Primary somatosensory cortex is actively involved in pain processing in human

Neural dynamics of pain modulation by emotional valence

Definitions of human pain acknowledge at least two dimensions of pain, affective and sensory, described as separable and thus potentially differentially modifiable. Using electroencephalography, we investigated perceptual and neural changes of emotional pain modulation in healthy individuals. Painful electrical stimuli were applied after presentation of priming emotional pictures (negative, neutral, positive) and followed by pain intensity and unpleasantness ratings. We found that perceptual and neural event-related potential responses to painful stimulation were significantly modulated by emotional valence. Specifically, pain unpleasantness but not pain intensity ratings were increased when pain was preceded by negative compared to neutral or positive pictures. Amplitudes of N2 were higher when pain was preceded by neutral compared to negative and positive pictures, and P2 amplitudes were higher for negative compared to neutral and positive pictures. In addition, a hierarchical regression analysis revealed that P2 alone and not N2, predicted pain perception. Finally, source analysis showed the anterior cingulate cortex and the thalamus as main spatial clusters accounting for the neural changes in pain processing. These findings provide evidence for a separation of the sensory and affective dimensions of pain and open new perspectives for mechanisms of pain modulation.

Increased Delta and Theta Power Density in Sickle Cell Disease Individuals with Chronic Pain Secondary to Hip Osteonecrosis: A Resting-State Eeg Study

PURPOSE

Identify the presence of a dysfunctional electroencephalographic (EEG) pattern in individuals with sickle cell disease (SCD) and hip osteonecrosis, and assess its potential associations with depression, anxiety, pain severity, and serum levels of brain-derived neurotrophic factor (BDNF).

METHODS

In this cross-sectional investigation, 24 SCD patients with hip osteonecrosis and chronic pain were matched by age and sex with 19 healthy controls. Resting-state EEG data were recorded using 32 electrodes for both groups. Power spectral density (PSD) and peak alpha frequency (PAF) were computed for each electrode across Delta, Theta, Alpha, and Beta frequency bands. Current Source Density (CSD) measures were performed utilizing the built-in Statistical nonparametric Mapping Method of the LORETA-KEY software.

RESULTS

Our findings demonstrated that SCD individuals exhibited higher PSD in delta and theta frequency bands when compared to healthy controls. Moreover, SCD individuals displayed increased CSD in delta and theta frequencies, coupled with decreased CSD in the alpha frequency within brain regions linked to pain processing, motor function, emotion, and attention. In comparison to the control group, depression symptoms, and pain intensity during hip abduction were positively correlated with PSD and CSD in the delta frequency within the parietal region. Depression symptoms also exhibited a positive association with PSD and CSD in the theta frequency within the same region, while serum BDNF levels showed a negative correlation with CSD in the alpha frequency within the left insula.

CONCLUSION

This study indicates that individuals with SCD experiencing hip osteonecrosis and chronic pain manifest a dysfunctional EEG pattern characterized by the persistence of low-frequency PSD during a resting state. This dysfunctional EEG pattern may be linked to clinical and biochemical outcomes, including depression symptoms, pain severity during movement, and serum BDNF levels.

Repetitive nociceptive stimulation increases spontaneous neural activation similar to nociception-induced activity in mouse insular cortex

Recent noninvasive neuroimaging technology has revealed that spatiotemporal patterns of cortical spontaneous activity observed in chronic pain patients are different from those in healthy subjects, suggesting that the spontaneous cortical activity plays a key role in the induction and/or maintenance of chronic pain. However, the mechanisms of the spontaneously emerging activities supposed to be induced by nociceptive inputs remain to be established. In the present study, we investigated spontaneous cortical activities in sessions before and after electrical stimulation of the periodontal ligament (PDL) by applying wide-field and two-photon calcium imaging to anesthetized GCaMP6s transgenic mice. First, we identified the sequential cortical activation patterns from the primary somatosensory and secondary somatosensory cortices to the insular cortex (IC) by PDL stimulation. We, then found that spontaneous IC activities that exhibited a similar spatiotemporal cortical pattern to evoked activities by PDL stimulation increased in the session after repetitive PDL stimulation. At the single-cell level, repetitive PDL stimulation augmented the synchronous neuronal activity. These results suggest that cortical plasticity induced by the repetitive stimulation leads to the frequent PDL stimulation-evoked-like spontaneous IC activation. This nociception-induced spontaneous activity in IC may be a part of mechanisms that induces chronic pain.

Self-referential processes and resting-state connectivity in breast cancer patients before and 1 year after chemotherapy

Modifications in the processing of information relevant to oneself have been reported in breast cancer (BC) patients. Here, we characterize the longitudinal changes to self-representations in BC patients and how they are related to intrinsic functional brain connectivity. We tested 16 BC patients before (T1) and 1 year after the end of chemotherapy (T2) along with 24 healthy control participants (HC) at similar time points. Participants underwent resting-state fMRI and completed the Questionnaire of Self-Representation (QSR), which evaluates self-assertion and self-esteem. Resting-state functional connectivity (RSFC) was calculated for regions implicated in self-referential processes (dorsomedial prefrontal cortex [dmPFC], posterior cingulate cortex [PCC], and dorsal anterior cingulate cortex [dACC]) and correlated with QSR scores. QSR scores were on average larger in patients compared with HC and did not vary over time. RSFC between the dACC and regions supporting body awareness (precentral/postcentral and supramarginal gyri, superior parietal lobule) decreased more between T1 and T2 in BC patients than in HC. BC patients had lower RSFC than HC between the dmPFC and the PCC, and regions supporting mental imagery (precuneus, lingual gyrus), at each time point, and a greater decrease from T1 and T2. QSR scores negatively correlated with RSFC. Patients described themselves as having greater self-awareness and positive self-image, reflecting a fighting spirit. In parallel, patients presented a decrease in cortical activity related to body awareness and mental imagery of self-representations over time that may be related to the positive self-image patients have and could reflect a temporary adaptive strategy.

Alterations of brain structural MRI are associated with outcome of surgical treatment in trigeminal neuralgia

BACKGROUND AND PURPOSE

To assess magnetic resonance imaging (MRI) alterations occurring in patients with trigeminal neuralgia (TN) and to explore the predictive ability of MRI for initial surgical outcome and long-term pain relief/recurrence after Gamma Knife radiosurgery (GKS).

METHODS

Thirty patients with idiopathic or classic TN, who underwent GKS and were followed for at least 24 months, were retrospectively included. Pre-treatment structural MRI and pre- and serial, postoperative clinical features were investigated. Fifteen age- and sex-matched healthy controls were also enrolled. Cortical thickness and gray matter (GM) volumes were assessed in TN patients relative to controls, as well as between patient subgroups according to treatment outcomes (initial responders/non-responders, patients with pain recurrence/long-lasting pain relief at the last follow-up). Clinical and MRI predictors of treatment outcomes were explored.

