Pain-related magnetic fields following painful CO2 laser stimulation in man

Performance Analysis of Optically Pumped 4He Magnetometers vs. Conventional SQUIDs: From Adult to Infant Head Models

Optically pumped magnetometers (OPMs) are new, room-temperature alternatives to superconducting quantum interference devices (SQUIDs) for measuring the brain's magnetic fields. The most used OPM in MagnetoEncephaloGraphy (MEG) are based on alkali atoms operating in the spin-exchange relaxation-free (SERF) regime. These sensors do not require cooling but have to be heated. Another kind of OPM, based on the parametric resonance of ⁴He atoms are operated at room temperature, suppressing the heat dissipation issue. They also have an advantageous bandwidth and dynamic range more suitable for MEG recordings. We quantitatively assessed the improvement (relative to a SQUID magnetometers array) in recording the magnetic field with a wearable ⁴He OPM-MEG system through data simulations. The OPM array and magnetoencephalography forward models were based on anatomical MRI data from an adult, a nine-year-old child, and 10 infants aged between one month and two years. Our simulations showed that a ⁴He OPMs array offers markedly better spatial specificity than a SQUID magnetometers array in various key performance areas (e.g., signal power, information content, and spatial resolution). Our results are also discussed regarding previous simulation results obtained for alkali OPM.

The Electromagnetic Will

The conscious electromagnetic information (cemi) field theory proposes that the seat of consciousness is the brain’s electromagnetic (EM) field that integrates information from trillions of firing neurons. What we call free will is its output. The cemi theory also proposes that the brain has two streams. Most actions are initiated by the first non-conscious stream that is composed of neurons that are insulated from EM field influences. These non-conscious involuntary actions are thereby invisible to our EM field-located thoughts. The theory also proposes that voluntary actions are driven by neurons that receive EM field inputs and are thereby visible to our EM field-located thoughts. I review the extensive evidence for EM field/ephaptic coupling between neurons and the increasing evidence that EM fields in the brain are a cause of behaviour. I conclude by arguing that though this EM field-driven will is not free, in the sense of being acausal, it nevertheless corresponds to the very real experience of our conscious mind being in control of our voluntary actions. Will is not an illusion. It is our experience of control by our EM field-located mind. It is an immaterial, yet physical, will.

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.

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.

Was it a pain or a sound? Across-species variability in sensory sensitivity

Natural selection has shaped the physiological properties of sensory systems across species, yielding large variations in their sensitivity. Here, we used laser stimulation of skin nociceptors, a widely used technique to investigate pain in rats and humans, to provide a vivid example of how ignoring these variations can lead to serious misconceptions in sensory neuroscience. In 6 experiments, we characterized and compared the physiological properties of the electrocortical responses elicited by laser stimulation in rats and humans. We recorded the electroencephalogram from the surface of the brain in freely moving rats and from the scalp in healthy humans. Laser stimuli elicited 2 temporally distinct responses, traditionally interpreted as reflecting the concomitant activation of different populations of nociceptors with different conduction velocities: small-myelinated Aδ-fibres and unmyelinated C-fibres. Our results show that this interpretation is valid in humans, but not in rats. Indeed, the early response recorded in rats does not reflect the activation of the somatosensory system, but of the auditory system by laser-generated ultrasounds. These results have wide implications: retrospectively, as they prompt for a reconsideration of a large number of previous interpretations of electrocortical rat recordings in basic, preclinical, and pharmacological research, and prospectively, as they will allow recording truly pain-related cortical responses in rats.

Normal laser-evoked cortical responses in patients with chronic hemibody pain

BACKGROUND

Patients with widespread unilateral chronic pain associated with recurrent herpes simplex virus (HSV) infections show functional and/or structural changes in the insula, anterior cingulate cortex, frontal and prefrontal cortices, as well as the thalamus, suggesting central dysfunction of the pain system in these patients. Central pain has been associated with attenuated laser-evoked cortical responses. We aimed to clarify whether the observed deficient activation of these areas to acute nociceptive stimuli is due to a lesion at a lower level of pain processing pathways.

METHODS

We explored the functional integrity of the ascending nociceptive pathways by recording the cortical-evoked responses to noxious laser stimulation using magnetoencephalography and electroencephalography in eight patients (age 41-51 years, mean 46) with recurrent HSV infections and a history of chronic, spontaneous, widespread unilateral pain, and in nine age-matched healthy control subjects.

RESULTS

The cortical-evoked fields of the HSV patients originating from the secondary somatosensory and posterior parietal cortices, as well as the evoked potentials recorded from the midline, did not differ from those of the control subjects, indicating functionally intact ascending nociceptive pathways.

CONCLUSIONS

The present results show that our patients with chronic hemibody pain do not show signs of spinothalamic tract lesion. This indicates normal processing of sensory aspects of painful stimuli, while higher pain processing areas show altered activation. We conclude that normal laser-evoked magnetic fields (LEF) or laser-evoked potentials (LEP) may not exclude central pain condition.

Multi-channel atomic magnetometer for magnetoencephalography: a configuration study

Atomic magnetometers are emerging as an alternative to SQUID magnetometers for detection of biological magnetic fields. They have been used to measure both the magnetocardiography (MCG) and magnetoencephalography (MEG) signals. One of the virtues of the atomic magnetometers is their ability to operate as a multi-channel detector while using many common elements. Here we study two configurations of such a multi-channel atomic magnetometer optimized for MEG detection. We describe measurements of auditory evoked fields (AEF) from a human brain as well as localization of dipolar phantoms and auditory evoked fields. A clear N100m peak in AEF was observed with a signal-to-noise ratio of higher than 10 after averaging of 250 stimuli. Currently the intrinsic magnetic noise level is 4fTHz(-1/2) at 10Hz. We compare the performance of the two systems in regards to current source localization and discuss future development of atomic MEG systems.

