Pain-related somatosensory evoked magnetic fields following lower limb stimulation

Comparison of the effective intensity of transcutaneous electrical nerve stimulation contralateral to a pain site for analgesia

[Purpose] This study aimed to compare the effectiveness of transcutaneous electrical nerve stimulation contralateral to the pain site for analgesia to identify the effective stimulation intensity. [Participants and Methods] Ten healthy adult females were recruited for the study. The same heat stimulation was applied to the left wrist joint of each participant to induce pain, serving as the control. Transcutaneous electrical nerve stimulation was then randomly administered to the right wrist, corresponding to the same dermatome contralateral to the painful site, at the intensities of comfortable stimulation, pain threshold, and maximum pain. The effect of transcutaneous electrical nerve stimulation was assessed using a Visual Analogue Scale and by analysis of heart rate variability. [Results] The Visual Analogue Scale score was significantly lower after stimulation with the maximum pain intensity than that for control, and there were no significant differences among the intensities of comfortable stimulation, pain threshold, and maximum pain. No significant differences were found among the groups in terms of high and low-to-high frequency components. [Conclusion] Transcutaneous electrical nerve stimulation at the maximum pain intensity to the dermatome area contralateral to that of the dorsal pain site of the left wrist was considered effective.

Comparison of the pain-relieving effects of transcutaneous electrical nerve stimulation applied at the same dermatome levels as the site of pain in the wrist joint

[Purpose] The purpose of this study was to develop a proposal for an effective interventional option for therapeutic stimulation sites by comparing the pain-relieving effect of transcutaneous electrical nerve stimulation (TENS) applied to the same dermatome level of the contralateral sites of the dorsal wrist joint with the pain or the neck, or both sites simultaneously. [Subjects and Methods] A control was first established by triggering pain in the left dorsal wrist joints of adult females by using heat stimulation. Three interventions were then performed, comprising the TENS to the contralateral wrist joint (CW) and to the neck (N) at the same dermatome level as the site of pain, and the TENS to both CW and N simultaneously (CWN). Levels of pain and cerebral blood flow were also measured. [Results] The pain levels of three interventions were found to be significantly decreased compared with the control; however, no significant differences in the levels of pain were seen between any combinations of three interventions. Furthermore, no significant differences were seen between any interventions in terms of cerebral blood flow. [Conclusion] The results suggest that in order for TENS to be effective, it is necessary to make effective use of the dermatome.

Nociceptive contribution to the evoked potentials after painful intramuscular electrical stimulation

Our study aimed at investigating the nociceptive contribution to the somatosensory evoked potentials after electrical intramuscular stimulation (mSEPs) at painful intensity. Scalp mSEPs were recorded in 10 healthy subjects after electrical stimulation of the left brachioradialis muscle at three intensities: non-painful (I2), slightly painful (I4) and moderately painful (I6). For each intensity, mSEPs were recorded in a neutral condition (NC) in which subjects did not have any task, and in an attention condition (AC) in which subjects were asked to count the number of stimuli. In both NC and AC, the N120 and P220 amplitudes were significantly higher at I6 than at I2. While the N120 amplitude did not vary between NC and AC, the P220 amplitude was significantly higher in AC than in NC at all stimulus intensities. Our results suggest that nociceptive inputs contribute to the N120 amplitude increase at painful stimulus intensity, while the P220 amplitude is more sensitive to changes of subjects' attention level. Therefore, the N120 amplitude increase to moderately painful stimuli, as compared to non-painful stimuli, may represent a marker of the activation of the muscular thin myelinated afferents.

Effects of spinal cord stimulation on the cortical somatosensory evoked potentials in failed back surgery syndrome patients

OBJECTIVE

To evaluate the functional activation of the somatosensory cortical regions in neuropathic pain patients during therapeutic spinal cord stimulation (SCS).

METHODS

In nine failed back surgery syndrome patients, the left tibial and the left sural nerves were stimulated in two sessions with intensities at motor and pain thresholds, respectively. The cortical somatosensory evoked potentials were analyzed using source dipole analysis based on 111 EEG signals.

RESULTS

The short-latency components of the source located in the right primary somatosensory cortex (SI: 43, 54 and 65ms) after tibial nerve stimulation, the mid-latency SI component (87ms) after sural nerve stimulation, and the mid-latency components in the right (approximately 161ms) and left (approximately 168ms) secondary somatosensory cortices (SII) were smaller in the presence of SCS than in absence of SCS. The long-latency source component arising from the mid-cingulate cortex (approximately 313ms) was smaller for tibial and larger for sural nerve stimuli during SCS periods compared to periods without SCS.