RESULTS

Cortical thinning of temporal, prefrontal, cingulate, somatosensory and occipital areas bilaterally was found in TN patients relative to controls. No cortical thickness and GM volume differences were observed when TN initial responders and non-responders were compared. Patients who experienced TN recurrence after initial pain relief were characterized by thicker parahippocampal and temporal cortices bilaterally and greater volume of right amygdala and hippocampus compared to patients with long-lasting pain relief. In TN patients, disease duration and baseline cortical thinning of right parahippocampal, left fusiform and middle temporal cortices were associated with poor outcome after GKS at the last follow-up (R² =0.57, p<0.001).

CONCLUSION

The study provides novel insights into structural brain alterations of TN patients, which might contribute to disease development and pain maintenance.

Abnormal Intrinsic Functional Interactions Within Pain Network in Cervical Discogenic Pain

Cervical discogenic pain (CDP) is mainly induced by cervical disc degeneration. However, how CDP modulates the functional interactions within the pain network remains unclear. In the current study, we studied the changed resting-state functional connectivities of pain network with 40 CDP patients and 40 age-, gender-matched healthy controls. We first defined the pain network with the seeds of the posterior insula (PI). Then, whole brain and seed-to-target functional connectivity analyses were performed to identify the differences in functional connectivity between CDP and healthy controls. Finally, correlation analyses were applied to reveal the associations between functional connectivities and clinical measures. Whole-brain functional connectivity analyses of PI identified increased functional connectivity between PI and thalamus (THA) and decreased functional connectivity between PI and middle cingulate cortex (MCC) in CDP patients. Functional connectivity analyses within the pain network further revealed increased functional connectivities between bilateral PI and bilateral THA, and decreased functional connectivities between left PI and MCC, between left postcentral gyrus (PoCG) and MCC in CDP patients. Moreover, we found that the functional connectivities between right PI and left THA, between left PoCG and MCC were negatively and positively correlated with the visual analog scale, respectively. Our findings provide direct evidence of how CDP modulates the pain network, which may facilitate understanding of the neural basis of CDP.

Feedforward and feedback pathways of nociceptive and tactile processing in human somatosensory system: A study of dynamic causal modeling of fMRI data

Nociceptive and tactile information is processed in the somatosensory system via reciprocal (i.e., feedforward and feedback) projections between the thalamus, the primary (S1) and secondary (S2) somatosensory cortices. The exact hierarchy of nociceptive and tactile information processing within this 'thalamus-S1-S2' network and whether the processing hierarchy differs between the two somatosensory submodalities remains unclear. In particular, two questions related to the ascending and descending pathways have not been addressed. For the ascending pathways, whether tactile or nociceptive information is processed in parallel (i.e., 'thalamus-S1' and 'thalamus-S2') or in serial (i.e., 'thalamus-S1-S2') remains controversial. For the descending pathways, how corticothalamic feedback regulates nociceptive and tactile processing also remains elusive. Here, we aimed to investigate the hierarchical organization for the processing of nociceptive and tactile information in the 'thalamus-S1-S2' network using dynamic causal modeling (DCM) combined with high-temporal-resolution fMRI. We found that, for both nociceptive and tactile information processing, both S1 and S2 received inputs from thalamus, indicating a parallel structure of ascending pathways for nociceptive and tactile information processing. Furthermore, we observed distinct corticothalamic feedback regulations from S1 and S2, showing that S1 generally exerts inhibitory feedback regulation independent of external stimulation whereas S2 provides additional inhibition to the thalamic activity during nociceptive and tactile information processing in humans. These findings revealed that nociceptive and tactile information processing have similar hierarchical organization within the somatosensory system in the human brain.

Role of the Anterior Cingulate Cortex in Translational Pain Research

As the most common symptomatic reason to seek medical consultation, pain is a complex experience that has been classified into different categories and stages. In pain processing, noxious stimuli may activate the anterior cingulate cortex (ACC). But the function of ACC in the different pain conditions is not well discussed. In this review, we elaborate the commonalities and differences from accumulated evidence by a variety of pain assays for physiological pain and pathological pain including inflammatory pain, neuropathic pain, and cancer pain in the ACC, and discuss the cellular receptors and signaling molecules from animal studies. We further summarize the ACC as a new central neuromodulation target for invasive and non-invasive stimulation techniques in clinical pain management. The comprehensive understanding of pain processing in the ACC may lead to bridging the gap in translational research between basic and clinical studies and to develop new therapies.

Abnormal Whole Brain Functional Connectivity Pattern Homogeneity and Couplings in Migraine Without Aura

Previous studies have reported abnormal amplitude of low-frequency fluctuation and regional homogeneity in patients with migraine without aura using resting-state functional magnetic resonance imaging. However, how whole brain functional connectivity pattern homogeneity and its corresponding functional connectivity changes in patients with migraine without aura is unknown. In the current study, we employed a recently developed whole brain functional connectivity homogeneity (FcHo) method to identify the voxel-wise changes of functional connectivity patterns in 21 patients with migraine without aura and 21 gender and age matched healthy controls. Moreover, resting-state functional connectivity analysis was used to reveal the changes of corresponding functional connectivities. FcHo analyses identified significantly decreased FcHo values in the posterior cingulate cortex (PCC), thalamus (THA), and left anterior insula (AI) in patients with migraine without aura compared to healthy controls. Functional connectivity analyses further found decreased functional connectivities between PCC and medial prefrontal cortex (MPFC), between AI and anterior cingulate cortex, and between THA and left precentral gyrus (PCG). The functional connectivities between THA and PCG were negatively correlated with pain intensity. Our findings indicated that whole brain FcHo and connectivity abnormalities of these regions may be associated with functional impairments in pain processing in patients with migraine without aura.

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.

Static and Dynamic Changes of Amplitude of Low-Frequency Fluctuations in Cervical Discogenic Pain

Cervical discogenic pain (CDP) is a clinically common pain syndrome caused by cervical disk degeneration. A large number of studies have reported that CDP results in brain functional impairments. However, the detailed dynamic brain functional abnormalities in CDP are still unclear. In this study, using resting-state functional magnetic resonance imaging, we explored the neural basis of CDP with 40 CDP patients and 40 age-, gender-matched healthy controls to delineate the changes of the voxel-level static and dynamic amplitude of low frequency fluctuations (ALFF). We found increased static ALFF in left insula (INS) and posterior precuneus (PCu), and decreased static ALFF in left precentral/postcentral gyrus (PreCG/PoCG), thalamus (THA), and subgenual anterior cingulate cortex in CPD patients compared to healthy controls. We also found decreased dynamic ALFF in left PreCG/PoCG, right posterior middle temporal gyrus, and bilateral THA. Moreover, we found that static ALFF in left PreCG/PoCG and dynamic ALFF in THA were significantly negatively correlated with visual analog scale and disease duration, respectively. Our findings provide the neurophysiological basis for CDP and facilitate understanding the neuropathology of CDP.