Increased brain responses during subjectively-matched mechanical pain stimulation in fibromyalgia patients as evidenced by MEG

OBJECTIVE

The precise pathophysiology of fibromyalgia, a syndrome characterized by chronic widespread pain, remains to be clarified. When subjected to the same amount of stimulation, patients show enhanced brain responses as compared to controls, providing evidence of central pain augmentation in this syndrome. We aimed to characterize brain response differences when stimulation is adjusted to elicit similar subjective levels of pain in both groups.

METHODS

Magnetoencephalography (MEG) was used to investigate the brain responses to pressure stimulation applied both above and below the pain threshold in nine patients and nine control subjects. A device was developed to deliver pressure pulses in a quantifiable and precise manner. The amount of pressure was adjusted to produce similar subjective pain in both groups.

RESULTS

A between-group comparison of differences between responses evoked by stimulation above and below the pain threshold was performed using cluster-based permutation testing. Increases in signal amplitude in somatosensory, temporal and parietal areas at short latencies, and in prefrontal areas at both short and long latencies, were found to be larger for patients than for control subjects.

CONCLUSION

Fibromyalgia patients show enhanced brain responses after reducing the amount of pressure to produce similar subjective levels of pain than to the control subjects.

SIGNIFICANCE

The present results suggest that central pain augmentation is present in fibromyalgia, not only when the objective level of stimulation is kept the same as for control subjects, but also when stimulation is adjusted to produce similar levels of pain in patients and controls.

Clonidine effects on pain evoked SII activity in humans

We investigated pain evoked activity in the human secondary sensory cortex (SII) following clonidine administration in six healthy volunteers using multi-channel magnetoencephalography (MEG). Pain was elicited by electrical shocks applied intracutaneously to the fingertip. Subjects rated pain intensity and perceptions of tiredness and passiveness by numerical ranking scales. Each subject underwent two investigations, one week apart from each other, with clonidine doses of 1.5 or 3.0microg/kg, administered intravenously in a random order and double-blinded. We applied a total number of seven blocks, each consisting of 60 painful stimuli, with one adaptation block, one pre-medication block, four post-medication blocks and one recovery block at the end of the session. MEG data were analysed by dipole reconstruction using CURRY(R) (Neuroscan, Hamburg) software package. Cortical activity in the contralateral SII cortex appeared with peak latencies of 118.5+/-10ms. This activity was significantly reduced by clonidine, in parallel with a reduction of pain intensity and enhancement of subjective tiredness and passiveness. There was, however, no significant correlation between MEG and subjective effects. Although both clonidine doses had similar effects, the higher dose induced longer changes. Results indicate that intravenous clonidine is able to relieve pain, but the exact mechanism of clonidine at the level of the SII cortex remains unclear. It is possible that clonidine interacts with the brainstem ascending system regulating vigilance and arousal which would explain the observed decrement of pain induced activity in SII. An additional more specific analgesic action at spinal level cannot be excluded.

The primary somatosensory cortex largely contributes to the early part of the cortical response elicited by nociceptive stimuli

Research on the cortical sources of nociceptive laser-evoked brain potentials (LEPs) began almost two decades ago (Tarkka and Treede, 1993). Whereas there is a large consensus on the sources of the late part of the LEP waveform (N2 and P2 waves), the relative contribution of the primary somatosensory cortex (S1) to the early part of the LEP waveform (N1 wave) is still debated. To address this issue we recorded LEPs elicited by the stimulation of four limbs in a large population (n=35). Early LEP generators were estimated both at single-subject and group level, using three different approaches: distributed source analysis, dipolar source modeling, and probabilistic independent component analysis (ICA). We show that the scalp distribution of the earliest LEP response to hand stimulation was maximal over the central-parietal electrodes contralateral to the stimulated side, while that of the earliest LEP response to foot stimulation was maximal over the central-parietal midline electrodes. Crucially, all three approaches indicated hand and foot S1 areas as generators of the earliest LEP response. Altogether, these findings indicate that the earliest part of the scalp response elicited by a selective nociceptive stimulus is largely explained by activity in the contralateral S1, with negligible contribution from the secondary somatosensory cortex (S2).

Features of somatosensory manifestations induced by intracranial electrical stimulations of the human insula

OBJECTIVE

To study the clinical manifestations induced by intracranial electrical stimulation of the insular cortex in epileptic patients submitted to invasive stereo-electroencephalography (SEEG) recordings.

METHODS

We retrospectively studied the clinical manifestations induced by intracerebral electrical stimulations of the insular cortex in 96 patients. In order to precisely localize the position of the electrodes a postoperative 3D CT scan or a 3D MRI was obtained and then the images were merged with the preoperative MRI in the same stereotactic referenced system.

RESULTS

A total of 341 electrical bipolar stimulations were performed. The most frequently induced symptom was a somatosensory manifestation (70%), mainly tingling and electric sensation involving the contralateral face and arm. Motor responses represented the 8% of the total amount, as well as auditory phenomena. Language dysfunction accounted for 2% of responses. Autonomic and gustatory phenomena represented respectively 1% of responses.

CONCLUSIONS

We found a great prevalence of somatosensory manifestations whereas other types of clinical modifications were extremely infrequent.

SIGNIFICANCE

Our data support a prominent somatosensory role of the human insular cortex and provide a precise characterization of the different types of sensory manifestations induced by intracranial electrical stimulation of the human insula.