CONCLUSIONS

SCS attenuates the somatosensory processing in the SI and SII. In the mid-cingulate cortex, the effect of SCS depends on the type of stimulation and nerve fibers involved.

SIGNIFICANCE

Results suggest that the effects of SCS on cortical somatosensory processing may contribute to a reduction of allodynia during SCS.

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.

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.

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.

Sensory perception during sleep in humans: a magnetoencephalograhic study

We reported the changes of brain responses during sleep following auditory, visual, somatosensory and painful somatosensory stimulation by using magnetoencephalography (MEG). Surprisingly, very large changes were found under all conditions, although the changes in each were not the same. However, there are some common findings. Short-latency components, reflecting the primary cortical activities generated in the primary sensory cortex for each stimulus kind, show no significant change, or are slightly prolonged in latency and decreased in amplitude. These findings indicate that the neuronal activities in the primary sensory cortex are not affected or are only slightly inhibited during sleep. By contrast, middle- and long-latency components, probably reflecting secondary activities, are much affected during sleep. Since the dipole location is changed (auditory stimulation), unchanged (somatosensory stimulation) or vague (visual stimulation) between the state of being awake and asleep, different regions responsible for such changes of activity may be one explanation, although the activated regions are very close to each other. The enhancement of activities probably indicates two possibilities, an increase in the activity of excitatory systems during sleep, or a decrease in the activity of some inhibitory systems, which are active in the awake state. We have no evidence to support either, but we prefer the latter, since it is difficult to consider why neuronal activities would be increased during sleep.

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.

Caudal cingulate cortex involvement in pain processing: an inter-individual laser evoked potential source localisation study using realistic head models

Electrophysiological studies have revealed a source of laser pain evoked potentials (LEPs) in cingulate cortex. However, few studies have used realistically shaped head models in the source analysis, which account for individual differences in anatomy and allow detailed anatomical localisation of sources. The aim of the current study was to accurately localise the cingulate source of LEPs in a group of healthy volunteers, using realistic head models, and to assess the inter-individual variability in anatomical location. LEPs, elicited by painful CO(2) laser stimulation of the right forearm, were recorded from 62 electrodes in five healthy subjects. Dipole source localisation (CURRY 4.0) was performed on the most prominent (P2) peak of each LEP data set, using head models derived from each subject's structural magnetic resonance image (MRI).For all subjects, the P2 LEP peak was best explained by a dipole whose origin was in cingulate cortex (mean residual variance was 3.9+/-2.4 %). For four out of five subjects, it was located at the border of the caudal division of left anterior cingulate cortex (area 24/32') with left posterior cingulate cortex (area 23/31). For the fifth subject the dipole was centred in right posterior cingulate cortex (area 31). This study demonstrates that the location of the cingulate source of LEPs is highly consistent across subjects, when analysed in this way, and supports the involvement of caudal cingulate regions in pain processing.

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.

Effects of sleep on pain-related somatosensory evoked potentials in humans

We investigated effects of sleep on pain-related somatosensory evoked potentials (SEP) following painful electrical stimulation of the left index finger. The biggest advantage of this method is that signals ascending through both A-beta fibers relating to touch and A-delta fibers relating to pain can be recorded simultaneously. While the subject was awake, non-painful stimulation evoked early- and middle latency components, N20, P30 and N60, at the C4 electrode, and painful stimulation evoked not only early- and middle latency components at the C4 but also later pain-specific components, N130 and P240, at the Cz electrode. During sleep, N20 and P30 did not show a significant change in amplitude, N60 showed a slight but significant amplitude reduction, and N130 and P240 significantly decreased in amplitude or disappeared, as compared with those while awake. Therefore, we speculate on the mechanisms generating each component as follows; (1) N20 and P30 are the primary components generated in SI ascending through A-beta fibers. (2) N60 is the secondary component generated in SI involving cognitive function to some degree. (3) N130-P240 are the pain-specific components ascending through A-delta fibers, and closely related to cognitive function, because they were much affected by consciousness, different from the components ascending through A-beta fibers.