Pain and reward circuits antagonistically modulate alcohol expectancy to regulate drinking

Expectancy of physical and social pleasure (PSP) promotes excessive drinking despite the potential aversive effects of misuse, suggesting an imbalance in the response to reward and pain in alcohol seeking. Here, we investigated the competing roles of the reward and pain circuits in PSP expectancy and problem drinking in humans. Using fMRI data during resting (n = 180) and during alcohol cue exposure (n = 71), we examined the antagonistic effects of the reward-related medial orbitofrontal cortex (mOFC) and pain-related periaqueductal gray (PAG) connectivities on PSP expectancy and drinking severity. The two regions' connectivity maps and strengths were characterized to assess their shared substrates and net relationship with PSP expectancy. We evaluated mediation and path models to further delineate how mOFC and PAG connectivities interacted through the shared substrates to differentially impact expectancy and alcohol use. During resting, whole-brain regressions showed mOFC connectivity in positive and PAG connectivity in negative association with PSP scores, with convergence in the precentral gyrus (PrCG). Notably, greater PAG-PrCG relative to mOFC-PrCG connectivity strength predicted lower PSP expectancy. During the alcohol cue exposure task, the net strength of the PAG vs. mOFC cue-elicited connectivity with the occipital cortex again negatively predicted PSP expectancy. Finally, mediation and path models revealed that the PAG and mOFC connectivities indirectly and antagonistically modulated problem drinking via their opposing influences on expectancy and craving. Thus, the pain and reward circuits exhibit functional antagonism such that the mOFC connectivity increases expectancy of drinking pleasure whereas the PAG serves to counter that effect.

Early gamma-oscillations as correlate of localized nociceptive processing in primary sensorimotor cortex

Recent studies put forward the idea that stimulus-evoked gamma-band oscillations (GBOs; 30-100 Hz) play a specific role in nociception. So far, evidence for the specificity of GBOs for nociception, their possible involvement in nociceptive sensory discriminatory abilities, and knowledge regarding their cortical sources is just starting to grow. To address these questions, we used electroencephalography (EEG) to record brain activity evoked by phasic nociceptive laser stimuli and tactile stimuli applied at different intensities to the right hand and foot of 12 healthy volunteers. The EEG was analyzed in the time domain to extract phase-locked event-related brain potentials (ERPs) and in three regions of interest in the time-frequency domain (delta/theta, 40-Hz gamma, 70-Hz gamma) to extract stimulus-evoked changes in the magnitude of non-phase-locked brain oscillations. Both nociceptive and tactile stimuli, matched with respect to subjective intensity, elicited phase locked ERPs of increasing amplitude with increasing stimulus intensity. In contrast, only nociceptive stimuli elicited a significant enhancement of GBOs (65-85 Hz, 150-230 ms after stimulus onset), whose magnitude encoded stimulus intensity, whereas tactile stimuli led to a GBO decrease. Following nociceptive hand stimulation, the topographical distribution of GBOs was maximal at contralateral electrode C3, whereas maximum activity following foot stimulation was recorded at the midline electrode Cz, compatible with generation of GBOs in the representations of the hand and foot of the primary sensorimotor cortex, respectively. The differential behavior of high-frequency GBOs and low-frequency 40-Hz GBOs is indicating different functional roles and regions in sensory processing.NEW & NOTEWORTHY Gamma-band oscillations show hand-foot somatotopy compatible with generation in primary sensorimotor cortex and are present following nociceptive but not tactile stimulation of the hand and foot in humans.

Alterations in grey matter density and functional connectivity in trigeminal neuropathic pain and trigeminal neuralgia: A systematic review and meta-analysis

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Activation likelihood estimation (ALE) shows to be a verified method to meta-analyze heterogeneous imaging results.

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From a broad variety of key regions, structural and functional changes were repeatedly found in the thalamus, the cingulate cortex and the middle frontal gyrus in TN/TNP patients.

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Future research should focus on these regions of interest in order to improve diagnostic imaging in TN/TNP.

Background

Various studies reported changes in grey matter volumes and modifications in functional connectivity of cortical and subcortical structures in patients suffering from trigeminal neuralgia (TN) and trigeminal neuropathic pain (TNP). This study meta-analyzed the concordant structural and functional changes in foci and provide further understanding of the anatomy and biology of TN/TNP.

Methods

Relevant articles on magnetic resonance imaging (MRI) and functional MRI in TN/TNP, published before August 2018, were searched for on PubMed and Embase. Following exclusion of unsuitable studies, a meta-analysis was performed using activation likelihood estimation (ALE).

Results

In total, 322 paper were identified, 11 of which could be included based on the predefined inclusion and exclusion criteria. Eight papers, totaling 279 subjects, discussing structural changes and four papers, totaling 102 subjects, discussing functional changes were included (i.e., one paper investigated both structural and functional alterations). ALE analysis showed that in TN/TNP, grey matter decreases are found in the thalamus, (anterior) cingulate gyrus, bilateral striatum, the superior-, middle- and transverse temporal gyrus, subcallosal gyrus, the bilateral insular cortex, the pre- and postcental gyrus, the middle frontal gyrus bilaterally and the anterior cerebellar lobe. Grey matter increases were seen in the periaqueductal grey (PAG). Increased resting state functional organization was found within the bilateral middle- and superior frontal gyri, the (posterior) cingulate cortex and the thalamus/pulvinar.

Conclusions

Structural and functional changes meta-analyzed in this paper may contribute to elucidating the central pathophysiological mechanisms involved in TN/TNP. These results may be used as biomarkers to predict the response to medication and, ideally, in the future to offer personalized treatments.

GABAergic cell transplants in the anterior cingulate cortex reduce neuropathic pain aversiveness

Dysfunction of inhibitory circuits in the rostral anterior cingulate cortex underlies the affective (aversive), but not the sensory-discriminative features (hypersensitivity) of the pain experience. To restore inhibitory controls, we transplanted inhibitory interneuron progenitor cells into the rostral anterior cingulate cortex in a chemotherapy-induced neuropathic pain model. The transplants integrated, exerted a GABA-A mediated inhibition of host pyramidal cells and blocked gabapentin preference (i.e. relieved ongoing pain) in a conditioned place preference paradigm. Surprisingly, pain aversiveness persisted when the transplants populated both the rostral and posterior anterior cingulate cortex. We conclude that selective and long lasting inhibition of the rostral anterior cingulate cortex, in the mouse, has a profound pain relieving effect against nerve injury-induced neuropathic pain. However, the interplay between the rostral and posterior anterior cingulate cortices must be considered when examining circuits that influence ongoing pain and pain aversiveness.

