Differential coding of pain intensity in the human primary and secondary somatosensory cortex

Functional exploration for neuropathic pain

Neuropathic pain (NP) may become refractory to conservative medical management, necessitating neurosurgical procedures in carefully selected cases. In this context, the functional neurosurgeon must have suitable knowledge of the disease he or she intends to treat, especially its pathophysiology. This latter factor has been studied thanks to advances in the functional exploration of NP, which will be detailed in this review. The study of the flexion reflex is a useful tool for clinical and pharmacological pain assessment and for exploring the mechanisms of pain at multiple levels. The main use of evoked potentials is to confirm clinical, or detect subclinical, dysfunction in peripheral and central somato-sensory pain pathways. LEP and SEP techniques are especially useful when used in combination, allowing the exploration of both pain and somato-sensory pathways. PET scans and fMRI documented rCBF increases to noxious stimuli. In patients with chronic NP, a decreased resting rCBF is observed in the contralateral thalamus, which may be reversed using analgesic procedures. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with amplification of the thalamic, insular and SII responses, concomitant to a paradoxical CBF decrease in ACC. Multiple PET studies showed that endogenous opioid secretion is very likely to occur as a reaction to pain. In addition, brain opioid receptors (OR) remain relatively untouched in peripheral NP, while a loss of ORs is most likely to occur in central NP, within the medial nociceptive pathways. PET receptor studies have also proved that antalgic Motor Cortex Stimulation (MCS), indicated in severe refractory NP, induces endogenous opioid secretion in key areas of the endogenous opioid system, which may explain one of the mechanisms of action of this procedure, since the secretion is proportional to the analgesic effect.

Ethnicity, catastrophizing, and qualities of the pain experience

OBJECTIVE

It is generally well established that catastrophizing exerts a potent influence on individuals' experience of pain and accompanying emotional distress. Further, preliminary evidence has shown that meaningful differences among various pain relevant outcomes (e.g., pain ratings, endogenous pain inhibitory processes) can be attributed to individuals' ethnic background. The mechanisms that might explain ethnic differences in pain outcomes are unclear, and it remains to be fully established whether the relation between ethnicity and pain response may be indirectly affected by pain catastrophizing.

DESIGN

In the current study, we examined differences in pain responses by ethnicity among healthy, young adults (N=62), and attempted to determine whether such an ethnicity-pain relation was mediated by catastrophizing using the standard Pain Catastrophizing Scale (PCS) and a modified version of the PCS reflecting situational catastrophizing during a cold pressor task.

RESULTS

Results showed that pain responses varied by ethnicity, as did reported catastrophizing. Catastrophizing mediated the relation between ethnicity and affective and sensory pain responses.

CONCLUSIONS

To better explicate our findings, we described the context in which these findings occurred following a "who, what, where, when, and why" approach. This approach provides an efficient description of how our findings align with previous research, while identifying future research that should clarify the theoretical underpinnings of catastrophizing and pain and also inform clinical intervention.

The pain matrix reloaded: a salience detection system for the body

Neuroimaging and neurophysiological studies have shown that nociceptive stimuli elicit responses in an extensive cortical network including somatosensory, insular and cingulate areas, as well as frontal and parietal areas. This network, often referred to as the "pain matrix", is viewed as representing the activity by which the intensity and unpleasantness of the perception elicited by a nociceptive stimulus are represented. However, recent experiments have reported (i) that pain intensity can be dissociated from the magnitude of responses in the "pain matrix", (ii) that the responses in the "pain matrix" are strongly influenced by the context within which the nociceptive stimuli appear, and (iii) that non-nociceptive stimuli can elicit cortical responses with a spatial configuration similar to that of the "pain matrix". For these reasons, we propose an alternative view of the functional significance of this cortical network, in which it reflects a system involved in detecting, orienting attention towards, and reacting to the occurrence of salient sensory events. This cortical network might represent a basic mechanism through which significant events for the body's integrity are detected, regardless of the sensory channel through which these events are conveyed. This function would involve the construction of a multimodal cortical representation of the body and nearby space. Under the assumption that this network acts as a defensive system signaling potentially damaging threats for the body, emphasis is no longer on the quality of the sensation elicited by noxious stimuli but on the action prompted by the occurrence of potential threats.

From the neuromatrix to the pain matrix (and back)

Pain is a conscious experience, crucial for survival. To investigate the neural basis of pain perception in humans, a large number of investigators apply noxious stimuli to the body of volunteers while sampling brain activity using different functional neuroimaging techniques. These responses have been shown to originate from an extensive network of brain regions, which has been christened the Pain Matrix and is often considered to represent a unique cerebral signature for pain perception. As a consequence, the Pain Matrix is often used to understand the neural mechanisms of pain in health and disease. Because the interpretation of a great number of experimental studies relies on the assumption that the brain responses elicited by nociceptive stimuli reflect the activity of a cortical network that is at least partially specific for pain, it appears crucial to ascertain whether this notion is supported by unequivocal experimental evidence. Here, we will review the original concept of the "Neuromatrix" as it was initially proposed by Melzack and its subsequent transformation into a pain-specific matrix. Through a critical discussion of the evidence in favor and against this concept of pain specificity, we show that the fraction of the neuronal activity measured using currently available macroscopic functional neuroimaging techniques (e.g., EEG, MEG, fMRI, PET) in response to transient nociceptive stimulation is likely to be largely unspecific for nociception.

Time Course of Activity in Itch-Related Brain Regions: A Combined MEG–fMRI Study

Functional neuroimaging studies have identified itch-related brain regions. However, no study has investigated the temporal aspect of itch-related brain processing. Here this issue was investigated using electrically evoked itch in ten healthy adults. Itch stimuli were applied to the left wrist and brain activity was measured using magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). In the MEG experiment, the magnetic responses evoked by the itch stimuli were observed in the contralateral and ipsilateral frontotemporal regions. The dipoles associated with the magnetic responses were mainly located in the contralateral (nine subjects) and ipsilateral (eight subjects) secondary somatosensory cortex (SII)/insula, which were also activated by the itch stimuli in the fMRI experiment. We also observed an itch-related magnetic response in the posterior part of the centroparietal region in six subjects. MEG and fMRI data showed that the magnetic response in this region was mainly associated with itch-related activation of the precuneus. The latency was significantly longer in the ipsilateral than that in the contralateral SII/insula, suggesting the difference to be associated with transmission in the callosal fibers. The timing of activation of the precuneus was between those of the contralateral and ipsilateral SII/insula. Other sources were located in the premotor, primary motor, and anterior cingulate cortices (one subject each). This study is the first to demonstrate part of the time course of itch-related brain processing. Combining methods with high temporal and spatial resolution (e.g., MEG and fMRI) would be useful to investigate the temporal aspect of the brain mechanism of itch.