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.

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.

Parallel spinal pathways generate the middle-latency N1 and the late P2 components of the laser evoked potentials

OBJECTIVE

To investigate the possible presence of multiple spino-thalamic pathways with different conduction velocities (CVs) in the human spinal cord.

METHODS

Laser evoked potentials (LEPs) were recorded in 10 healthy subjects after stimulation of the dorsal midline at four vertebral level: C5, T2, T6, and T10. This method allowed us to minimize the influence of the conduction in the peripheral fibers and to calculate the spinal CV in two different ways: (1) the reciprocal of the slope of the regression line was obtained from the latencies of the different LEP components, and (2) the distance between C5 and T10 was divided by the latency difference of the responses at the two sites. In particular, we considered the middle-latency N1 potential (latencies of around 135, 150, 157, and 171 ms after stimulation at C5, T2, T6, and T10 levels, respectively), which is generated in the second somatosensory (SII) area, and the late P2 response (latencies of around 336, 344, 346, and 362 ms after stimulation at C5, T2, T6, and T10 levels, respectively), which is generated in the anterior cingulate cortex (ACC).

RESULTS

The calculated CV of the spinal fibers generating the N1 potential (around 9 m/s) was significantly different (P<0.05) from the one of the pathway producing the P2 response (around 13 m/s).

CONCLUSIONS

Our results suggest that the N1 and the P2 LEP components are generated by two parallel spinal pathways.

SIGNIFICANCE

Both the N1 and P2 potentials should be recorded in the clinical routine since a dissociated abnormality of either response may be found in lesions of the nociceptive system not only in the brain, but also at spinal cord level.

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.

Mécanismes cérébraux impliqués dans l’interaction entre la douleur et les émotions

INTRODUCTION

Pain is an unpleasant and intrusive sensation, warning of actual or potential tissue damage. Over the last fifteen years, functional cerebral imaging research has demonstrated the involvement of many cerebral structures in the experience of pain.

BACKGROUND

Intimately linked to the notion of suffering, the affective dimension of pain relies on neurophysiological systems partly distinct anatomically from those involved more specifically in its sensory dimension. Some pathways convey nociceptive information to the somatosensory cortex and the insula, contributing to the sensory aspects of pain (e.g.: sensory intensity), and secondarily, to its affective dimension. Other pathways project directly to the anterior cingulate cortex, the insula, the amygdala and to the prefrontal cortices, which are structures involved in the affective dimension of pain (unpleasantness of pain and regulation of autonomic and behavioral responses). Interestingly, these latter regions are an integral part of the cerebral emotional networks.

PERSPECTIVES AND CONCLUSION

This close anatomical relationship between pain and emotions circuits could explain the powerful emotional impact of pain as well as the reciprocal modulatory effect of emotions on pain observed in clinical and experimental studies. More specifically, this modulatory effect might reflect interactions between emotional and nociceptive systems in the prefrontal and cingulate cortices, ventral striatum, amygdala and hippocampal regions. Taken together, these observations further attest to the emotional nature of pain experience.

Anticipatory electroencephalography alpha rhythm predicts subjective perception of pain intensity

This high-resolution electroencephalography (EEG) study tested the hypothesis that the suppression of rolandic alpha power before predictable painful stimulation affects the subject's subsequent evaluation of pain intensity, as a reflection of the influence of expectancy processes on painful stimulus processing. High-resolution EEG data were recorded (126 channels) from 10 healthy adult volunteers during the expectancy of a painful CO(2)-laser stimulation at the right wrist. Surface laplacian estimation enhanced the EEG spatial information content over 6 scalp regions of interest (left frontal, right frontal, left central, right central, left parietal, and right parietal areas). Spectral power was computed for 3 alpha sub-bands with reference to the individual alpha frequency peak (about 5-7 Hz for alpha 1, 7-9 Hz for alpha 2, and 9-11 Hz for alpha 3). The suppression of the alpha power before the painful stimulation [as reflected by the event-related desynchronization (ERD)] indexed the anticipatory cortical processes. Results showed maximum (negative) correlations between the alpha 2 and alpha 3 ERD amplitude at the left central area and the subjective evaluation of pain intensity (P < .001). The stronger the anticipatory alpha 2 and alpha 3 ERD, the higher the subjective evaluation of pain intensity. For alpha 3, that correlation was confirmed even when the effect of habituation across the recording session was taken into account. These results suggest that the anticipatory suppression of the alpha rhythms over the contralateral primary sensorimotor cortex predicts subsequent subjects' evaluation of pain intensity, in line with its crucial role for the discrimination of that intensity.

PERSPECTIVE

This electroencephalographic study showed that anticipatory activation/deactivation of sensorimotor cortex roughly predicts subjective evaluation of pain. This motivates further investigation on possible implications for the understanding of central chronic pain. Chronic pain patients might exaggerate the anticipatory activation of sensorimotor cortex to negligible pain stimuli.

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.