Differing Spontaneous Brain Activity in Healthy Adults with Two Different Body Constitutions: A Resting-State Functional Magnetic Resonance Imaging Study

Traditional Chinese medicine (TCM) practitioners assess body constitution (BC) as a treatment basis for maintaining body homeostasis. We investigated patterns in spontaneous brain activity in different BC groups using resting-state functional magnetic resonance imaging (rsfMRI) and determined the relationship between these patterns and quality of life (QOL). Thirty-two healthy individuals divided into two groups (body constitution questionnaire (BCQ)-gentleness [BCQ-G] and BCQ-deficiency [BCQ-D]) based on the body constitution questionnaire (BCQ) underwent rsfMRI to analyze regional homogeneity (ReHo) and the amplitude of low-frequency fluctuation (ALFF). The World Health Organization Quality of Life Instruments (brief edition) scale was used to evaluate the QOL. The BCQ-G group (n = 18) had significantly greater ReHo values in the right postcentral gyrus and lower ALFF values in the brainstem than the BCQ-D group (n = 14). In the BCQ-D group, decreased ReHo of the postcentral gyrus correlated with better physiological functioning; increased ALFF in the brainstem correlated with poor QOL. BCQ-subgroup analysis revealed a nonsignificant correlation between ReHo and Yang deficiency/phlegm and stasis (Phl & STA). Nonetheless, the BCQ-D group showed a positive correlation between ALFF and Phl & STA in the parahippocampus. This study identified differences between BCQ-G and BCQ-D types of healthy adults based on the rsfMRI analysis. The different BCQ types with varied brain endophenotypes may elucidate individualized TCM treatment strategies.

TrpM8-mediated somatosensation in mouse neocortex

Somatosensation is a complex sense mediated by more than a dozen distinct neural subtypes in the periphery. Although pressure and touch sensation have been mapped to primary somatosensory cortex in rodents, it has been controversial whether pain and temperature inputs are also directed to this area. Here we use a well-defined somatosensory modality, cool sensation mediated by peripheral TrpM8-receptors, to investigate the neural substrate for cool perception in the mouse neocortex. Using activation of cutaneous TrpM8 receptor-expressing neurons, we identify candidate neocortical areas responsive for cool sensation. Initially, we optimized TrpM8 stimulation and determined that menthol, a selective TrpM8 agonist, was more effective than cool stimulation at inducing expression of the immediate-early gene c-fos in the spinal cord. We developed a broad-scale brain survey method for identification of activated brain areas, using automated methods to quantify c-fos immunoreactivity (fos-IR) across animals. Brain areas corresponding to the posterior insular cortex and secondary somatosensory (S2) show elevated fos-IR after menthol stimulation, in contrast to weaker activation in primary somatosensory cortex (S1). In addition, menthol exposure triggered fos-IR in piriform cortex, the amygdala, and the hypothalamus. Menthol-mediated activation was absent in TrpM8-knock-out animals. Our results indicate that cool somatosensory input broadly drives neural activity across the mouse brain, with neocortical signal most elevated in the posterior insula, as well as S2 and S1. These findings are consistent with data from humans indicating that the posterior insula is specialized for somatosensory information encoding temperature, pain, and gentle touch.

Transcranial Static Magnetic Field Stimulation over the Primary Motor Cortex Induces Plastic Changes in Cortical Nociceptive Processing

Transcranial static magnetic field stimulation (tSMS) is a novel and inexpensive, non-invasive brain stimulation (NIBS) technique. Here, we performed non-invasive modulation of intra-epidermal electrical stimulation-evoked potentials (IES-EPs) by applying tSMS or sham stimulation over the primary motor (M1) and somatosensory (S1) cortices in 18 healthy volunteers for 15 min. We recorded EPs after IES before, right after, and 10 min after tSMS. The IES-EP amplitude was significantly reduced immediately after tSMS over M1, whereas tSMS over S1 and sham stimulation did not affect the IES-EP amplitude. Thus, tSMS may affect cortical nociceptive processing. Although the results of intervention for experimental acute pain in healthy subjects cannot be directly translated into the clinical situation, tSMS may be a potentially useful NIBS method for managing chronic pain, in addition to standard of care treatments.

Functional magnetic resonance imaging reveals differences in brain activation in response to thermal stimuli in diabetic patients with and without diabetic peripheral neuropathy

Introduction

Diabetes affects both the peripheral and central nervous systems. The aim of this study was to explore the changes in brain activity in response to thermal stimuli in diabetic patients with and without diabetic peripheral neuropathy (DPN) using functional magnetic resonance imaging (fMRI).

Methods

A total of 36 right-handed volunteers were enrolled: eight patients with Type-2 diabetes mellitus and DPN, 13 patients with Type-2 diabetes mellitus lacking DPN (NDPN patients), and 15 healthy volunteers (HV). Blood oxygenation level-dependent baseline scans were performed, first without any stimuli, and then with four sessions of thermal stimuli (0, 10, 34, and 44°C, in a random order) applied to the lateral side of the right lower extremity. There was a 240-s rest interval between each thermal stimulation. Each stimulation session consisted of three cycles of 30 s of stimulation followed by 30 s of rest. After each stimuli session, the participant rated pain and itch perception on a visual analog scale. The fMRI data series were analyzed by using Statistical Parametric Mapping 8 and Data Processing Assistant for Resting-State fMRI.

Results

In response to temperature stimuli, DPN patients showed stronger activation than HV and NDPN patients, not only in brain areas that participate in somatosensory pathways (right insula, left caudate nucleus, frontal gyrus, and cingulate cortex), but also in the cognition-related cerebral areas (right temporal lobe, left hippocampus, and left fusiform gyrus). Activation of vermis 1–3 was greater in NDPN patients than in HV in response to 0°C stimulation.

Conclusions

fMRI may be useful for the early detection of central nervous system impairment caused by DPN. Our results indicate that central nervous system impairment related to diabetic neuropathy may not be limited to motion- and sensation-related cortical regions. Cognition-associated cerebral regions such as the hippocampus and fusiform gyrus are also affected by functional changes caused by DPN. This suggests that fMRI can detect the early stages of cognitive impairment in DPN patients before the symptoms become clinically significant.