Pain-related somatosensory evoked magnetic fields induced by controlled ballistic mechanical impacts

The purpose of this study was to investigate cortical processing of painful compared with tactile mechanical stimulation by means of magnetoencephalography (MEG) using the novel technique of mechanical impact loading. A light, hard projectile is accelerated pneumatically in a guiding barrel and elicits a brief sensation of pain when hitting the skin in free flight. Controllable noxious and innocuous impact velocities facilitate the generation of different, predetermined stimulus intensities. The authors applied painful as well as tactile mechanical impacts to the dorsum of the second, third, and fourth digit of the nondominant hand. Pain-related somatosensory evoked magnetic fields (SSEFs) were compared with those following tactile stimulation in seven healthy volunteers. Contralateral primary sensory cortical area activation was observed within the first 70 msec after tactile as well as painful stimulus intensities. Only painful impacts elicited SSEF responses assigned to the bilateral secondary sensory cortical regions and to the middle part of the contralateral cingulate gyrus, which were active at latency ranges of 55 to 155 msec and 90 to 220 msec respectively. Additional long-latency responses occurred in these cortical areas as long as 280 msec after painful stimulation in three subjects. In contrast to tactile stimulation, painful mechanical impacts elicited SSEF responses in cortical areas demonstrated to be involved in central pain processing by previous MEG and neuroimaging studies. Because of its similarity to natural noxious stimuli and the possibility of adjustable painful and tactile impact velocities, the technique of mechanical impact loading provides a useful method for the neurophysiologic evaluation of cortical pain perception.

A Review of Functional Imaging of the Brain and Pain

Functional imaging of the brain is a current reality using positron emission tomography and functional magnetic imaging. This article reviews many of the reports that have emerged in the past several years using these techniques in the analysis of pain experience. The areas of the brain that appear to be functioning during the experience of pain are discussed, and the variances in findings between studies are described. The implications of the findings are noted. Although much has been learned through these techniques, it is clear that further research is needed before clinicians can use these diagnostic studies for therapeutic purposes.

The somatosensory evoked magnetic fields

Averaged magnetoencephalography (MEG) following somatosensory stimulation, somatosensory evoked magnetic field(s) (SEF), in humans are reviewed. The equivalent current dipole(s) (ECD) of the primary and the following middle-latency components of SEF following electrical stimulation within 80-100 ms are estimated in area 3b of the primary somatosensory cortex (SI), the posterior bank of the central sulcus, in the hemisphere contralateral to the stimulated site. Their sites are generally compatible with the homunculus which was reported by Penfield using direct cortical stimulation during surgery. SEF to passive finger movement is generated in area 3a or 2 of SI, unlike with electrical stimulation. Long-latency components with peaks of approximately 80-120 ms are recorded in the bilateral hemispheres and their ECD are estimated in the secondary somatosensory cortex (SII) in the bilateral hemispheres. We also summarized (1) the gating effects on SEF by interference tactile stimulation or movement applied to the stimulus site, (2) clinical applications of SEF in the fields of neurosurgery and neurology and (3) cortical plasticity (reorganization) of the SI. SEF specific to painful stimulation is also recorded following painful stimulation by CO(2) laser beam. Pain-specific components are recorded over 150 ms after the stimulus and their ECD are estimated in the bilateral SII and the limbic system. We introduced a newly-developed multi (12)-channel gradiometer system with the smallest and highest quality superconducting quantum interference device (micro-SQUID) available to non-invasively detect the magnetic fields of a human peripheral nerve. Clear nerve action fields (NAFs) were consistently recorded from all subjects.

Activation of the somatosensory cortex during Abeta-fiber mediated hyperalgesia. A MSI study

OBJECTIVE

To investigate the neural activation in the primary somatosensory cortex (SI) that is induced by capsaicin-evoked secondary Abeta-fiber-mediated hyperalgesia with magnetic source imaging (MSI) in healthy humans.

BACKGROUND

Dynamic mechanical hyperalgesia, i.e. pain to innocuous light touching, is a symptom of painful neuropathies. Animal experiments suggest that alterations in central pain processing occur so that tactile stimuli conveyed in Abeta low threshold mechanoreceptive afferents become capable of activating central pain signalling neurons. A similar state of central sensitization can be experimentally produced with capsaicin.

METHODS

In six individuals the somatosensory evoked magnetic fields (SEFs) induced by non-painful electrical stimulation of Abeta-afferents at the forearm skin were recorded. Capsaicin was injected adjacent to the stimulation site to induce secondary dynamic Abeta-hyperalgesia. Thereafter, the SEFs induced by the identical electrical stimulus applied within the secondary hyperalgesic skin were analyzed. The electrical stimulus was subsequently perceived as painful without changing the stimulus intensity and location. Latencies, anatomical source location and amplitudes of SEFs during both conditions were compared.