Electroacupuncture modulates cortical activities evoked by noxious somatosensory stimulations in human

A noninvasive high-resolution imaging technique of cerebral electric activities has been developed to directly link scalp potential measurement with the magnetic resonance images of the subjects, which is very helpful for the elucidation of the cortical processing following various stimulations. Here, we used a 64-channel Neuroscan ESI-128 system to explore the specific cortical activities elicited by electroacupuncture (EA) acupoint in normal volunteers and the modulatory effect of EA on cortical activities evoked by noxious somatosensory stimulation. A specific later-latency somatosensory-evoked potential (SEP, P150) located in bilateral anterior cingulated cortex was observed after EA acupoint but not non-acupoint. Two pain-specific SEP components (P170 and N280), located in bilateral suprasylvian operculum and anterior cingulated cortex respectively were observed following painful median nerve stimulation. Binding EA acupoint with painful median nerve stimulation, the amplitudes of P170 and N280 appeared to be attenuated significantly, 2D topography exhibited tremendous decrease of cortical activation between 120 ms and 296 ms in latency, and visual analogue scale (VAS) changes also showed a similar pattern to the change of amplitude. The bilateral anterior cingulated cortex recruited following acupoint stimuli might, to some extent, suggest that EA has the specific physiological effects. Decrease of pain-induced cortical activation by EA acupoint was considered to be mainly due to an interaction of the signals in anterior cingulated cortex ascending from the pain stimulation and EA.

Human brain mechanisms of pain perception and regulation in health and disease

CONTEXT

The perception of pain due to an acute injury or in clinical pain states undergoes substantial processing at supraspinal levels. Supraspinal, brain mechanisms are increasingly recognized as playing a major role in the representation and modulation of pain experience. These neural mechanisms may then contribute to interindividual variations and disabilities associated with chronic pain conditions.

OBJECTIVE

To systematically review the literature regarding how activity in diverse brain regions creates and modulates the experience of acute and chronic pain states, emphasizing the contribution of various imaging techniques to emerging concepts.

DATA SOURCES

MEDLINE and PRE-MEDLINE searches were performed to identify all English-language articles that examine human brain activity during pain, using hemodynamic (PET, fMRI), neuroelectrical (EEG, MEG) and neurochemical methods (MRS, receptor binding and neurotransmitter modulation), from January 1, 1988 to March 1, 2003. Additional studies were identified through bibliographies.

STUDY SELECTION

Studies were selected based on consensus across all four authors. The criteria included well-designed experimental procedures, as well as landmark studies that have significantly advanced the field.

DATA SYNTHESIS

Sixty-eight hemodynamic studies of experimental pain in normal subjects, 30 in clinical pain conditions, and 30 using neuroelectrical methods met selection criteria and were used in a meta-analysis. Another 24 articles were identified where brain neurochemistry of pain was examined. Technical issues that may explain differences between studies across laboratories are expounded. The evidence for and the respective incidences of brain areas constituting the brain network for acute pain are presented. The main components of this network are: primary and secondary somatosensory, insular, anterior cingulate, and prefrontal cortices (S1, S2, IC, ACC, PFC) and thalamus (Th). Evidence for somatotopic organization, based on 10 studies, and psychological modulation, based on 20 studies, is discussed, as well as the temporal sequence of the afferent volley to the cortex, based on neuroelectrical studies. A meta-analysis highlights important methodological differences in identifying the brain network underlying acute pain perception. It also shows that the brain network for acute pain perception in normal subjects is at least partially distinct from that seen in chronic clinical pain conditions and that chronic pain engages brain regions critical for cognitive/emotional assessments, implying that this component of pain may be a distinctive feature between chronic and acute pain. The neurochemical studies highlight the role of opiate and catecholamine transmitters and receptors in pain states, and in the modulation of pain with environmental and genetic influences.

CONCLUSIONS

The nociceptive system is now recognized as a sensory system in its own right, from primary afferents to multiple brain areas. Pain experience is strongly modulated by interactions of ascending and descending pathways. Understanding these modulatory mechanisms in health and in disease is critical for developing fully effective therapies for the treatment of clinical pain conditions.

Intracerebral pain processing in a Yoga Master who claims not to feel pain during meditation

We recorded magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) following noxious laser stimulation in a Yoga Master who claims not to feel pain when meditating. As for background MEG activity, the power of alpha frequency bands peaking at around 10 Hz was much increased during meditation over occipital, parietal and temporal regions, when compared with the non-meditative state, which might mean the subject was very relaxed, though he did not fall asleep, during meditation. Primary pain-related cortical activities recorded from primary (SI) and secondary somatosensory cortices (SII) by MEG were very weak or absent during meditation. As for fMRI recording, there were remarkable changes in levels of activity in the thalamus, SII-insula (mainly the insula) and cingulate cortex between meditation and non-meditation. Activities in all three regions were increased during non-meditation, similar to results in normal subjects. In contrast, activities in all three regions were weaker during meditation, and the level was lower than the baseline in the thalamus. Recent neuroimaging and electrophysiological studies have clarified that the emotional aspect of pain perception mainly involves the insula and cingulate cortex. Though we cannot clearly explain this unusual condition in the Yoga Master, a change of multiple regions relating to pain perception could be responsible, since pain is a complex sensory and emotional experience.

Electrophysiological studies on human pain perception

OBJECTIVE

We reviewed the recent progress in electrophysiological studies using electroencephalography (EEG), magnetoencephalography (MEG) and repetitive transcranial magnetic stimulation (rTMS) on human pain perception.

METHODS

For recording activities following A delta fiber stimulation relating to first pain, several kinds of lasers such as CO2, Tm:YAG and argon lasers are now widely used. The activity is frequently termed laser evoked potential (LEP), and we reviewed previous basic and clinical reports on LEP. We also introduced our new method, epidermal stimulation (ES), which is useful for recording brain activities by the signals ascending through A delta fibers. For recording activities following C fiber stimulation relating to second pain, several methods have been used but weak CO2 laser stimuli applied to tiny areas of the skin were recently used.

RESULTS

EEG and MEG findings following C fiber stimulation were similar to those following A delta fiber stimulation except for a longer latency. Finally, we reviewed the effect of rTMS on acute pain perception. rTMS alleviated acute pain induced by intracutaneous injection of capsaicin, which activated C fibers, but it enhanced acute pain induced by laser stimulation, which activated A delta fibers.