Pain syndromes and the parietal lobe

Pain was considered to be integrated subcortically during most of the 20^th century, and it was not until 1956 that focal injury to the parietal opercular-insular cortex was shown to produce selective loss of pain senses. The parietal operculum and adjacent posterior insula are the main recipients of spinothalamic afferents in primates. The innermost operculum appears functionally associated with the posterior insula and can be segregated histologically, somatotopically and neurochemically from the more lateral S2 areas. The Posterior Insula and Medial Operculum (PIMO) encompass functional networks essential to initiate cortical nociceptive processing. Destruction of this region selectively abates pain sensations; direct stimulation generates acute pain, and epileptic foci trigger painful seizures. Lesions of the PIMO have also high potential to develop central pain with dissociated loss of pain and temperature. The PIMO region behaves as a somatosensory area on its own, which handles phylogenetically old somesthetic capabilities based on thinly myelinated or unmyelinated inputs. It integrates spinothalamic-driven information - not only nociceptive but also innocuous heat and cold, crude touch, itch, and possibly viscero-somatic interoception. Conversely, proprioception, graphesthesia or stereognosis are not processed in this area but in S1 cortices. Given its anatomo-functional properties, thalamic connections, and tight relations with limbic and multisensory cortices, the region comprising the inner parietal operculum and posterior insula appears to contain a third somatosensory cortex contributing to the spinothalamic attributes of the final perceptual experience.

Functional Evaluation of Small Fiber Pathways in Primary Restless Legs Syndrome: Aδ Pathway Study

STUDY OBJECTIVES

The aim of this study was to provide a neurophysiological evaluation of the function of large and small fibers, particularly the peripheral part of the thermonociceptive Aδ pathway in patients with primary restless legs syndrome (RLS).

METHODS

The main evaluation was based on an analysis of the parameters of laser-evoked potentials (LEPs), N2 and P2 components, and an assessment of thermonociceptive thresholds (pain thresholds; PThs). Routine nerve conduction studies (NCS) were also performed.

RESULTS

No essential or important differences of clinical significance were observed in the parameters of large fiber conduction between the study and the control groups. Prolonged latencies of N2 and P2 potentials were obtained during foot stimulation in patients with primary RLS when compared to controls (N2, P2-lower right limb, and N2-lower left limb). We also observed higher amplitudes of LEPs evaluated as P2 and N2-P2 potentials in patients with primary RLS in comparison with the control group. Significantly higher (normal distribution P < .05) thermonociceptive thresholds in both lower and upper limbs were found in the RLS group.

CONCLUSIONS

On the basis of the analysis of LEPs and their comparison with the respective results from the control group, the presence of functional disability of the thermonociceptive Aδ pathway was confirmed in patients with primary RLS. The results indicated the presence of changes in the conduction of small fiber pathways in the pathomechanism of idiopathic RLS.

Negative mood influences default mode network functional connectivity in patients with chronic low back pain: implications for functional neuroimaging biomarkers

The default mode network (DMN) has been proposed as a biomarker for several chronic pain conditions. Default mode network functional connectivity (FC) is typically examined during resting-state functional neuroimaging, in which participants are instructed to let thoughts wander. However, factors at the time of data collection (eg, negative mood) that might systematically impact pain perception and its brain activity, influencing the application of the DMN as a pain biomarker, are rarely reported. This study measured whether positive and negative moods altered DMN FC patterns in patients with chronic low back pain (CLBP), specifically focusing on negative mood because of its clinical relevance. Thirty-three participants (CLBP = 17) underwent resting-state functional magnetic resonance imaging scanning before and after sad and happy mood inductions, and rated levels of mood and pain intensity at the time of scanning. Two-way repeated-measures analysis of variances were conducted on resting-state functional connectivity data. Significant group (CLBP > healthy controls) × condition (sadness > baseline) interaction effects were identified in clusters spanning parietal operculum/postcentral gyrus, insular cortices, anterior cingulate cortex, frontal pole, and a portion of the cerebellum (PFDR < 0.05). However, only 1 significant cluster covering a portion of the cerebellum was identified examining a two-way repeated-measures analysis of variance for happiness > baseline (PFDR < 0.05). Overall, these findings suggest that DMN FC is affected by negative mood in individuals with and without CLBP. It is possible that DMN FC seen in patients with chronic pain is related to an affective dimension of pain, which is important to consider in future neuroimaging biomarker development and implementation.

Human primary somatosensory cortex is differentially involved in vibrotaction and nociception

The role of the primary somatosensory cortex (S1) in vibrotaction is well established. In contrast, its involvement in nociception is still debated. Here we test whether S1 is similarly involved in the processing of nonnociceptive and nociceptive somatosensory input in humans by comparing the aftereffects of high-definition transcranial direct current stimulation (HD-tDCS) of S1 on the event-related potentials (ERPs) elicited by nonnociceptive and nociceptive somatosensory stimuli delivered to the ipsilateral and contralateral hands. Cathodal HD-tDCS significantly affected the responses to nonnociceptive somatosensory stimuli delivered to the contralateral hand: both early-latency ERPs from within S1 (N20 wave elicited by transcutaneous electrical stimulation of median nerve) and late-latency ERPs elicited outside S1 (N120 wave elicited by short-lasting mechanical vibrations delivered to index fingertip, thought to originate from bilateral operculo-insular and cingulate cortices). These results support the notion that S1 constitutes an obligatory relay for the cortical processing of nonnociceptive tactile input originating from the contralateral hemibody. Contrasting with this asymmetric effect of HD-tDCS on the responses to nonnociceptive somatosensory input, HD-tDCS over the sensorimotor cortex led to a bilateral and symmetric reduction of the magnitude of the N240 wave of nociceptive laser-evoked potentials elicited by stimulation of the hand dorsum. Taken together, our results demonstrate in humans a differential involvement of S1 in vibrotaction and nociception.NEW & NOTEWORTHY Whereas the role of the primary somatosensory cortex (S1) in vibrotaction is well established, its involvement in nociception remains strongly debated. By assessing, in healthy volunteers, the effect of high-definition transcranial direct current stimulation over S1, we demonstrate a differential involvement of S1 in vibrotaction and nociception.

Abnormality of spontaneous brain activities in patients with chronic neck and shoulder pain: A resting-state fMRI study

Objectives

Chronic gneck and shoulder pain (CNSP) is a common clinical symptom of cervical spondylotic radiculopathy. Several studies using resting-state functional magnetic resonance imaging (rs-fMRI) have reported that most chronic pain diseases are accompanied by structural and functional changes in the brain. However, few rs-fMRI studies have examined CNSP. The current study investigated cerebral structural and functional changes in CNSP patients.

Methods

In total, 25 CNSP patients and 20 healthy volunteers participated in the study. 3D-T1W and rs-fMRI images were acquired. Voxel-based morphometry analysis was applied to structural images, and regional homogeneity (ReHo) was extracted from rs-fMRI. Statistical analysis was performed on post-processing images and ReHo parameter maps.

Results

The results revealed no significant differences in brain structure between the two groups. In the patient group, ReHo values were significantly increased in the bilateral middle frontal gyrus and decreased in the left insula, superior frontal gyrus, middle cingulate gyrus, supplementary motor area, right postcentral gyrus, and superior parietal lobule.

Conclusions

This initial structural and rs-fMRI study of CNSP revealed characteristic features of spontaneous brain activity of CNSP patients. These findings may be helpful for increasing our understanding of the neuropathology of CNSP.