Steady-state activation in somatosensory cortex after changes in stimulus rate during median nerve stimulation

Passive electrical stimulation activates various human somatosensory cortical systems including the contralateral primary somatosensory area (SI), bilateral secondary somatosensory area (SII) and bilateral insula. The effect of stimulation frequency on blood oxygenation level-dependent (BOLD) activity remains unclear. We acquired 3-T functional magnetic resonance imaging (fMRI) in eight healthy volunteers during electrical median nerve stimulation at frequencies of 1, 3 and 10 Hz. During stimulation BOLD signal changes showed activation in the contralateral SI, bilateral SII and bilateral insula. Results of fMRI analysis showed that these areas were progressively active with the increase of rate of stimulation. As a major finding, the contralateral SI showed an increase of peak of BOLD activation from 1 to 3 Hz but reached a plateau during 10-Hz stimulation. Our finding is of interest for basic research and for clinical applications in subjects unable to perform cognitive tasks in the fMRI scanner.

Pain and non-pain processing during hypnosis: a thulium-YAG event-related fMRI study

The neural mechanisms underlying the antinociceptive effects of hypnosis still remain unclear. Using a parametric single-trial thulium-YAG laser fMRI paradigm, we assessed changes in brain activation and connectivity related to the hypnotic state as compared to normal wakefulness in 13 healthy volunteers. Behaviorally, a difference in subjective ratings was found between normal wakefulness and hypnotic state for both non-painful and painful intensity-matched stimuli applied to the left hand. In normal wakefulness, non-painful range stimuli activated brainstem, contralateral primary somatosensory (S1) and bilateral insular cortices. Painful stimuli activated additional areas encompassing thalamus, bilateral striatum, anterior cingulate (ACC), premotor and dorsolateral prefrontal cortices. In hypnosis, intensity-matched stimuli in both the non-painful and painful range failed to elicit any cerebral activation. The interaction analysis identified that contralateral thalamus, bilateral striatum and ACC activated more in normal wakefulness compared to hypnosis during painful versus non-painful stimulation. Finally, we demonstrated hypnosis-related increases in functional connectivity between S1 and distant anterior insular and prefrontal cortices, possibly reflecting top-down modulation.

Oscillatory characteristics of nociceptive responses in the SII cortex

OBJECTIVE

This study is aimed to explore the frequency characteristics of pain-evoked neuromagnetic responses in the secondary somatosensory (SII) cortices.

METHODS

Thulium-laser nociceptive stimuli to the left hand dorsum of 10 right-handed healthy adults. The pain stimuli were rated as mild, moderate, and severe levels according to subjects' reports on a 10-point visual analog scale. We analyzed their cortical responses with wavelet-based frequency analyses and equivalent current dipole (ECD) modeling.

RESULTS

For each pain level, we found an increase of theta (4-8 Hz) and alpha (8-13 Hz) power in bilateral SII areas at 180-210 ms after stimulus onset. The power was larger for the moderate than for the mild pain level (p < 0.05), but there was no statistical power difference of these oscillations between moderate and severe pain stimulus conditions (p = 0.7). Within the SII area, we did not observe particular difference in theta and alpha ECD locations between varying pain level conditions.

CONCLUSIONS

The 4-13 Hz activities, peaking from 180 to 210 ms, are oscillatory correlates of SII activation in response to nociceptive stimulation, but their power may code the magnitude of pain stimuli only up to moderate level, as rated subjectively. This measure could be potentially used to evaluate SII activation in further pain studies.

Quality discrimination for noxious stimuli in secondary somatosensory cortex: a MEG-study

A complex cortical network is believed to encode the multi-dimensionality of the human pain experience. In the present study, we used magnetoencephalography (MEG) to examine whether the cortical processing of noxious stimuli with different psychophysical properties differs in primary (S1) and secondary (S2) somatosensory cortices. Noxious low (condition 1) and high (condition 2) current density stimulations of equal stimulus intensities were applied at the left forearm in 12 subjects in a randomised order. Concomitantly, subjects had to evaluate the corresponding sensory-discriminative and affective-motivational pain dimensions. MEG revealed an increased activation of bilateral secondary somatosensory cortices (S2) during condition 2 compared to condition 1. Higher activations of bilateral S2 were significantly correlated with higher scores for the sensory-discriminative component during condition 2. In contrast, corresponding scores for the affective-motivational pain dimension did not differ between both conditions. Therefore, concerning the sensory dimension of the human pain experience we conclude that the S2 cortex is involved in the encoding of quality discrimination.

Common neural systems for contact heat and laser pain stimulation reveal higher-level pain processing

Our current knowledge of pain-related neuronal responses is largely based on experimental pain studies using contact heat or nontactile laser painful stimulation. Both stimuli evoke pain, yet they differ considerably in their physical and perceptual properties. In sensory cortex, cerebral responses to either stimulus should therefore substantially differ. However, given that both stimuli evoke pain, we hypothesized that at a certain subset of cortical regions the different physical properties of the stimuli become less important and are therefore activated by both stimuli. In contrast, regions with clearly dissociable activity may belong to "lower-level" pain processing mechanisms depending on the physical properties of the administered stimuli. We used functional magnetic resonance (fMRI) to intraindividually compare pain-related activation patterns between laser and contact heat stimulation using four different intensities of laser and contact heat stimuli. Common and dissociable neural responses were identified by correlating perceived pain intensities with blood oxygenation level dependent (BOLD) signal changes. Only neuronal responses to stimuli that were perceived as painful were analyzed. Pain-related BOLD signal increases independent of stimulus modality were detected in the anterior insula, anterior cingulate cortex, medial secondary somatosensory cortex, and the prefrontal cortex. These similarities are likely to reflect higher-level pain processing, which is largely independent of the single physical parameters that determine the painful nature of the stimuli.