RESULTS

Non-painful electrical stimulation of Abeta-afferents induced SEFs in SI at latencies between 20 and 150 ms. Stimulation of Abeta-afferents within the capsaicin-induced secondary hyperalgesic skin induced SEFs at identical latencies and locations as compared with the stimulation of Abeta-afferents within normal skin. The amplitudes, i.e., the magnetic dipole strengths of the SEFs were higher during Abeta-hyperalgesia.

CONCLUSIONS

Acute application of capsaicin produces an increase in the excitability of central neurons, e.g., in SI. This might be due to sensitization of central neurons so that normally innocuous stimuli activate pain signalling neurons or cortical neurons might increase their receptive fields.

Korrelate der kortikalen Schmerzverarbeitung - eine Übersicht

Zusammenfassung: Schmerz ist ein kompliziertes Resultat verschiedener neuronaler Aktivitäten unseres Gehirns und nicht nur das einfache Ergebnis der Tätigkeit des peripheren nozizeptiven Systems. Schmerz resultiert aus dem Zusammenspiel verschiedener Module im Gehirn, die sich in verschiedenen Hirnarealen befinden. Er wird durch Erwartungen, Lernprozesse, Erfahrungen und Coping modifiziert. Elektrophysiologische Begleiterscheinungen, die mit der zentralnervösen Schmerzverarbeitung assoziiert sind, erlauben dabei eine Charakterisierung der ablaufenden Informationsverarbeitungsprozesse. Neben der grundlagentheoretischen Bedeutung spielt hier die Evaluation verschiedener Therapieansätze eine herausragende Rolle. Darüber hinaus konnte mit Hilfe der Hirnelektrizität nachgewiesen werden, daß auch die kortikalen Module des nozizeptiven Systems im Zusammenhang mit Schmerzverarbeitung funktionell reorganisiert werden. Die relativ neuen quellenanalytischen Ansätze lassen einen weiteren, deutlichen Erkenntnisgewinn über die Rolle einzelner Hirnstrukturen bei der Verarbeitung und Behandlung von Schmerz erwarten.

Pain-Related somatosensory evoked potentials

The authors reviewed basic and clinical reports of pain-related somatosensory evoked potentials (SSEP) after high-intensity electrical stimulation [pain SSEP(E)] and painful laser stimulation [pain SSEP(L)]. The conduction velocity of peripheral nerves for both pain SSEP(E) and pain SSEP(L) is approximately 10 to 15 m/second, in a range of Adelta fibers. The generator sources are considered to be the secondary somatosensory cortex and insula, and the limbic system, including the cingulate cortex, amygdala, or hippocampus of the bilateral hemispheres. The latencies and amplitudes are clearly affected by vigilance, attention-distraction, and various kinds of stimulation applied simultaneously with pain. Abnormalities of pain SSEP(L) reflect an impairment of pain-temperature sensation, probably relating to dysfunction of A5 fibers of the peripheral nerve and spinothalamic tract. In contrast, conventional SSEP after nonpainful electrical stimulation reflects an impairment of tactile, vibratory, and deep sensation, probably relating to dysfunction of Aalpha or Abeta fibers of the peripheral nerve and dorsal column. Therefore, combining the study of pain SSEP(L) and conventional SSEP is useful to detect physiologic abnormalities, and sometimes subclinical abnormalities, of patients with peripheral and central nervous system lesions.

After-effect of transcutaneous electrical nerve stimulation (TENS) on pain-related evoked potentials and magnetic fields in normal subjects

OBJECTIVES

The after-effect of transcutaneous electrical nerve stimulation (TENS) on pain-related brain responses was investigated using electroencephalography (EEG) and magnetoencephalography (MEG).

METHODS

We studied 13 healthy volunteers for the main experiment and 7 for the control experiment. The pain-related evoked cerebral potentials (PREP) at Cz and magnetic cortical fields (PRCF) on both hemispheres following painful electrical finger stimulation were simultaneously recorded before and after TENS on the right forearm of the median nerve territory at 50 Hz for 30 min. PREP and PRCF were similarly recorded without TENS in the control experiment.