Diabetic peripheral neuropathy may not be as its name suggests: evidence from magnetic resonance imaging

Diabetic peripheral neuropathy (DPN) affects up to 50% of patients with diabetes and is a major cause of morbidity and increased mortality. Its clinical manifestations include distressing painful neuropathic symptoms and insensitivity to trauma that result in foot ulcerations and amputations. Several recent studies have implicated poor glycemic control, duration of diabetes, hyperlipidemia (particularly hypertryglyceridaemia), elevated albumin excretion rates, and obesity as risk factors for the development of DPN. However, similar data are not available for painful DPN. Moreover, although there is now strong evidence for the importance of peripheral nerve microvascular disease in the pathogenesis of DPN, peripheral structural biomarkers of painful DPN are lacking. However, there is now emerging evidence for the involvement of the central nervous system in both painful and painless DPN afforded by magnetic resonance imaging. This review will focus on this emerging evidence for central changes in DPN, hitherto considered a peripheral nerve disease only.

Distinct Somatic Discrimination Reflected by Laser-Evoked Potentials Using Scalp EEG Leads

Discrimination is an important function in pain processing of the somatic cortex. The involvement of the somatic cortex has been studied using equivalent dipole analysis and neuroimaging, but the results are inconsistent. Scalp electroencephalography (EEG) can reflect functional changes of particular brain regions underneath a lead. However, the responses of EEG leads close to the somatic cortex in response to pain have not been systematically evaluated. The present study applied CO₂ laser stimulation to the dorsum of the left hand. Laser-evoked potentials (LEPs) of C4, T3, and T4 leads and pain ratings in response to four stimulus intensities were analyzed. LEPs started earlier at the C4 and T4 leads. The onset latency and peak latency of LEPs for C4 and T4 leads were the same. Only 10 of 22 subjects (45 %) presented equivalent current dipoles within the primary somatosensory or motor cortices. LEP amplitudes of these leads increased as stimulation intensity increased. The stimulus–response pattern of the C4 lead was highly correlated with pain rating. In contrast, an S-shaped stimulus–response curve was obtained for the T3 and T4 leads. The present study provides supporting evidence that particular scalp channels are able to reflect the functional characteristics of their underlying cortical areas. Our data strengthen the clinical application of somatic-cortex-related leads for pain discrimination.

New Insights in Trigeminal Anatomy: A Double Orofacial Tract for Nociceptive Input

Orofacial pain in patients relies on the anatomical pathways that conduct nociceptive information, originating from the periphery towards the trigeminal sensory nucleus complex (TSNC) and finally, to the thalami and the somatosensorical cortical regions. The anatomy and function of the so-called trigeminothalamic tracts have been investigated before. In these animal-based studies from the previous century, the intracerebral pathways were mapped using different retro- and anterograde tracing methods. We review the literature on the trigeminothalamic tracts focusing on these animal tracer studies. Subsequently, we related the observations of these studies to clinical findings using fMRI trials. The intracerebral trigeminal pathways can be subdivided into three pathways: a ventral (contralateral) and dorsal (mainly ipsilateral) trigeminothalamic tract and the intranuclear pathway. Based on the reviewed evidence we hypothesize the co-existence of an ipsilateral nociceptive conduction tract to the cerebral cortex and we translate evidence from animal-based research to the human anatomy. Our hypothesis differs from the classical idea that orofacial pain arises only from nociceptive information via the contralateral, ventral trigeminothalamic pathway. Better understanding of the histology, anatomy and connectivity of the trigeminal fibers could contribute to the discovery of a more effective pain treatment in patients suffering from various orofacial pain syndromes.

Quantitative sensory testing of temperature, pain, and touch in adults with Down syndrome

The spinothalamic pathway mediates sensations of temperature, pain, and touch. These functions seem impaired in children with Down syndrome (DS), but have not been extensively examined in adults. The objective of the present study was to compare the spinothalamic-mediated sensory functions between adults with DS and adults from the general population and to examine in the DS group the relationship between the sensory functions and level of intellectual functioning. Quantitative sensory testing (QST) was performed in 188 adults with DS (mean age 37.5 years) and 142 age-matched control participants (median age 40.5 years). Temperature, pain, and touch were evaluated with tests for cold-warm discrimination, sharp-dull discrimination (pinprick), and tactile threshold, respectively. Level of intellectual functioning was estimated with the Social Functioning Scale for Intellectual Disability (intellectual disability level) and the Wechsler Preschool and Primary Scale of Intelligence--Revised (intelligence level). Overall, the difference in spinothalamic-mediated sensory functions between the DS and control groups was not statistically significant. However, DS participants with a lower intelligence level had a statistically significant lower performance on the sharp-dull discrimination test than DS participants with higher intelligence level (adjusted p=.006) and control participants (adjusted p=.017). It was concluded that intellectual functioning level is an important factor to take into account for the assessment of spinothalamic-mediated sensory functioning in adults with DS: a lower level could coincide with impaired sensory functioning, but could also hamper QST assessment.

A meta-analysis of site-specific effects of cathodal transcranial direct current stimulation on sensory perception and pain

The primary aim of our meta-analysis was to evaluate the effects of cathodal transcranial direct current stimulation (c-tDCS) on sensory and pain thresholds (STh and PTh) in healthy individuals and pain level (PL) in patients with chronic pain. Electronic databases were searched for c-tDCS studies. Methodological quality was evaluated using the PEDro and Downs and Black (D&B) assessment tools. C-tDCS of the primary motor cortex (S1) increases both STh (P<0.001, effect size of 26.84%) and PTh (P<0.001, effect size of 11.62%). In addition, c-tDCS over M1 led to STh increase (P<0.005, effect size of 30.44%). Likewise, PL decreased significantly in the patient group following application of c-tDCS. The small number of studies precluded subgroup analysis. Nevertheless, meta-analysis showed that in all groups (except c-tDCS of S1) active c-tDCS and sham stimulation produced significant differences in STh/PTh in healthy and PL in patient group. This review provides evidence for the site-specific effectiveness of c-tDCS in increasing STh/PTh in healthy individuals and decreasing PL in patients with chronic pain. However, due to small sample sizes in the included studies, our results should be interpreted with caution. Given that the level of blinding was not considered in the inclusion criteria, the results of the current study should be interpreted with caution.

How does anodal transcranial direct current stimulation of the pain neuromatrix affect brain excitability and pain perception? A randomised, double-blind, sham-control study

Background

Integration of information between multiple cortical regions of the pain neuromatrix is thought to underpin pain modulation. Although altered processing in the primary motor (M1) and sensory (S1) cortices is implicated in separate studies, the simultaneous changes in and the relationship between these regions are unknown yet. The primary aim was to assess the effects of anodal transcranial direct current stimulation (a-tDCS) over superficial regions of the pain neuromatrix on M1 and S1 excitability. The secondary aim was to investigate how M1 and S1 excitability changes affect sensory (STh) and pain thresholds (PTh).