CONCLUSIONS

One promising approach in the near future is to analyze the change of a frequency band. This method will probably be used for evaluation of continuous tonic pain such as cancer pain, which evoked response studies cannot evaluate.

Modulation of motor-cortex oscillatory activity by painful Adelta- and C-fiber stimuli

Spontaneous approximately 20-Hz oscillations, arising predominantly from the primary motor cortex (MI), are readily observed by magnetoencephalography (MEG). Prior studies have indicated that the level of the approximately 20-Hz rhythm reflects the functional state of the MI cortex: increased 20-Hz level is associated with increased inhibition and suppression of the rhythm with excitation of MI. Close interaction is suggested between pain and the motor system by the association of chronic pain with motor dysfunction and by the alleviation of pain by motor-cortex stimulation. We therefore explored the effect of noxious input on motor-cortex functions by recording MEG signals from nine healthy subjects during selective laser stimulation of Adelta- and C-fibers of the hand. The approximately 20-Hz level was suppressed in the contralateral MI cortex in all nine subjects after painful Adelta- and C-fiber stimuli (P < 0.001). The suppression started 180 +/- 10 ms (mean +/- SEM) after Adelta-fiber stimuli and 820 +/- 30 ms after C-fiber stimuli, and peaked 160-170 ms later. Similar, but about 50% weaker, suppression of the approximately 20-Hz oscillations occurred in seven out of nine subjects in the ipsilateral MI. These results suggest automatic, lateralized, excitation of the MI cortex by noxious Adelta- and C-fiber input.

Common cortical network for first and second pain

We measured, with whole-scalp magnetoencephalography, evoked fields from 10 healthy subjects to 1-ms thulium-laser stimuli that selectively activated nociceptive nerve fibers. The stimuli were delivered to the dorsum of the subject's left hand. The earliest cortical responses peaked at 165 +/- 7 ms, agreeing with the conduction velocity of Adelta-fibers. To stimulate unmyelinated C-fibers, we modified the method of Bragard et al. [Bragard, D., Chen, A.C., Plaghki, L., 1996. Direct isolation of ultra-late (C-fibre) evoked brain potentials by CO2 laser stimulation of tiny cutaneous surface areas in man. Neurosci. Lett. 209, 81-84], by decreasing the total energy of the laser beam and by restricting the size of the stimulated skin area to 0.2-0.3 mm2. The earliest cortical responses to these stimuli peaked at 811 +/- 14 ms. Bilateral activation of the SII cortices was detected in all 10 subjects to Adelta and in 8 subjects to C stimuli, emphasizing the importance of the SII cortex in processing of pain. Additional activation was observed in the posterior parietal cortex (PPC), probably related to sensorimotor coordination targeted to produce precise motor acts that reduce or prevent the pain; the PPC activation may have been accentuated by the required continuous evaluation of the perceived pain. In contrast to some earlier studies, we did not observe activation of the primary somatosensory cortex (SI). Additional activations to both types of stimuli were detected in the cingulate cortex (three subjects) and in the bilateral insular cortex (two subjects). These results implicate that the nociceptive inputs mediated by the Adelta- and C-fibers are processed in a common cortical network in different time windows. Reliable temporospatial characterization of cortical responses to first and second pain offers a unique tool for basic and clinical neuroscience to study the two distinctive pain fiber systems at cortical level.

Contingent Negative Variation in the Parasylvian Cortex Increases During Expectancy of Painful Sensorimotor Events: A Magnetoencephalographic Study

Previous evidence relating to somatosensory-evoked magnetic fields has shown that the human parasylvian cortex (PC) is affected by ongoing painful sensorimotor interactions. In the present magnetoencephalographic study, the activity of the PC was investigated to evaluate the hypothesis of anticipatory processes preceding painful sensorimotor interactions. Sensorimotor interactions were induced by warned painful electrical stimulations at the left hand concomitant with a motor task of the right hand. The anticipatory activity of the PC was probed via contingent negative variation. Compared with the control nonpainful condition, the anticipation of the painful sensorimotor interactions increased the PC activity over the hemisphere ipsilateral to the stimulation. Dipole modeling indicated that the center of gravity of the anticipatory activity in the PC was located in the secondary somatosensory cortex. These results suggest that anticipation of painful sensorimotor interactions engages the human PC, especially in the hemisphere ipsilateral to upcoming painful stimuli and contralateral to preparatory motor commands.

Regional intensive and temporal patterns of functional MRI activation distinguishing noxious and innocuous contact heat

Cortical responses to painful and nonpainful heat were measured using functional magnetic resonance imaging (fMRI) region of interest analysis (ROI) of primary somatosensory cortex (S1), secondary somatosensory cortex (S2), anterior cingulate (ACC), supplementary motor area (SMA), insula, and inferior frontal gyrus (IFG). Previous studies indicated that innocuous and noxious stimuli of different modalities produce responses with different time courses in S1 and S2. The aim of this study was to 1) determine whether temporally distinct nociceptive blood oxygen level-dependent (BOLD) responses are evoked in multiple somatosensory processing cortical areas and 2) whether these responses discriminate small noxious stimulus intensity differences. Thirty-three subjects underwent fMRI scanning while receiving three intensities of thermal stimuli, ranging from innocuous warm (41 degrees C) to 1 degrees C below tolerance, applied to the dorsum of the left foot. Innocuous and noxious responses were distinguishable in contralateral S1, the mid-ACC, and SMA. The peak of the nociceptive response was temporally delayed from the innocuous response peak by 6-8 s. Responses to noxious but not to innocuous stimuli were observed in contralateral posterior insula. Responses to innocuous and noxious stimuli were not statistically different in contralateral S2. In contralateral S1 only, the nociceptive response could differentiate heat stimuli separated by 1 degrees C. These results show that 1) multiple cortical areas have temporally distinguishable innocuous and noxious responses evoked by a painfully hot thermode, 2) the nociceptive processing properties vary across cortical regions, and 3) nociceptive responses in S1 discriminate between painful temperatures at a level unmatched in other cortical areas.