Area-specific representation of mechanical nociceptive stimuli within SI cortex of squirrel monkeys

While functional imaging studies in humans have consistently reported activation of primary somatosensory cortex (SI) with painful stimuli, the specific roles of subdivisions of areas 3a, 3b, and 1 within SI during pain perception are largely unknown, particularly in the representation of mechanical evoked pain. In this study, we investigated how modality, location, and intensity of nociceptive stimuli are represented within SI by using high-spatial resolution optical imaging of intrinsic signals in Pentothal-anesthetized squirrel monkeys. Perceptually comparable mechanical nociceptive and innocuous tactile stimuli were delivered by indenting the glabrous skin of the distal finger pads with 0.2 and 2mm diameter probes, respectively. Within each of areas 3a, 3b, and 1, activations to mechanical nociceptive stimulation of individual distal finger pads were spatially distinct and somatotopically organized. We observed differential cortical activation patterns. Areas 3a, 3b, and 1 were all activated during mechanical nociceptive stimulation and were modulated by nociceptive stimulus intensity. However, with innocuous tactile stimulation, mainly areas 3b and 1 exhibited response modulation with different levels of stimulation. In summary, mechanical nociceptive inputs are area-specific and topographically represented within SI. We propose that all areas of SI are implicated in encoding the features of mechanical nociception, where areas 3a and 3b are distinctively involved in coding nociceptive and pressure sensation components of stimulation.

Identifying brain activity specifically related to the maintenance and perceptual consequence of central sensitization in humans

Central sensitization (CS) refers to an increase in the excitability of spinal dorsal horn neurons that results from, and far outlasts the initiating nociceptive input. Here, functional magnetic resonance imaging was used to examine whether supraspinal activity might contribute to the maintenance of CS in humans. A crossover parametric design was used to distinguish and control for brain activity that is related to the consequence of increased pain experienced during CS. When the intensity of pain during CS and normal states were matched, only activity within the brainstem, including the mesencephalic pontine reticular formation, and the anterior thalami remained increased during CS. Further analyses revealed that activity in the isolated brainstem area correlated positively with the force of noxious stimulation only during CS, whereas activity in the isolated thalamic area was not modulated parametrically in either CS or normal states. Additionally, the mean activity in the isolated brainstem area was increased only during CS, whereas the mean activity in the isolated thalamic area was increased in both states, albeit less so in the normal state. Together, these findings suggest a specific role of the brainstem for the maintenance of CS in humans. Regarding brain areas related to the consequence of increased pain perception during CS, we found that only cortical activity, mainly in the primary somatosensory area, was significantly correlated with intensity of pain that was attributable to both the force of noxious stimulation used and state in which noxious stimulation was applied.

Effects of attention on visceral stimulus intensity encoding in the male human brain

BACKGROUND & AIMS

Hypervigilance is considered important in pain perception in functional gastrointestinal disorders. Nonetheless, a comprehensive assessment of the influence of attention on brain processing of visceral sensation has not been performed. We investigated the effects of attention on esophageal pain perception and brain activity.

METHODS

Twelve healthy male volunteers (age range, 21-32 years) underwent 4 functional magnetic resonance imaging scans incorporating 4 levels of esophageal stimulation (ES), ranging from nonpainful to painful, during which they completed a task aimed at distracting them from the esophageal stimulus. The volunteers were then scanned a fifth time, during painful stimulation without distraction.

RESULTS

Following ES during distraction, there was a significant linear trend (P < .05) in which the intensity of cerebral activation in the primary somatosensory cortex (SI) (bilateral) and left mid-anterior cingulate cortex (ACC) increased with stimulation intensity. When pain was delivered during distraction, there was a significant reduction in pain ratings, accompanied by significant decreases (P < .05) in brain activity in the right ACC and right prefrontal cortex. There was no effect of distraction on SI activity (P < .05).

CONCLUSIONS

Our results suggest that the SI is involved in processing sensory-discriminative aspects of visceral sensation. In contrast, activity in the mid-ACC suggests that this region is multifunctional because it appears to be involved in sensory and cognitive appraisal of visceral pain; the right prefrontal cortex seems to be involved in only cognitive responses to pain.

Noxious counterirritation in patients with advanced osteoarthritis of the knee reduces MCC but not SII pain generators: A combined use of MEG and EEG

Chronic pain is mainly a result of two processes: peripheral and central sensitization, which can result in neuroplastic changes. Previous psychophysical studies suggested a decrease of the so-called pain-inhibiting-pain effect (DNIC) in chronic pain patients. We aimed to study the DNIC effect on the neuronal level using magnetoencephalography and electroencephalography in 12 patients suffering from advanced unilateral knee osteoarthritis (OA). DNIC was induced in patients by provoking the typical OA pain by a slightly hyperextended joint position, while they received short electrical pain stimuli. Although the patients did not report a reduction of electrical pain perception, the cingulate gyrus showed a decrease of activation during provoked OA pain, while activity in the secondary somatosensory cortex did not change. Based on much stronger DNIC induction at comparable intensities of an acute counterirritant pain in healthy subjects this result suggests a deficit of DNIC in OA patients. We suggest that the strength of DNIC is subject to neuronal plasticity of descending inhibitory pain systems and diminishes during the development of a chronic pain condition.

Noxious Lingual Stimulation Influences the Excitability of the Face Primary Motor Cerebral Cortex (Face MI) in the Rat

The mechanisms whereby orofacial pain affects motor function are poorly understood. The aims were to determine whether 1) lingual algesic chemical stimulation affected face primary motor cerebral cortex (face MI) excitability defined by intracortical microstimulation (ICMS); and 2) any such effects were limited to the motor efferent MI zones driving muscles in the vicinity of the noxious stimulus. Ketamine-anesthetized Sprague–Dawley male rats were implanted with electromyographic (EMG) electrodes into anterior digastric, masseter, and genioglossus muscles. In 38 rats, three microelectrodes were located in left face MI at ICMS-defined sites for evoking digastric and/or genioglossus responses. ICMS thresholds for evoking EMG activity from each site were determined every 15 min for 1 h, then the right anterior tongue was infused (20 μl, 120 μl/h) with glutamate (1.0 M, n = 18) or isotonic saline ( n = 7). Subsequently, ICMS thresholds were determined every 15 min for 4 h. In intact control rats ( n = 13), ICMS thresholds were recorded over 5 h. Only left and right genioglossus ICMS thresholds were significantly increased (≤350%) in the glutamate infusion group compared with intact and isotonic saline groups ( P < 0.05). These dramatic effects of glutamate on ICMS-evoked genioglossus activity contrast with its weak effects only on right genioglossus activity evoked from the internal capsule or hypoglossal nucleus. This is the first documentation that intraoral noxious stimulation results in prolonged neuroplastic changes manifested as a decrease in face MI excitability. These changes appear to occur predominantly in those parts of face MI that provide motor output to the orofacial region receiving the noxious stimulation.