Methods

Twelve healthy participants received 20 min a-tDCS under five different conditions including a-tDCS of M1, a-tDCS of S1, a-tDCS of DLPFC, sham a-tDCS, and no-tDCS. Excitability of dominant M1 and S1 were measured before, immediately, and 30 minutes after intervention respectively. Moreover, STh and PTh to peripheral electrical and mechanical stimulation were evaluated. All outcome measures were assessed at three time-points of measurement by a blind rater.

Results

A-tDCS of M1 and dorsolateral prefrontal cortex (DLPFC) significantly increased brain excitability in M1 (p < 0.05) for at least 30 min. Following application of a-tDCS over the S1, the amplitude of the N20-P25 component of SEPs increased immediately after the stimulation (p < 0.05), whilst M1 stimulation decreased it. Compared to baseline values, significant STh and PTh increase was observed after a-tDCS of all three stimulated areas. Except in M1 stimulation, there was significant PTh difference between a-tDCS and sham tDCS.

Conclusion

a-tDCS of M1 is the best spots to enhance brain excitability than a-tDCS of S1 and DLPFC. Surprisingly, a-tDCS of M1 and S1 has diverse effects on S1 and M1 excitability. A-tDCS of M1, S1, and DLPFC increased STh and PTh levels. Given the placebo effects of a-tDCS of M1 in pain perception, our results should be interpreted with caution, particularly with respect to the behavioural aspects of pain modulation.

Trial Registration

Australian New Zealand Clinical Trials, ACTRN12614000817640, http://www.anzctr.org.au/.

Central mechanisms of itch

Itch is a complex sensory and emotional experience. Functional brain imaging studies have been performed to identify brain regions associated with this complex experience, and these studies reported that several brain regions are activated by itch stimuli. The possible roles of these regions in itch perception and difference in cerebral mechanism between healthy subjects and chronic itch patients are discussed in this review article. Additionally, the central itch modulation system and cerebral mechanisms of contagious itch, pleasurable sensation evoked by scratching have also been investigated in previous brain imaging studies. We also discuss how these studies advance our understanding of these mechanisms.

Changes in cerebellar activity and inter-hemispheric coherence accompany improved reading performance following Quadrato Motor Training

Dyslexia is a multifactorial reading deficit that involves multiple brain systems. Among other theories, it has been suggested that cerebellar dysfunction may be involved in dyslexia. This theory has been supported by findings from anatomical and functional imaging. A possible rationale for cerebellar involvement in dyslexia could lie in the cerebellum’s role as an oscillator, producing synchronized activity within neuronal networks including sensorimotor networks critical for reading. If these findings are causally related to dyslexia, a training regimen that enhances cerebellar oscillatory activity should improve reading performance. We examined the cognitive and neural effects of Quadrato Motor Training (QMT), a structured sensorimotor training program that involves sequencing of motor responses based on verbal commands. Twenty-two adult Hebrew readers (12 dyslexics and 10 controls) were recruited for the study. Using Magnetoencephalography (MEG), we measured changes in alpha power and coherence following QMT in a within-subject design. Reading performance was assessed pre- and post-training using a comprehensive battery of behavioral tests. Our results demonstrate improved performance on a speeded reading task following one month of intensive QMT in both the dyslexic and control groups. Dyslexic participants, but not controls, showed significant increase in cerebellar oscillatory alpha power following training. In addition, across both time points, inter-hemispheric alpha coherence was higher in the dyslexic group compared to the control group. In conclusion, the current findings suggest that the combination of motor and language training embedded in QMT increases cerebellar oscillatory activity in dyslexics and improves reading performance. These results support the hypothesis that the cerebellum plays a role in skilled reading, and begin to unravel the underlying mechanisms that mediate cerebellar contribution in cognitive and neuronal augmentation.

Regional gray matter volume and anxiety-related traits interact to predict somatic complaints in a non-clinical sample

Somatic complaints can be important features of an individual's expression of anxiety. Anxiety-related traits are also risk factors for somatic symptoms. However, it is not known which neuroanatomical mechanisms may be responsible for this relationship. In this study, our first step was to use voxel-based morphometry (VBM) approaches to investigate the neuroanatomical basis underlying somatic complaints in a large sample of healthy subjects. We found a significant positive correlation between somatic complaints and parahippocampal gyrus (PHG) volume adjacent to the entorhinal cortex. Further analysis revealed that the interaction between PHG volume/entorhinal cortex and neuroticism-anxiety (N-Anx) predicted somatic complaints. Specifically, somatic complaints were associated with higher N-Anx for individuals with increased PHG volume. These findings suggest that increased PHG volume and higher trait anxiety can predict vulnerability to somatic complaints in the general population.

Diagnostic classification based on functional connectivity in chronic pain: model optimization in fibromyalgia and rheumatoid arthritis

RATIONALE AND OBJECTIVES

The combination of functional magnetic resonance imaging (fMRI) of the brain with multivariate pattern analysis (MVPA) has been proposed as a possible diagnostic tool. Goal of this investigation was to identify potential functional connectivity (FC) differences in the salience network (SN) and default mode network (DMN) between fibromyalgia syndrome (FMS), rheumatoid arthritis (RA), and controls (HC) and to evaluate the diagnostic applicability of derived pattern classification approaches.

MATERIALS AND METHODS

The resting period during an fMRI examination was retrospectively analyzed in women with FMS (n = 17), RA (n = 16), and HC (n = 17). FC was calculated for SN and DMN subregions. Classification accuracies of discriminative MVPA models were evaluated with cross-validation: (1) inferential test of a single method, (2) explorative model optimization.

RESULTS

No inferentially tested model was able to classify subjects with statistically significant accuracy. However, the diagnostic ability for the differential diagnostic problem exhibited a trend to significance (accuracy: 69.7%, P = .086). Optimized models in the explorative analysis reached accuracies up to 73.5% (FMS vs. HC), 78.8% (RA vs. HC), and 78.8% (FMS vs. RA) whereas other models performed at or below chance level. Comparable support vector machine approaches performed above average for all three problems.

CONCLUSIONS

Observed accuracies are not sufficient to reliably differentiate between FMS and RA for diagnostic purposes. However, some indirect evidence in support of the feasibility of this approach is provided. This exploratory analysis constitutes a fundamental model optimization effort to be based on in further investigations.

Motor cortex stimulation suppresses cortical responses to noxious hindpaw stimulation after spinal cord lesion in rats

BACKGROUND

Motor cortex stimulation (MCS) is a potentially effective treatment for chronic neuropathic pain. The neural mechanisms underlying the reduction of hyperalgesia and allodynia after MCS are not completely understood.