Amplitudes of laser evoked potential recorded from primary somatosensory, parasylvian and medial frontal cortex are graded with stimulus intensity

Intensity encoding of painful stimuli in many brain regions has been suggested by imaging studies which cannot measure electrical activity of the brain directly. We have now examined the effect of laser stimulus intensity (three energy levels) on laser evoked potentials (LEPs) recorded directly from the human primary somatosensory (SI), parasylvian, and medial frontal cortical surfaces through subdural electrodes implanted for surgical treatment of medically intractable epilepsy. LEP N2* (early exogenous/stimulus-related potential) and LEP P2** (later endogenous potential) amplitudes were significantly related to the laser energy levels in all regions, although differences between regions were not significant. Both LEP peaks were also significantly correlated with the pain intensity evoked by the laser stimulus, excepting N2* over the parasylvian region. Peak latencies of both LEP peaks were independent of laser energy levels. N2* and P2** amplitudes of the maxima in all regions showed significant positive linear correlations with laser energy, excepting N2* over the parasylvian region. The lack of correlation of parasylvian cortical N2* with laser energy and pain intensity may be due to the unique anatomy of this region, or the small sample, rather than the lack of activation by the laser. Differences in thresholds of the energy correlation with amplitudes were not significant between regions. These results suggest that both exogenous in endogenous potentials evoked by painful stimuli, and recorded over SI, parasylvian, and medial frontal cortex of awake humans, encode the intensity of painful stimuli and correlate with the pain evoked by painful stimuli.

Cutaneous Painful Laser Stimuli Evoke Responses Recorded Directly From Primary Somatosensory Cortex in Awake Humans

Negative and positive laser evoked potential (LEP) peaks (N2*, P2**) were simultaneously recorded from the primary somatosensory (SI), parasylvian, and medial frontal (MF: anterior cingulate and supplementary motor area) cortical surfaces through subdural electrodes implanted for the surgical treatment of intractable epilepsy. Distribution of the LEP N2*and P2**peaks was estimated to be in cortical areas (SI, parasylvian, and MF) identified by anatomic criteria, by their response to innocuous vibratory stimulation of a finger (v-SEP), and to electrical stimulation of the median nerve (e-SEP). The maximum of the LEP N2*peak was located on the CS, medial (dorsal) to the finger motor area, as determined by cortical stimulation, and to the finger somatosensory area, as determined from the e-SEP and v-SEP. This finding suggests that the generator source of the LEP N2*peak in SI was different from that of e-SEP or v-SEP in Brodmann's areas 3b or 1. In parasylvian and MF, polarity reversal was often observed, indicating tangential current sources in these regions. In contrast to e-SEP and v-SEP, the LEP N2*latency over SI was not shorter than that over the parasylvian region. The amplitude of N2*was larger over SI than over MF and the latencies of the LEP peaks in those 2 regions were different. These findings provide evidence for a significant LEP generator in the postcentral gyrus, perhaps SI cortex, that is situated outside the tactile homunculus in SI and that receives its input arising from nociceptors simultaneously with parasylvian and MF cortex.

Effects of distraction on magnetoencephalographic responses ascending through C-fibers in humans

OBJECTIVE

Using magnetoencephalography (MEG), we evaluated the cerebral regions relating to second pain perception ascending through C-fibers and investigated the effect of distraction on each region.

METHODS

Thirteen normal subjects participated in this study. CO2 laser pulses were delivered to the dorsum of the left hand to selectively activate C-fibers. The MEG responses were analyzed using a multi-dipole model.

RESULTS

(1) primary somatosensory cortex (SI), and (2) secondary somatosensory cortex (SII)--insula were the main generators for the primary component, 1M, whose mean peak latency was 744 ms. In addition to (1) and (2), (3) cingulate cortex and (4) medial temporal area (MT) were also activated for the subsequent component, 2M, whose mean peak latency was 947 ms. During a mental calculation task (Distraction), all 6 sources were significantly reduced in amplitude, but the SII-insula (P < 0.01) and cingulate cortex (P < 0.001) were more sensitive than the SI (P < 0.05) and MT (P < 0.05).

CONCLUSIONS

We confirmed that SI in the contralateral hemisphere and SII-insula, cingulate cortex and MT in bilateral hemispheres play a major role in second pain perception, and all sites were much affected by a change of attention, indicating that these regions are related to the cognitive aspect of second pain perception.

SIGNIFICANCE

The SI, SII, cingulate and MT were activated during the C-fiber-related MEG response, and responses in these regions were significantly diminished during mental distraction.