Neurophysiology of the cortical pain network: revisiting the role of S1 in subjective pain perception via standardized low-resolution brain electromagnetic tomography (sLORETA)

Multiple studies have supported the usefulness of standardized low-resolution brain electromagnetic tomography (sLORETA) in localizing generators of scalp-recorded potentials. The current study implemented sLORETA on pain event-related potentials, primarily aiming at validating this technique for pain research by identifying well-known pain-related regions. Subsequently, we pointed at investigating the still-debated and ambiguous topic of pain intensity coding at these regions, focusing on their relative impact on subjective pain perception. sLORETA revealed significant activations of the bilateral primary somatosensory (SI) and anterior cingulate cortices and of the contralateral operculoinsular and dorsolateral prefrontal (DLPFC) cortices (P < .05 for each). Activity of these regions, excluding DLPFC, correlated with subjective numerical pain scores (P < .05 for each). However, a multivariate regression analysis (R = .80; P = .024) distinguished the contralateral SI as the only region whose activation magnitude significantly predicted the subjective perception of noxious stimuli (P = .020), further substantiated by a reduced regression model (R = .75, P = .008). Based on (1) correspondence of the pain-activated regions identified by sLORETA with the acknowledged imaging-based pain-network and (2) the contralateral SI proving to be the most contributing region in pain intensity coding, we found sLORETA to be an appropriate tool for relevant pain research and further substantiated the role of SI in pain perception.

PERSPECTIVE

Because the literature of pain intensity coding offers inconsistent findings, the current article used a novel tool for revisiting this controversial issue. Results suggest that it is the activation magnitude of SI, which solely establishes the significant correlation with subjective pain ratings, in accordance with the classical clinical thinking, relating SI lesions to diminished perception of pain. Although this study cannot support a causal relation between SI activation magnitude and pain perception, such relation might be insinuated.

Determinants of laser-evoked EEG responses: pain perception or stimulus saliency?

Although laser-evoked electroencephalographic (EEG) responses are increasingly used to investigate nociceptive pathways, their functional significance remains unclear. The reproducible observation of a robust correlation between the intensity of pain perception and the magnitude of the laser-evoked N1, N2, and P2 responses has led some investigators to consider these responses a direct correlate of the neural activity responsible for pain intensity coding in the human cortex. Here, we provide compelling evidence to the contrary. By delivering trains of three identical laser pulses at four different energies, we explored the modulation exerted by the temporal expectancy of the stimulus on the relationship between intensity of pain perception and magnitude of the following laser-evoked brain responses: the phase-locked N1, N2, and P2 waves, and the non-phase-locked laser-induced synchronization (ERS) and desynchronization (ERD). We showed that increasing the temporal expectancy of the stimulus through stimulus repetition at a constant interstimulus interval 1) significantly reduces the magnitudes of the laser-evoked N1, N2, P2, and ERS; and 2) disrupts the relationship between the intensity of pain perception and the magnitude of these responses. Taken together, our results indicate that laser-evoked EEG responses are not determined by the perception of pain per se, but are mainly determined by the saliency of the eliciting nociceptive stimulus (i.e., its ability to capture attention). Therefore laser-evoked EEG responses represent an indirect readout of the function of the nociceptive system.

Parallel input makes the brain run faster

In serial sensory processing, information flows from the thalamus via primary sensory cortices to higher-order association areas. However, association cortices also receive, albeit weak, direct thalamocortical sensory inputs of unknown function. For example, while information proceeds from primary (SI) to secondary (SII) somatosensory cortex in a serial fashion, both areas are known to receive direct thalamocortical sensory input. The present study examines the potential roles of such parallel input arrangements. The subjects were presented with median nerve somatosensory stimuli with the instruction to respond with the contralateral hand. The locations and time courses of the activated brain areas were first identified with magnetoencephalography (MEG). In a subsequent session, these brain areas were modulated with single-pulse transcranial magnetic stimulation (TMS) at 15-210 ms after the somatosensory stimulus while electroencephalography (EEG) was recorded. TMS pulses at 15-40 ms post-stimulus significantly speeded up reaction times and somatosensory-evoked responses, with largest facilitatory effects when the TMS pulse was given to contralateral SII at about 20 ms. To explain the results, we propose that the early somatosensory-evoked physiological SII activation exerts an SII-->SI influence that facilitates the reciprocal SI-->SII pathway - with TMS to SII we apparently amplified this mechanism. The results suggest that the human brain may utilize parallel inputs to facilitate long-distance cortico-cortical connections, resulting in accelerated processing and speeded reaction times. This arrangement could also allow very early top-down modulation of the bottom-up stream of sensory information.

Attenuation of N2 amplitude of laser-evoked potentials by theta burst stimulation of primary somatosensory cortex

Theta burst stimulation (TBS) is a special repetitive transcranial magnetic stimulation (rTMS) paradigm, where bursts of low-intensity stimuli are applied in the theta frequency. The aim of this study was to investigate the effect of neuronavigated TBS over primary somatosensory cortex (SI) on laser-evoked potentials (LEPs) and acute pain perception induced with Tm : YAG laser stimulation. The amplitude changes of the N1, N2, and P2 components of LEPs and related subjective pain rating scores of 12 healthy subjects were analyzed prior to and following continuous TBS (cTBS), intermittent TBS (iTBS), intermediate TBS (imTBS), and sham stimulation. Our results demonstrate that all active TBS paradigms significantly diminished the amplitude of the N2 component, when the hand contralateral to the site of TBS was laser-stimulated. Sham stimulation condition had no significant effect. The subjective pain perception also decreased during the experimental sessions, but did not differ significantly from the sham stimulation condition. The main finding of our study is that TBS over SI diminished the amplitude of the N2 component evoked from the contralateral side without any significant analgesic effects. Furthermore, imTBS produced responses similar to those observed by other forms of TBS induced excitability changes in the SI.