RESULTS

The PREP components, N150 and P220, were significantly attenuated after TENS, compared to those before TENS (P<0.01, two-way repeated ANOVA). However, there was no consistent change of the PRCF components. Eleven of 13 subjects reported no change of pain sensation after TENS. There was no change of PREP in control experiment without TENS.

CONCLUSIONS

The results indicated that TENS reduced PREP following painful electrical stimulation, and that the origin of PREP was, at least partially, different from that of PRCF which was not changed after TENS. An after-effect of TENS significantly affected the generation process of PREP, but it was not enough to relieve the subjective painful feeling.

Effects of distraction on pain-related somatosensory evoked magnetic fields and potentials following painful electrical stimulation

We aimed to compare the effects of distraction on pain-related somatosensory evoked magnetic fields (pain SEF) following painful electrical stimulation with simultaneous recordings of evoked potentials (pain SEP). Painful electrical stimuli were applied to the right index finger of eleven healthy subjects. A table with 25 random two-digit numbers was shown to the subjects, who were asked to add 5 numbers of each line in their mind (calculation task) or to memorize the numbers (memorization task) during the recording. In the SEF recording, 3 short-latency components within 50 ms of the stimulation were generated in the primary sensory cortex (SI) of the hemisphere contralateral to the stimulated finger. Middle-latency components between 100 and 250 ms after the stimuli were recorded from the secondary somatosensory cortex (SII) in the bilateral hemispheres or the cingulate cortex. No SEF components were significantly affected by either task. In the SEP recording, the middle-latency components (N140 and P230) were identified as being maximal around the vertex. Amplitudes of the N140 and P230 were not affected by each task, but the peak-to-peak amplitude (N140-P230) was significantly decreased by both the calculation and memorization tasks, particularly by the former. Subjective pain rating was decreased in both the calculation and memorization tasks, particularly in the former. We concluded that distraction tasks reduced activities in the limbic system, in which the middle-latency EEG component probably generated, while neither the short-latency SEF components generated in SI nor the primary pain-related SEF components generated in SII-insula are affected.

Primary somatosensory cortex is actively involved in pain processing in human

We recorded somatosensory evoked magnetic fields (SEFs) by a whole head magnetometer to elucidate cortical receptive areas involved in pain processing, focusing on the primary somatosensory cortex (SI), following painful CO(2) laser stimulation of the dorsum of the left hand in 12 healthy human subjects. In seven subjects, three spatially segregated cortical areas (contralateral SI and bilateral second (SII) somatosensory cortices) were simultaneously activated at around 210 ms after the stimulus, suggesting parallel processing of pain information in SI and SII. Equivalent current dipole (ECD) in SI pointed anteriorly in three subjects whereas posteriorly in the remaining four. We also recorded SEFs following electric stimulation of the left median nerve at wrist in three subjects. ECD of CO(2) laser stimulation was located medial-superior to that of electric stimulation in all three subjects. In addition, by direct recording of somatosensory evoked potentials (SEPs) from peri-Rolandic cortex by subdural electrodes in an epilepsy patient, we identified a response to the laser stimulation over the contralateral SI with the peak latency of 220 ms. Its distribution was similar to, but slightly wider than, that of P25 of electric SEPs. Taken together, it is postulated that the pain impulse is received in the crown of the postcentral gyrus in human.

Neurophysiological evaluation of pain

Neurophysiological techniques for the evaluation of pain in humans have made important advances in the last decade. A number of features of neuroanatomy and physiology of nociception qualifies pain as a multidimensional phenomenon which is rather unique among the sensory systems and which poses a number of technical and procedural requirements for its appropriate diagnostic assessment. Various stimulation techniques to induce defined pain in humans and used in combination with the methodology of evoked electrical brain potentials and magnetic fields are presented. Most recent knowledge gathered from scalp topography and dipole source analysis of pain-relevant evoked potentials and fields is discussed. Particular emphasis is put upon laser-evoked potentials and their application for diagnosis, pathophysiological description and monitoring of patients with neurological disorders and abnormal pain states. Future perspectives in this growing field of research are discussed briefly.

