connectivity was examined for 4 regions: anterior insula (aIC), posterior insula (pIC), anterior mid-cingulate cortex (aMCC) and subgenual anterior cingulate cortex (sgACC).

RESULTS

In MDD, SC correlated positively with pain endurance and negatively with threshold intensity. Furthermore, SC correlated with the connectivity of aIC to the supramarginal gyrus, pIC to the paracingulate gyrus, aMCC to the paracingulate gyrus, and sgACC to the dorsolateral prefrontal cortex. These correlations were stronger in MDD compared to controls. Only threshold intensity mediated the correlation between SC and connectivity strength.

LIMITATIONS

Resting-state scans provided an indirect assessment of SC and the pain network.

CONCLUSIONS

These findings highlight point to a neural network underlying SC that is associated with pain processing. This supports the potential clinical utility of pain response measurement as a method to investigate markers of suicide risk.

How fMRI Analysis Using Structural Equation Modeling Techniques Can Improve Our Understanding of Pain Processing in Fibromyalgia

PURPOSE

The purpose of this study was to investigate the utility of data-driven analyses of functional magnetic resonance imaging (fMRI) data, by means of structural equation modeling, for the investigation of pain processing in fibromyalgia (FM).

PATIENTS AND METHODS

Datasets from two separate pain fMRI studies involving healthy controls (HC) and participants with FM were re-analyzed using both a conventional model-driven approach and a data-driven approach, and the results from these analyses were compared. The first dataset contained 15 women with FM and 15 women as healthy controls. The second dataset contained 15 women with FM and 11 women as healthy controls.

RESULTS

Consistent with previous studies, the model-driven analyses did not identify differences in pain processing between the HC and FM study groups in both datasets. On the other hand, the data-driven analyses identified significant group differences in both datasets.

CONCLUSION

Data-driven analyses can enhance our understanding of pain processing in healthy controls and in clinical populations by identifying activity associated with pain processing specific to the clinical groups that conventional model-driven analyses may miss.

A nociresponsive specific area of human somatosensory cortex within BA3a: BA3c?

It is well recognized that in primates, including humans, noxious body stimulation evokes a neural response in the posterior bank of the central sulcus, in Brodmann cytoarchitectonic subdivisions 3b and 1 of the primary somatosensory cortex. This response is associated with the 1st/sharp pain and contributes to sensory discriminative aspects of pain perception and spatial localization of the noxious stimulus. However, neurophysiological studies in New World monkeys predict that in humans noxious stimulation also evokes a separate neural response-mediated by C-afferent drive and associated with the 2nd/burning pain-in the depth of the central sulcus in Brodmann area 3a (BA3a) at the transition between the somatosensory and motor cortices. To evoke such a response, it is necessary to use multi-second duration noxious stimulation, rather than brief laser pulses. Given the limited human pain-imaging literature on cortical responses induced by C-nociceptive input specifically within BA3a, here we used high spatial resolution 7T fMRI to study the response to thermonoxious skin stimulation. We observed the predicted response of BA3a in the depth of the central sulcus in five human volunteers. Review of the available evidence suggests that the nociresponsive region in the depth of the central sulcus is a structurally and functionally distinct cortical area that should not be confused with proprioceptive BA3a. It is most likely engaged in interoception and control of the autonomic nervous system, and contributes to the sympathetic response to noxious stimulation, arguably the most intolerable aspect of pain experience. Ablation of this region has been shown to reduce pain sensibility and might offer an effective means of ameliorating some pathological pain conditions.

Endogenous opioidergic dysregulation of pain in fibromyalgia: a PET and fMRI study

Endogenous opioid system dysfunction potentially contributes to chronic pain in fibromyalgia (FM), but it is unknown if this dysfunction is related to established neurobiological markers of hyperalgesia. We previously reported that µ-opioid receptor (MOR) availability was reduced in patients with FM as compared with healthy controls in several pain-processing brain regions. In the present study, we compared pain-evoked functional magnetic resonance imaging with endogenous MOR binding and clinical pain ratings in female opioid-naive patients with FM (n = 18) using whole-brain analyses and regions of interest from our previous research. Within antinociceptive brain regions, including the dorsolateral prefrontal cortex (r = 0.81, P < 0.001) and multiple regions of the anterior cingulate cortex (all r > 0.67; all P < 0.02), reduced MOR availability was associated with decreased pain-evoked neural activity. Additionally, reduced MOR availability was associated with lower brain activation in the nucleus accumbens (r = 0.47, P = 0.050). In many of these regions, pain-evoked activity and MOR binding potential were also associated with lower clinical affective pain ratings. These findings are the first to link endogenous opioid system tone to regional pain-evoked brain activity in a clinical pain population. Our data suggest that dysregulation of the endogenous opioid system in FM could lead to less excitation in antinociceptive brain regions by incoming noxious stimulation, resulting in the hyperalgesia and allodynia commonly observed in this population. We propose a conceptual model of affective pain dysregulation in FM.