Gamma oscillations in human primary somatosensory cortex reflect pain perception

Successful behavior requires selection and preferred processing of relevant sensory information. The cortical representation of relevant sensory information has been related to neuronal oscillations in the gamma frequency band. Pain is of invariably high behavioral relevance and, thus, nociceptive stimuli receive preferred processing. Here, by using magnetoencephalography, we show that selective nociceptive stimuli induce gamma oscillations between 60 and 95 Hz in primary somatosensory cortex. Amplitudes of pain-induced gamma oscillations vary with objective stimulus intensity and subjective pain intensity. However, around pain threshold, perceived stimuli yielded stronger gamma oscillations than unperceived stimuli of equal stimulus intensity. These results show that pain induces gamma oscillations in primary somatosensory cortex that are particularly related to the subjective perception of pain. Our findings support the hypothesis that gamma oscillations are related to the internal representation of behaviorally relevant stimuli that should receive preferred processing.

Pain is a highly subjective sensation of inherent behavioral importance and is therefore expected to receive enhanced processing in relevant brain regions. We show that painful stimuli induce high-frequency oscillations in the electrical activity of the human primary somatosensory cortex. Amplitudes of these pain-induced gamma oscillations were more closely related to the subjective perception of pain than to the objective stimulus attributes. They correlated with participants' ratings of pain and were stronger for laser stimuli that caused pain, compared with the same stimuli when no pain was perceived. These findings indicate that gamma oscillations may represent an important mechanism for processing behaviorally relevant sensory information.

Magnetoencephalography reveals that gamma oscillations in the somatosensory cortex correlate with the subjective rating of pain and are stronger for laser stimuli that cause pain, compared with the same stimuli when no pain is perceived.

Cortical brain responses during passive nonpainful median nerve stimulation at low frequencies (0.5-4 Hz): an fMRI study

Previous findings have shown that the human somatosensory cortical systems that are activated by passive nonpainful electrical stimulation include the contralateral primary somatosensory area (SI), bilateral secondary somatosensory area (SII), and bilateral insula. The present study tested the hypothesis that these areas have different sensitivities to stimulation frequency in the condition of passive stimulation. Functional MRI (fMRI) was recorded in 24 normal volunteers during nonpainful electrical median nerve stimulations at 0.5, 1, 2, and 4 Hz repetition rates in separate recording blocks in pseudorandom order. Results of the blood oxygen level-dependent (BOLD) effect showed that the contralateral SI, the bilateral SII, and the bilateral insula were active during these stimulations. As a major finding, only the contralateral SI increased its activation with the increase of the stimulus frequency at the mentioned range. The fact that nonpainful median-nerve electrical stimuli at 4 Hz induces a larger BOLD response is of interest both for basic research and clinical applications in subjects unable to perform cognitive tasks in the fMRI scanner.

Phosphorylation of c-Jun N-terminal kinase isoforms and their different roles in spinal cord dorsal horn and primary somatosensory cortex

The present study was undertaken to investigate whether isoforms of c-Jun N-terminal kinase (JNK 46 kDa and 54 kDa), one component of the mitogen-activated protein kinase (MAPK) family, might show region-related differential activation patterns in both naïve and pain-experiencing rats. In naïve rats, no significant difference was observed in total expression level of the two JNK isoforms between spinal cord and primary somatosensory cortex (S1 area). However, phosphorylated JNK 46 kDa was normally expressed in the S1 area, but not in the spinal cord, while neither of the two structures contained phosphorylated JNK 54 kDa. Subcutaneous bee venom (BV)-induced persistent pain stimulation resulted in a significant increase in the phosphorylation of both JNK isoforms in each area for a long period (lasting at least 48 h). Nevertheless, JNK 46 kDa exhibited a much higher activation than JNK 54 kDa in the spinal cord, whereas the same noxious stimulation elicited evident activation of JNK 54 kDa in the S1 area, leaving JNK 46 kDa less affected. Intraplantar injection of sterile saline solution, causing acute and transient pain, produced almost the same changes in activation profile of the two JNK isoforms as found in the BV-treated rats. These results implicate that individual members of the JNK family may be associated with specific regions of nociceptive processing. Also, the two JNK isoforms are supposed to function differently according to their locations within the rat central nervous system.

Brain imaging of analgesic and antihyperalgesic effects of cyclooxygenase inhibition in an experimental human pain model: a functional MRI study

One of the most distressing symptoms of many neuropathic pain syndromes is the enhanced pain sensation to tactile or thermal stimulation (hyperalgesia). In the present study we used functional magnetic resonance imaging (fMRI) and explored brain activation patterns during acute impact pain and mechanical hyperalgesia in the human ultraviolet (UV)-B model. To investigate pharmacological modulation, we examined potential differential fMRI correlates of analgesic and antihyperalgesic effects of two intravenous cyclooxygenase inhibitors, i.e. parecoxib and acetylsalicylic acid (ASA). Fourteen healthy volunteers participated in this double-blinded, randomized and placebo-controlled crossover study. Tactile stimuli and mechanical impact hyperalgesia were tested at the site of a UV-B irradiation and acute mechanical pain was tested at a site distant from the irradiated skin. These measurements were conducted before and 30 min after a 5-min intravenous infusion of either saline (placebo), parecoxib 40 mg or ASA 1000 mg. Acute mechanical pain and mechanical hyperalgesia led to widespread activations of brain areas known to comprise the human pain matrix. Analgesic effects were found in primary (S1) and secondary (S2) somatosensory cortices, parietal association cortex (PA), insula, anterior parts of the cingulate cortex and prefrontal cortices. These brain areas were also modulated under antihyperalgesic conditions. However, we observed a greater drug-induced modulation of mainly PA and inferior frontal cortex during mechanical hyperalgesia; during acute mechanical pain there was a greater modulation of mainly bilateral S2. Therefore, the results of the present study suggest that there is a difference in the brain areas modulated by analgesia and antihyperalgesia.