OBJECTIVE

To investigate the neural mechanisms responsible for analgesic effects after MCS. We test the hypothesis that MCS attenuates evoked blood oxygen-level dependent signals in cortical areas involved in nociceptive processing in an animal model of chronic neuropathic pain.

METHODS

We used adult female Sprague-Dawley rats (n = 10) that received unilateral electrolytic lesions of the right spinal cord at the level of C6 (SCL animals). In these animals, we performed magnetic resonance imaging (fMRI) experiments to study the analgesic effects of MCS. On the day of fMRI experiment, 14 days after spinal cord lesion, the animals were anesthetized and epidural bipolar platinum electrodes were placed above the left primary motor cortex. Two 10-min sessions of fMRI were performed before and after a session of MCS (50 μA, 50 Hz, 300 μs, for 30 min). During each fMRI session, the right hindpaw was electrically stimulated (noxious stimulation: 5 mA, 5 Hz, 3 ms) using a block design of 20 s stimulation off and 20 s stimulation on. A general linear model-based statistical parametric analysis was used to analyze whole brain activation maps. Region of interest (ROI) analysis and paired t-test were used to compare changes in activation before and after MCS in these ROI.

RESULTS

MCS suppressed evoked blood oxygen dependent signals significantly (Family-wise error corrected P < 0.05) and bilaterally in 2 areas heavily implicated in nociceptive processing. These areas consisted of the primary somatosensory cortex and the prefrontal cortex.

CONCLUSIONS

These findings suggest that, in animals with SCL, MCS attenuates hypersensitivity by suppressing activity in the primary somatosensory cortex and prefrontal cortex.

Unmasking the obligatory components of nociceptive event-related brain potentials

It has been hypothesized that the human cortical responses to nociceptive and nonnociceptive somatosensory inputs differ. Supporting this view, somatosensory-evoked potentials (SEPs) elicited by thermal nociceptive stimuli have been suggested to originate from areas 1 and 2 of the contralateral primary somatosensory (S1), operculo-insular, and cingulate cortices, whereas the early components of nonnociceptive SEPs mainly originate from area 3b of S1. However, to avoid producing a burn lesion, and sensitize or fatigue nociceptors, thermonociceptive SEPs are typically obtained by delivering a small number of stimuli with a large and variable interstimulus interval (ISI). In contrast, the early components of nonnociceptive SEPs are usually obtained by applying many stimuli at a rapid rate. Hence, previously reported differences between nociceptive and nonnociceptive SEPs could be due to differences in signal-to-noise ratio and/or differences in the contribution of cognitive processes related, for example, to arousal and attention. Here, using intraepidermal electrical stimulation to selectively activate Aδ-nociceptors at a fast and constant 1-s ISI, we found that the nociceptive SEPs obtained with a long ISI are no longer identified, indicating that these responses are not obligatory for nociception. Furthermore, using a blind source separation, we found that, unlike the obligatory components of nonnociceptive SEPs, the obligatory components of nociceptive SEPs do not receive a significant contribution from a contralateral source possibly originating from S1. Instead, they were best explained by sources compatible with bilateral operculo-insular and/or cingulate locations. Taken together, our results indicate that the obligatory components of nociceptive and nonnociceptive SEPs are fundamentally different.

Theta burst stimulation applied over primary motor and somatosensory cortices produces analgesia unrelated to the changes in nociceptive event-related potentials

Continuous theta burst stimulation (cTBS) applied over the primary motor cortex (M1) can alleviate pain although the neural basis of this effect remains largely unknown. Besides, the primary somatosensory cortex (S1) is thought to play a pivotal role in the sensori-discriminative aspects of pain perception but the analgesic effect of cTBS applied over S1 remains controversial. To investigate cTBS-induced analgesia we characterized, in two separate experiments, the effect of cTBS applied either over M1 or S1 on the event-related brain potentials (ERPs) and perception elicited by nociceptive (CO2 laser stimulation) and non-nociceptive (transcutaneous electrical stimulation) somatosensory stimuli. All stimuli were delivered to the ipsilateral and contralateral hand. We found that both cTBS applied over M1 and cTBS applied over S1 significantly reduced the percept elicited by nociceptive stimuli delivered to the contralateral hand as compared to similar stimulation of the ipsilateral hand. In contrast, cTBS did not modulate the perception of non-nociceptive stimuli. Surprisingly, this side-dependent analgesic effect of cTBS was not reflected in the amplitude modulation of nociceptive ERPs. Indeed, both nociceptive (N160, N240 and P360 waves) and late-latency non-nociceptive (N140 and P200 waves) ERPs elicited by stimulation of the contralateral and ipsilateral hands were similarly reduced after cTBS, suggesting an unspecific effect, possibly due to habituation or reduced alertness. In conclusion, cTBS applied over M1 and S1 reduces similarly the perception of nociceptive inputs originating from the contralateral hand, but this analgesic effect is not reflected in the magnitude of nociceptive ERPs.

Magnetic resonance imaging of the central nervous system in diabetic neuropathy

Diabetic 'peripheral' neuropathy (DPN) is one of the common sequelae to the development of both type-1 and type-2 diabetes mellitus. Neuropathy has a major negative impact on quality of life. Abnormalities in both peripheral vasculature and nerve function are well documented and, in addition, evidence is emerging regarding changes within the central nervous system (CNS) that are concomitant with the presence of DPN. The often-resistant nature of DPN to medical treatment highlights the need to understand the role of the CNS in neuropathic symptomatology and progression, as this may modulate therapeutic approaches. Advanced neuroimaging techniques, especially those that can provide quantitative measures of structure and function, can provide objective markers of CNS status. With that comes great potential for not only furthering our understanding of involvement of the CNS in neuropathic etiology but also most importantly aiding the development of new and more effective, targeted, analgesic interventions.

Fine-grained nociceptive maps in primary somatosensory cortex

Topographic maps of the receptive surface are a fundamental feature of neural organization in many sensory systems. While touch is finely mapped in the cerebral cortex, it remains controversial how precise any cortical nociceptive map may be. Given that nociceptive innervation density is relatively low on distal skin regions such as the digits, one might conclude that the nociceptive system lacks fine representation of these regions. Indeed, only gross spatial organization of nociceptive maps has been reported so far. However, here we reveal the existence of fine-grained somatotopy for nociceptive inputs to the digits in human primary somatosensory cortex (SI). Using painful nociceptive-selective laser stimuli to the hand, and phase-encoded functional magnetic resonance imaging analysis methods, we observed somatotopic maps of the digits in contralateral SI. These nociceptive maps were highly aligned with maps of non-painful tactile stimuli, suggesting comparable cortical representations for, and possible interactions between, mechanoreceptive and nociceptive signals. Our findings may also be valuable for future studies tracking the time course and the spatial pattern of plastic changes in cortical organization involved in chronic pain.