Initially intact neural responses to pain in autism are diminished during sustained pain

Pain assessments typically depend on self-report of the pain experience. Yet, in individuals with autism spectrum disorders, this can be an unreliable due to communication difficulties. Importantly, observations of behavioral hypo- and hyperresponsivity to pain suggest altered pain sensitivity in autism spectrum disorder. Neuroimaging may provide insight into mechanisms underlying pain behaviors. The neural pain signature reliably responds to painful stimulation and is modulated by other outside regions, affecting the pain experience. In this first functional magnetic resonance imaging study of pain in autism spectrum disorder, we investigated neural responses to pain in 15 adults with autism spectrum disorder relative to a typical comparison group (n = 16). We explored temporal and spatial properties of the neural pain signature and its modulators during sustained heat pain. The two groups had indistinguishable pain ratings and neural pain signature responses during acute pain; yet, we observed strikingly reduced neural pain signature response in autism spectrum disorder during sustained pain and after stimulus offset. The posterior cingulate cortex, a neural pain signature modulating region, mirrored this late signal reduction in autism spectrum disorder. Intact early responses, followed by diminished late responses to sustained pain, may reflect altered pain coping or evaluation in autism spectrum disorder. Evidence of a dichotomous neural response to initial versus protracted pain may clarify the coexistence of both hypo- and hyperresponsiveness to pain in autism spectrum disorder.

Human Posterior Insula Functional Connectivity Differs Between Electrical Pain and the Resting State

The objective in this study was to directly compare MRI-based functional connectivity between conditions of rest and painful electrical nerve stimulation for key regions involved in pain processing: the anterior and posterior insula and the anterior cingulate cortex. Electric nerve stimulation, rated 7/10 for pain, was delivered to the right index finger of 14 healthy pain-free adult volunteers in four 30-sec blocks and continuously for 2 min. Functional connectivity maps obtained at rest and during both pain tasks were compared using seed time courses from the left anterior and posterior insula and anterior cingulate. Significant Pain versus Rest connectivity differences were consistently shown for the posterior insula, notably to the posterior cingulate and precuneus, while minimal and inconsistent differences were observed for the anterior insula and anterior cingulate. This study reinforces the known differences that can occur with changes in seed region selection in functional connectivity analysis. It also presents preliminary evidence that functional connectivity for the left posterior insula can potentially differentiate the presence of acute right-sided electrical pain from the nonpainful resting state.

Behavior, neuropsychology and fMRI

Cognitive neuroscientists in the late 20th century began the task of identifying the part(s) of the brain concerned with normal behavior as manifest in the psychological capacities as affective powers, reasoning, behaving purposively and the pursuit of goals, following introduction of the 'functional magnetic resonance imaging' (fMRI) method for identifying brain activity. For this research program to be successful two questions require satisfactory answers. First, as the fMRI method can currently only be used on stationary subjects, to what extent can neuropsychological tests applicable to such stationary subjects be correlated with normal behavior. Second, to what extent can correlations between the various neuropsychological tests on the one hand, and sites of brain activity determined with fMRI on the other, be regarded as established. The extent to which these questions have yet received satisfactory answers is reviewed, and suggestions made both for improving correlations of neuropsychological tests with behavior as well as with the results of fMRI-based observations.

Optimizing and Interpreting Insular Functional Connectivity Maps Obtained During Acute Experimental Pain: The Effects of Global Signal and Task Paradigm Regression

The insula is uniquely located between the temporal and parietal cortices, making it anatomically well-positioned to act as an integrating center between the sensory and affective domains for the processing of painful stimulation. This can be studied through resting-state functional connectivity (fcMRI) imaging; however, the lack of a clear methodology for the analysis of fcMRI complicates the interpretation of these data during acute pain. Detected connectivity changes may reflect actual alterations in low-frequency synchronous neuronal activity related to pain, may be due to changes in global cerebral blood flow or the superimposed task-induced neuronal activity. The primary goal of this study was to investigate the effects of global signal regression (GSR) and task paradigm regression (TPR) on the changes in functional connectivity of the left (contralateral) insula in healthy subjects at rest and during acute painful electric nerve stimulation of the right hand. The use of GSR reduced the size and statistical significance of connectivity clusters and created negative correlation coefficients for some connectivity clusters. TPR with cyclic stimulation gave task versus rest connectivity differences similar to those with a constant task, suggesting that analysis which includes TPR is more accurately reflective of low-frequency neuronal activity. Both GSR and TPR have been inconsistently applied to fcMRI analysis. Based on these results, investigators need to consider the impact GSR and TPR have on connectivity during task performance when attempting to synthesize the literature.

The time-course of cortico-limbic neural responses to air hunger

Several studies have mapped brain regions associated with acute dyspnea perception. However, the time-course of brain activity during sustained dyspnea is unknown. Our objective was to determine the time-course of neural activity when dyspnea is sustained. Eight healthy subjects underwent brain blood oxygen level dependent functional magnetic imaging (BOLD-fMRI) during mechanical ventilation with constant mild hypercapnia (∼ 45 mm Hg). Subjects rated dyspnea (air hunger) via visual analog scale (VAS). Tidal volume (V(T)) was alternated every 90 s between high VT (0.96 ± 0.23 L) that provided respiratory comfort (12 ± 6% full scale) and low V(T) (0.48 ± 0.08 L) which evoked air hunger (56 ± 11% full scale). BOLD signal was extracted from a priori brain regions and combined with VAS data to determine air hunger related neural time-course. Air hunger onset was associated with BOLD signal increases that followed two distinct temporal profiles within sub-regions of the anterior insula, anterior cingulate and prefrontal cortices (cortico-limbic circuitry): (1) fast, BOLD signal peak <30s and (2) slow, BOLD signal peak >40s. BOLD signal during air hunger offset followed fast and slow temporal profiles symmetrical, but inverse (signal decreases) to the time-courses of air hunger onset. We conclude that differential cortico-limbic circuit elements have unique contributions to dyspnea sensation over time. We suggest that previously unidentified sub-regions are responsible for either the acute awareness or maintenance of dyspnea. These data enhance interpretation of previous studies and inform hypotheses for future dyspnea research.