Reproducibility of human brain activity evoked by esophageal stimulation using functional magnetic resonance imaging

Functional MRI is a popular tool for investigating central processing of visceral pain in healthy and clinical populations. Despite this, the reproducibility of the neural correlates of visceral sensation by use of functional MRI remains unclear. The aim of the present study was to address this issue. Seven healthy right-handed volunteers participated in the study. Blood oxygen level-dependent contrast images were acquired at 1.5 T while subjects received nonpainful and painful phasic balloon distensions ("on-off" block design, 10 stimuli per "on" period, 0.3 Hz) to the distal esophagus. This procedure was repeated on two further occasions to investigate reproducibility. Painful stimulation resulted in highly reproducible activation over three scanning sessions in the anterior insula, primary somatosensory cortex, and anterior cingulate cortex. A significant decrease in strength of activation occurred from session 1 to session 3 in the anterior cingulate cortex, primary somatosensory cortex, and supplementary motor cortex, which may be explained by an analogous decrease in pain ratings. Nonpainful stimulation activated similar brain regions to painful stimulation, but with greater variability in signal strength and regions of activation between scans. Painful stimulation of the esophagus produces robust activation in many brain regions. A decrease in subjective perception of pain and brain activity from the first to the final scan suggests that serial brain imaging studies may be affected by habituation. These findings indicate that for brain imaging studies that require serial scanning, development of experimental paradigms that control for the effect of habituation is necessary.

Early cortical activities evoked by noxious stimulation in humans

Lasers can selectively activate the nociceptors of A-delta fibers. Since nociceptors in the skin are activated via temperature conduction by the laser beam, a latency jittering of cortical responses among trials would affect results obtained with a conventional averaging (C-AVE) technique. We therefore used a new method, latency-adjusted averaging (L-AVE), to investigate cortical responses to noxious laser stimulation in normal subjects. L-AVE was done by averaging trials after adjusting the latency so that the peak latency of an activity in the temporal region of all trials matched on the time axis. Both in C-AVE and in L-AVE, clear activations were found in the contralateral primary somatosensory cortex (SI) and bilateral parasylvian regions, whose activities peaked 163-181 ms after the stimulation. In addition to these three main activities, weak activities peaking at around 109-119 ms could be identified in only L-AVE in similar cortical regions. Since the direction of the source differed between early and main activities, we considered that the early weak activities were cancelled out by the later main activities with an opposite orientation. The results suggested that early cortical processing of noxious information occurs earlier than previous neurophysiological studies have estimated and that the temporal sequence of activations should be reconsidered.

Duration of the cue-to-pain delay increases pain intensity: a combined EEG and MEG study

Expectation of pain is an important adaptive process enabling individuals to avoid bodily harm. It reflects the linking of past experience and environmental cues with imminent threat. In the present study, we examined changes in perceived pain contingent upon variation of the interval between an auditory cue and a subsequent painful laser stimulus. The duration of the cue-to-stimulus delay was systematically varied between 2, 4 and 6 s. Pain intensity and evoked brain responses measured by EEG and MEG recordings were analysed. Pain ratings from 15 subjects increased with longer cue-to-pain delays, accompanied by an increase in activity of the midcingulate cortex (MCC), as modelled from evoked EEG potential maps. On the other hand, MEG-based source activity in secondary somatosensory (SII) cortex remained unaffected by manipulation of the cue-to-stimulus interval. We conclude that activity in limbic structures such as MCC play a key role in the temporal dynamics of recruitment of expectation towards pain. Although this reaction is adaptive if the individual is able to avoid the stimulus, it is maladaptive if such opportunity is not present.

Empathy for Pain and Touch in the Human Somatosensory Cortex

Although feeling pain and touch has long been considered inherently private, recent neuroimaging and neurophysiological studies hint at the social implications of this experience. Here we used somatosensory-evoked potentials (SEPs) to investigate whether mere observation of painful and tactile stimuli delivered to a model would modulate neural activity in the somatic system of an onlooker. Viewing video clips showing pain and tactile stimuli delivered to others, respectively, increased and decreased the amplitude of the P45 SEP component that reflects the activity of the primary somatosensory cortex (S1). These modulations correlated with the intensity but not with the unpleasantness of the pain and touch ascribed to the model or the aversion induced in the onlooker by the video clips. Thus, modulation of S1 activity contingent upon observation of others' pain and touch may reflect the mapping of sensory qualities of observed painful and tactile stimuli. Results indicate that the S1 is not only involved in the actual perception of pain and touch but also plays an important role in extracting somatic features from social interactions.

Neural activity related to self- versus externally generated painful stimuli reveals distinct differences in the lateral pain system in a parametric fMRI study

Self-generated sensory stimulation can be distinguished from externally generated stimulation that is otherwise identical. To determine how the brain differentiates external from self-generated noxious stimulation and which structures of the lateral pain system use neural signals to predict the sensory consequences of self-generated painful stimulation, we used functional magnetic resonance imaging to examine healthy human subjects who received thermal-contact stimuli with noxious and non-noxious temperatures on the resting right hand in random order. These stimuli were internally (self-generated) or externally generated. Two additional conditions served as control conditions: to account for stimulus onset uncertainty, acoustic stimuli preceding the same thermal stimuli were used with variable or fixed delays but without any stimulus-eliciting movements. Whereas graded pain-related activity in the insula and secondary somatosensory cortex (SII) was independent of how the stimulus was generated, it was attenuated in the primary somatosensory cortex (SI) during self-generated stimulation. These data agree with recent concepts of the parallel processing of nociceptive signals to the primary and secondary somatosensory cortices. They also suggest that brain areas that encode pain intensity do not distinguish between internally or externally applied noxious stimuli, i.e., this adaptive biological mechanism prevents harm to the individual. The attenuated activation of SI during self-generated painful stimulation might be a result of the predictability of the sensory consequences of the pain-related action.