Brain generators of laser-evoked potentials: from dipoles to functional significance

In this work we review data on cortical generators of laser-evoked potentials (LEPs) in humans, as inferred from dipolar modelling of scalp EEG/MEG results, as well as from intracranial data recorded with subdural grids or intracortical electrodes. The cortical regions most consistently tagged as sources of scalp LERs are the suprasylvian region (parietal operculum, SII) and the anterior cingulate cortex (ACC). Variability in opercular sources across studies appear mainly in the anterior-posterior direction, where sources tend to follow the axis of the Sylvian fissure. As compared with parasylvian activation described in functional pain imaging studies, LEP opercular sources tended to cluster at more superior sites and not to involve the insula. The existence of suprasylvian opercular LEPs has been confirmed by both epicortical (subdural) and intracortical recordings. In dipole-modelling studies, these sources appear to become active less than 150 ms post-stimulus, and remain in action for longer than opercular responses recorded intracortically, thus suggesting that modelled opercular dipoles reflect a "lumped" activation of several sources in the suprasylvian region, including both the operculum and the insula. Participation of SI sources to explain LEP scalp distribution remains controversial, but evidence is emerging that both SI and opercular sources may be concomitantly activated by laser pulses, with very similar time courses. Should these data be confirmed, it would suggest that a parallel processing in SI and SII has remained functional in humans for noxious inputs, whereas hierarchical processing from SI toward SII has emerged for other somatosensory sub-modalities. The ACC has been described as a source of LEPs by virtually all EEG studies so far, with activation times roughly corresponding to scalp P2. Activation is generally confined to area 24 in the caudal ACC, and has been confirmed by subdural and intracortical recordings. The inability of most MEG studies to disclose such ACC activity may be due to the radial orientation of ACC currents relative to scalp. ACC dipole sources have been consistently located between the VAC and VPC lines of Talairach's space, near to the cingulate subsections activated by motor tasks involving control of the hand. Together with the fact that scalp activities at this latency are very sensitive to arousal and attention, this supports the hypothesis that laser-evoked ACC activity may underlie orienting reactions tightly coupled with limb withdrawal (or control of withdrawal). With much less consistency than the above-mentioned areas, posterior parietal, medial temporal and anterior insular regions have been occasionally tagged as possible contributors to LEPs. Dipoles ascribed to medial temporal lobe may be in some cases re-interpreted as being located at or near the insular cortex. This would make sense as the insular region has been shown to respond to thermal pain stimuli in both functional imaging and intracranial EEG studies.

“Gating” effects of simultaneous peripheral electrical stimulations on human secondary somatosensory cortex: a whole-head MEG study

The secondary somatosensory cortex (SII) is strongly involved in the processing of somatosensory tactile and nociceptive sensations. We investigated the effect on SII responses of simultaneous painful and nonpainful electrical stimulations delivered to the thumb and little finger. According to the "bimodal" (i.e., nociceptive, tactile) organization of SII, it was expected that simultaneous painful and nonpainful stimulations would lead to modality interference with a marked reduction ("gating") of somatosensory evoked fields (SEFs) generated in SII. Eight different stimulus conditions were studied. Two conditions were simultaneous "unimodal" (thumb and little finger nonpainful; thumb and little finger painful) and two conditions were simultaneous "bimodal" (thumb nonpainful and little finger painful; thumb painful and little finger nonpainful). As a reference, four conditions included stimulations at single sites (thumb nonpainful, little finger nonpainful, thumb painful, little finger painful). The gating phenomenon was defined as the percentage of difference between the intensities of SII activation after simultaneous compared to the sum of the separate stimulations. Results showed that simultaneous stimulations induced gating effects on SEFs generated by SII. No significant gating differences were observed after the two unimodal stimulations, suggesting a negligible effect of global energy on gating. Instead, the gating effects on bilateral SII activity were stronger after simultaneous bimodal when compared to unimodal stimulations. Our findings hint that there could be a greater level of integration/convergence of painful and nonpainful stimuli in SII with respect to SI. Future studies should explore if it could have an important role in exploring pain relief.

A comparative magnetoencephalographic study of cortical activations evoked by noxious and innocuous somatosensory stimulations

We recorded somatosensory-evoked magnetic fields and potentials produced by painful intra-epidermal stimulation (ES) and non-painful transcutaneous electrical stimulation (TS) applied to the left hand in 12 healthy volunteers to compare cortical responses to noxious and innocuous somatosensory stimulations. Our results revealed that cortical processing following noxious and innocuous stimulations was strikingly similar except that the former was delayed approximately 60 ms relative to the latter, which was well explained by a difference in peripheral conduction velocity mediating noxious (Adelta fiber) and innocuous (Abeta fiber) inputs. The first cortical activity evoked by both ES and TS was in the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulated side. The following activities were in the bilateral secondary somatosensory cortex (SII), insular cortex, cingulate cortex, anterior medial temporal area and ipsilateral SI. The source locations did not differ between the two stimulus modalities except that the dipole for insular activity following ES was located more anterior to that following TS. Both ES and TS evoked vertex potentials consisting of a negativity followed by a positivity at a latency of 202 and 304 ms, and 134 and 243 ms, respectively. The time course of the vertex potential corresponded to that of the activity of the medial temporal area. Our results suggested that cortical processing was similar between noxious and innocuous stimulation in SI and SII, but different in insular cortex. Our data also implied that activities in the amygdala/hippocampal formation represented common effects of noxious and tactile stimulations.