Functional MRI study of thalamic and cortical activations evoked by cutaneous heat, cold, and tactile stimuli

Positron emission tomography studies have provided evidence for the involvement of the thalamus and cortex in pain and temperature perception. However, the involvement of these structures in pain and temperature perception of individual subjects has not been studied in detail with high spatial resolution imaging. As a first step toward this goal, we have used functional magnetic resonance imaging (fMRI) to locate discrete regions of the thalamus, insula, and second somatosensory cortex (S2) modulated during innocuous and noxious thermal stimulation. Results were compared with those obtained during tactile stimulation of the palm. High resolution functional images were acquired on a 1.5 T echospeed GE MR system with an in-plane resolution of 1.7 mm. A modified peltier-type thermal stimulator was used to deliver innocuous cool and warm and noxious cold and hot stimuli for 40-60 s to the thenar eminence of normal male and female volunteers. Experimental paradigms consisted of four repetitions of interleaved control and task stimuli. A pixel by pixel statistical analysis of images obtained during each task versus control (e.g., noxious heat vs. warm, warm vs. neutral temperature, etc.) was used to determine task-related activations. Painful thermal stimuli activated discrete regions within the lateral and medial thalamus, and insula, predominantly in the anterior insula in most subjects, and the contralateral S2 in 50% of subjects. The innocuous thermal stimuli did not activate the S2 in any of the subjects but activated the thalamus and posterior insula in 50% of subjects. By comparison, innocuous tactile stimulation consistently activated S2 bilaterally and the contralateral lateral thalamus. These data also demonstrate that noxious thermal and innocuous tactile-related activations overlap in S2. The data also suggest that innocuous and noxious-related activations may overlap within the thalamus but may be located in different regions of the insula. Therefore, we provide support for a role of the anterior insula, S2, and thalamus in the perception of pain; whereas the posterior insula appears to be involved in tactile and innocuous temperature perception. These data demonstrate the feasibility of using fMRI for studies of pain, temperature, and mechanical stimuli in individual subjects, even in small regions such as thalamic nuclei. However, the intersubject variability should be considered in future single subject imaging studies and studies that rely on averaged group responses.

Hierarchical somatosensory processing

Recent studies of the postcentral and additional somatosensory cortices support a hierarchical scheme for information processing. In the postcentral gyrus, the complexity of receptive field properties increases with caudal progression from area 1. It has been reported that the anterior bank of the intraparietal sulcus, the caudalmost part of the postcentral gyrus, is responsible for the systematic integration of bilateral body parts, as well as of somatic and visual information.

Simple and novel method for measuring conduction velocity of A delta fibers in humans

We report a simple new method for measuring the conduction velocity (CV) of A delta fibers in normal subjects. A large positive component of somatosensory evoked potential (SEP) whose peak latency was approximately 250 ms was clearly recorded only when strong electrical stimulation causing a definite painful feeling was applied to the skin. The CV of the peripheral nerve was calculated by measuring the latency difference of this component between the distal-stimulated SEP and proximal-stimulated SEP and the distance between two stimulus sites. The CV was approximately 11.4 m/s, (range 8.8-15.9 m/s), in the range of A delta fibers. The sleep effect on pain-related SEP was also observed in 3 subjects. The amplitude of pain-related SEP decreased with the progress of sleep stage. This simple and novel method is available in most clinics and should be very useful in investigating the physiologic functions of peripheral nerves in patients as well as normal subjects.

Magnetoencephalographic study of intracerebral interactions caused by bilateral posterior tibial nerve stimulation in man

We studied somatosensory evoked magnetic fields (SEFs) following stimulation of bilateral posterior tibial nerves ('bilateral' waveform) in normal subjects to determine the inter- and intra-hemispheric interference effects caused by activation of sensory areas in bilateral hemispheres. Activated areas in the primary and second sensory cortices (SI and SII) in each hemisphere following bilateral stimulation were clearly identified by estimation of the double best-fitted equivalent current dipoles (ECD) using the spherical head model, and the large inter-individual differences were identified. SEFs following the right posterior tibial nerve stimulation and those following the left stimulation were summated ('summated' waveform). The 'difference' waveform was induced by a subtraction of 'bilateral' waveforms from the 'summated' waveform. Short-latency deflections showed no consistent changes between the 'summated' and 'bilateral' waveforms, but the long-latency deflection, the N100m-P100m, in the 'bilateral' waveform was significantly (P < 0.02) reduced in amplitude as compared with the 'summated' waveform. The differences were clearly identified in the 'difference' waveform, in which the main deflections, U100m-D100m, were found. The ECDs of the short-latency deflections were located in SI contralateral to the stimulated nerve, but the ECDs of the N100m-P100m were located in bilateral SII which are considered to receive ascending signals from the body bilaterally. Therefore, some inhibitory interactions might take place in SII by receiving inputs from the body bilaterally.