Imaging pain modulation by subanesthetic S-(+)-ketamine

Little is known about the effects of low-dose S-(+)-ketamine on the cerebral processing of pain. We investigated the effects of subanesthetic IV S-(+)-ketamine doses on the perception of experimental painful heat stimuli. Healthy volunteers were evaluated with functional magnetic resonance imaging (fMRI) while receiving the painful stimuli in conjunction with placebo and increasing doses (0.05, 0.1, 0.15 mg x kg(-1) x h(-1)) of ketamine infusion. Vital variables were monitored and all subjects rated pain intensity and unpleasantness on a numerical rating scale. Alterations in consciousness were measured using a psycho-behavioral questionnaire. Pain unpleasantness declined as ketamine dosage was increased (55.1% decrease, placebo versus 0.15 mg x kg(-1) x h(-1) ketamine). Pain intensity ratings also decreased with increasing ketamine dosage but to a lesser extent (23.1% decrease). During placebo administration, a typical pain activation network (thalamus, insula, cingulate, and prefrontal cortex) was found, whereas decreased pain perception with ketamine was associated with a dose-dependent reduction of pain-induced cerebral activations. Analysis of the dose-dependent ketamine effects on pain processing showed a decreasing activation of the secondary somatosensory cortex (S2), insula and anterior cingulate cortex. This part of the anterior cingulate cortex (midcingulate cortex) has been linked with the affective pain component that underlines the potency of ketamine in modulating affective pain processing.

Secondary somatosensory cortex is important for the sensory-discriminative dimension of pain: a functional MRI study

A complex cortical network is believed to encode the multidimensionality of the human pain experience. In the present study, we used functional magnetic resonance imaging (fMRI) to examine whether the brain's processing of noxious stimuli differs with different psychophysical properties. Painful mechanical impact and heat stimulations of equal stimulus intensity were applied to the forearm of 14 subjects in a randomized order. Concomitantly, subjects had to evaluate the corresponding sensory-discriminative and affective-motivational pain dimensions. fMRI revealed an increased activation of bilateral secondary somatosensory cortices (S2) during mechanical impact pain compared with heat pain. Activations in S2 were significantly correlated with scores for the sensory-discriminative component during mechanical impact pain. By contrast, corresponding scores for the affective-motivational pain dimension did not differ between both conditions. In summary, we conclude that S2 plays an important role in the sensory-discriminative dimension of pain.

Multiple pathways for noxious information in the human spinal cord

To investigate the pathways of noxious information in the spinal cord in humans, we recorded cortical potentials following the stimulation of A-delta fibers using a YAG laser applied to two cutaneous points on the back at the C7 and Th10 level, 4cm to the right of the vertebral spinous process. A multiple source analysis showed that four sources were activated; the primary somatosensory cortex (SI), bilateral parasylvian region (Parasylvian), and cingulate cortex. The activity of the cingulate cortex had two components (N2/P2). The mean peak latencies of the activities obtained by C7 and Th10 stimulation were 166.9 and 186.0 ms (SI), 144.3 and 176.8 ms (contralateral Parasylvian), 152.7 and 185.5 ms (ipsilateral Parasylvian), 186.2 and 215.8 ms (N2), and 303.0 and 332.3 ms (P2). Estimated spinal conduction velocities (CVs) of the respective activities were 16.8, 9.3, 8.7, 10.1 and 10.7 m/s. CV of SI was significantly faster than the others (P<0.05). Therefore, our results suggested that noxious signals were conveyed through at least two distinct pathways of the spinal cord probably reaching distinct groups of thalamic nuclei. Further studies are required to clarify the functional significance of these two pathways.

Neuromagnetic SII responses do not fully reflect pain scale

To elucidate the role of somatosensory cortices in coding pain magnitude, we recorded the neuromagnetic responses of ten subjects to mild, moderate, and severe pain stimulation by delivering thulium-laser pulses on the dorsum of the left hand. The stimulus intensities for producing different pain levels were determined individually, and the mean values across subjects were 255, 365, and 490 mJ for mild, moderate, and severe pain, respectively. We obtained 40 responses for each intensity condition, and analyzed the averaged cortical signals by multi-dipole modeling. All subjects showed consistent activation over the bilateral secondary somatosensory (SII) cortices for each intensity level, peaking around 150-230 ms, with 15-ms earlier on the contralateral hemisphere. The SII dipole strength was significantly larger for the moderate than for the mild pain stimulation, but lacked further increase as the pain magnitude elevated to the severe level. In contrast, the primary somatosensory cortical response was detected in only half of our subjects, and thus it seemed difficult to evaluate its role in pain intensity coding. Our results suggest that activation strength in human SII cortices reflects the magnitude of peripheral noxious inputs only up to the moderate level, and some other cerebral correlates may get involved in sensing a further increment of pain magnitude.

Cortical oscillatory changes occurring during somatosensory and thermal stimulation

Brief somatosensory stimuli are followed by amplitude decreases (event-related desynchronization, ERD) of the 10 and 20 Hz oscillations over the bilateral primary sensorimotor cortices, and by post-stimulus synchronization (event-related synchronization, ERS) of the 20 Hz oscillations in the contralateral primary sensorimotor cortex and in the supplementary motor area (SMA). The 10 and 20 Hz ERD differentiate weak and strong somatosensory stimuli but not fine intensity gradations, and the ipsilateral ERD is especially sensitive to habituation. Stimulus anticipation, motor imagery, action viewing as well as voluntary movements modulate the stimulus-related changes of cortical oscillations. Noxious laser stimuli, selectively activating Adelta and/or C fibers, and innocuous warm and cold stimuli are associated with 10 and 20 Hz ERD but not with the post-stimulus 20 Hz ERS suggesting that the post-stimulus ERS is only related to neuronal transmission in the lemniscal system. It is proposed that phase-unlocked cortical oscillations modulate the preparedness of a particular sensory channel for upcoming somatosensory processing.