Left-hemisphere dominance in early nociceptive processing in the human parasylvian cortex

Pain perception comprises sensory and emotional dimensions. While the emotional experience is thought to be represented in the right hemisphere, we here report a left-hemisphere dominance for the early sensory component of pain perception using brain electrical source analysis of laser-evoked potentials. Ten right-handed subjects underwent several series of laser radiant heat stimuli to pairs of parallel lines on the dorsum of the left or right hand. Stimulus location and intensity were randomised independently. The sensory-discriminative aspects of pain were emphasised by asking the subjects to perform either a spatial or an intensity discrimination task and were contrasted with active distraction by mental arithmetics. Pain ratings obtained after each of the laser stimulus series revealed an analgesic effect of distraction (27%, P < 0.001). Four equivalent dipole sources were active in the latency range of 100-200 ms (bilateral operculoinsular cortex, midcingulate gyrus, postcentral gyrus). The sources in the operculoinsular cortex exhibited (a) the shortest peak latency (155 +/- 6 ms), (b) the most pronounced enhancement during spatial and intensity discrimination tasks compared to active distraction (43%, P < 0.001), and (c) a significantly stronger source activity in the left hemisphere independent of stimulation side (23%, P < 0.05). The distribution of these sources extended into the dorsal insula. The postcentral source had the longest peak latency (180 +/- 7 ms); its source strength was task-dependent (25%, P = 0.051) but exhibited no hemisphere dominance. The midcingulate source had an intermediate peak latency (169 +/- 7 ms). Its source strength was modulated by tasks, but this modulation was significant only in the latency range >200 ms (46%, P < 0.001). These findings suggest a dominant role of the left frontal operculum and adjacent dorsal insula in the early sensory-discriminative dimensions of pain processing. This region has been proposed to be the cortical projection target of nociceptive pathways from the spinal cord to the ventroposteroinferior and ventromedial (its posterior part: VMpo) thalamic nuclei.

Effects of sleep on pain-related somatosensory evoked magnetic fields in humans

We investigated the effects of sleep on pain-related somatosensory evoked magnetic fields (SEFs) following painful electrical stimulation to identify the mechanisms generating them in both fast A-beta fibers relating to touch and slow A-delta fibers relating to pain. While the subjects were awake, non-painful and painful electrical stimulations were applied, and while asleep, painful stimulation was applied to the left index finger. During awake, five components (1M-5M) were identified following both non-painful and painful stimulation, but the 4M and 5M at around 70-100 ms and 140-180 ms, respectively, were significantly enhanced following painful stimulation. During sleep, 1M and 2M generated in the primary somatosensory cortex (SI) did not show a significant change, 3M in SI showed a slight but significant amplitude reduction, and 4M and 5M generated in both SI and the secondary somatosensory cortex (SII) were significantly decreased in amplitude or disappeared. The 4M and 5M are complicated components generated in SI and SII ascending through both A-beta fibers and A-delta fibers. They are specifically enhanced by painful stimulation due to an increase of signals ascending through A-delta fibers, and are markedly decreased during sleep, because they much involve cognitive function.

Dipole source analysis of laser-evoked subdural potentials recorded from parasylvian cortex in humans

The location of the human nociceptive area(s) near the Sylvian fissure is still controversial in spite of evidence from imaging and evoked potential studies that noxious heat stimuli activate somatosensory areas in that region. Some studies have suggested the secondary somatosensory cortex (SII) on the upper bank of the Sylvian fissure posterior to the central sulcus, others the anterior insula or parietal area 7b. In this study, we applied dipole source analysis techniques to laser-evoked potentials (LEPs) that were recorded from subdural grid electrodes in three patients. As a functional marker, auditory-evoked potentials (AEPs) with a generator on the opposite bank of the Sylvian fissure were recorded from the same electrodes. The LEP global field power (GFP), a measure of spatial variance, showed a first peak at about 150 ms latency, corresponding to the latency of the N1 recorded from the scalp. In contrast to scalp recordings, the amplitude of the first GFP peak recorded from the grid was larger than the second peak (P2). This finding suggests that the generator of N1, but not that of later LEP components, was close to the subdural grids. When a regional source was fitted to the first GFP peak, its location was within the frontoparietal operculum in all patients. On average, the LEP source was 13 mm anterior, 6 mm superior, and 2 mm medial of the AEP source. This relative location also suggests a source within the frontoparietal operculum overlying the insula. At the latency of the first GFP peak, source orientation pointed inward, suggesting a generator within the inner vertical surface of the operculum. Somatotopy was assessed in one patient and was consistent with that of the projection area of the presumed nociceptive thalamic nucleus posterior part of the ventromedial nucleus, but differed from that of SII. These findings suggest that the nociceptive area in human parasylvian cortex that is activated most rapidly by noxious heat pulses may be separate from the tactile SII area.

Cerebral responses following stimulation of unmyelinated C-fibers in humans: electro- and magneto-encephalographic study

There are two kinds of pain, a sharp pain ascending through Adelta fibers (first pain) and a second burning pain ascending though C fibers (second pain). By using a novel method, the application of a low intensity CO(2) laser beam to a tiny area of skin using a very thin aluminum plate with numerous tiny holes as a spatial filter, we succeeded in selectively stimulating unmyelinated C fibers of the skin in humans, and could record consistent and clear brain responses using electroencephalography (EEG) and magnetoencephalography (MEG). The conduction velocity (CV) of the C fibers of the peripheral nerve and spinal cord, probably spinothalamic tract (STT), is approximately 1-4 m/s, which is significantly slower than that of Adelta (approximately 10-15 m/s) and Abeta fibers (approximately 50-70 m/s). This method should be very useful for clinical application. Following C fiber stimulation, primary and secondary somatosensory cortices (SI and SII) are simultaneously activated in the cerebral hemisphere contralateral to the stimulation, and then, SII in the hemisphere ipsilateral to the stimulation is activated. These early responses are easily detected by MEG. Then, probably limbic systems such as insula and cingulate cortex are activated, and those activities reflected in EEG components. Investigations of the cortical processing in pain perception including both first and second pain should provide a better understanding of pain perception and, therefore, contribute to pain relief in clinical medicine.