Current source density analysis of laser heat-evoked intra-cortical field potentials in the primary somatosensory cortex of rats

The role of the primary somatosensory cortex in thermal pain perception has been established. However, the cortical circuitry that mediates the thermo-nociceptive information processing has not been elucidated. The aim of present study was to investigate the intracortical synaptic currents in primary somatosensory cortex evoked by short laser pulses and to determine their transmission pathway. Noxious CO2 laser pulse stimuli or innocuous electrical and mechanical stimuli were delivered to the hind paw of halothane-anesthetized rats. Multi-channel field potentials were recorded simultaneously in primary somatosensory cortex and laminar-specific transmembrane currents were analyzed using a current source density method. A distinct spatial-temporal pattern of intra-cortical sink source currents was evoked by laser pulse stimuli. The amplitude of the early component was graded by laser energy output and influenced by contralateral signals, whereas the late components were not intensity-dependent and exhibited bilateral excitation. Intra-cortical current flows revealed that synaptic activation occurred initially at layers IV and VI separately and then was relayed transynaptically to the more superficial and the deeper layers. Latency, amplitude and intracortical distributions of the activated intra-cortical currents evoked by noxious stimuli differed significantly from those evoked by innocuous stimuli. Conduction velocity data together with the results of tetrodotoxin, capsaicin and morphine treatments indicated that the early and late components were mediated separately by A-delta and C fibers. Our results suggest that large and small diameter thermal nociceptive afferents generated laminar-specific intracortical synaptic currents in primary somatosensory cortex and that these excitatory synaptic currents were conveyed separately by lateral and medial thalamic nuclei.

Differential coding of hyperalgesia in the human brain: A functional MRI study

Neuropathic pain can be both ongoing or stimulus-induced. Stimulus-induced pain, also known as hyperalgesia, can be differentiated into primary and secondary hyperalgesia. The former results from sensitization of peripheral nociceptive structures, the latter involves sensitization processes within the central nervous system (CNS). Hypersensitivity towards heat stimuli, i.e. thermal hyperalgesia, is a key feature of primary hyperalgesia, whereas secondary hyperalgesia is characterized by hypersensitivity towards mechanical (e.g. pin-prick) stimulation. Using functional magnetic resonance imaging (fMRI), we investigated if brain activation patterns associated with primary and secondary hyperalgesia might differ. Thermal and pin-prick hyperalgesia were induced on the left forearm in 12 healthy subjects by topical capsaicin (2.5%, 30 min) application. Equal pain intensities of both hyperalgesia types were applied during fMRI experiments, based on previous quantitative sensory testing. Simultaneously, subjects had to rate the unpleasantness of stimulus-related pain. Pin-prick hyperalgesia (i.e. subtraction of brain activations during pin-prick stimulation before and after capsaicin exposure) led to activations of primary and secondary somatosensory cortices (S1 and S2), associative-somatosensory cortices, insula and superior and inferior frontal cortices (SFC, IFC). Brain areas activated during thermal hyperalgesia (i.e. subtraction of brain activations during thermal stimulation before and after capsaicin exposure) were S1 and S2, insula, associative-somatosensory cortices, cingulate cortex (GC), SFC, middle frontal cortex (MFC) and IFC. When compared to pin-prick hyperalgesia, thermal hyperalgesia led to an increased activation of bilateral anterior insular cortices, MFC, GC (Brodmann area 24' and 32') and contralateral SFC and IFC, despite equal pain intensities. Interestingly, stronger activations of GC, contralateral MFC and anterior insula significantly correlated to higher ratings of the stimulus-related unpleasantness. We conclude that thermal and mechanical hyperalgesia produce substantially different brain activation patterns. This is linked to different psychophysical properties.

Attenuation of Sensory and Affective Responses to Heat Pain: Evidence for Contralateral Mechanisms

Attenuation of responses to repeated sensory events has been thoroughly studied in many modalities; however, attenuation of pain perception has not yet benefitted from such extensive investigation. Described here are two psychophysical studies that examined the effects of repeated exposure to thermal stimuli, assessing potential attenuation of the perception of pain and its possible spatial specificity. Twenty-two subjects were presented thermal stimuli to the volar surface of the right and left forearms. Twelve subjects in study 1 received the same stimuli and conditions on each of five daily experimental sessions, whereas 10 subjects in study 2 received thermal stimuli, which were restricted to one side for four daily sessions and then applied to the other side on the fifth session. Ratings of warmth intensity, pain intensity, and pain unpleasantness were recorded while the subjects performed a thermal sensory discrimination task. Results of study 1 demonstrate that repeated stimulation with noxious heat can lead to long-term attenuation of pain perception; results of study 2 extend these findings of attenuation to both pain intensity and unpleasantness and show that this effect is highly specific to the exposed body side for both aspects of the pain experience. We suggest that the functional plasticity underlying this attenuation effect lies in brain areas with a strong contralateral pattern of pain-related activation.

Pain suppresses spontaneous brain rhythms

The neuronal activity of the resting human brain is dominated by spontaneous oscillatory activity of primary visual, somatosensory and motor areas. These spontaneous brain rhythms are related to the functional state of a system. A higher amplitude of oscillatory activity is thought to reflect an idling state, whereas a lower amplitude is associated with activation and higher excitability of the specific system. Here, we used magnetoencephalography to investigate the effects of pain on spontaneous brain rhythms. Our results show that a focally applied brief painful stimulus globally suppresses spontaneous oscillations in somatosensory, motor and visual areas. This global suppression contrasts with the regionally specific suppressions of other modalities and shows that pain induces a widespread change in cortical function and excitability. This global change in excitability may reflect the alerting function of pain which opens the gates for processing of and reacting to stimuli of existential relevance.

Differential generators for N20m and P35m responses to median nerve stimulation

To study the spatial and behavioral dynamics of cortical sources for N20m and P35m at varying stimulus intensities, we measured neuromagnetic cortical responses to left electric median nerve stimulation at the wrist in 17 male healthy adults. The stimulus intensity levels were individually determined according to sensory threshold (ST) for perceiving electric pulses. Using equivalent current dipole (ECD) modeling, we analyzed the peak latencies, amplitudes, and locations of ECDs from 14 subjects for N20m and P35m elicited at 2 ST, 3 ST, and 4 ST. Compared with N20m, P35m was localized 3.3 +/- 0.6 mm more superiorly at 2-4 ST, and 2.9 +/- 1.2 mm more medially at 3-4 ST. Superimposed over subjects' own MR images, N20m ECDs were localized in the area of 3b contralateral to stimulus side in all 17 subjects at 3 ST, whereas P35m ECDs were localized either in the postcentral (in 14 subjects) or in the precentral areas (in 3 subjects). We found no clear correlation between N20m and P35m in terms of peak latencies as well as the corresponding growth of activation strengths along with stepwise increase in stimulus intensity. Our results imply that the two early SEF components, N20m and P35m, have differential cortical generators, with distinctive neurophysiological behaviors in response to varying stimulus intensity levels.