Topography of somatosensory evoked magnetic fields following posterior tibial nerve stimulation

The Effect of Various Spinal Neurostimulation Paradigms on the Supraspinal Somatosensory Evoked Response: A Systematic Review

INTRODUCTION

Spinal neurostimulation is a therapy for otherwise intractable chronic pain. Spinal neurostimulation includes stimulation of the spinal cord (SCS), dorsal root ganglion (DRGS), and dorsal root entry zone (DREZS). New paresthesia-free neurostimulation paradigms may rely on different mechanisms of action from those of conventional tonic neurostimulation. The aim of this systematic review is to assess the existing knowledge on the effect of spinal neurostimulation on somatosensory processing in patients with chronic pain. We therefore reviewed the existing literature on the effect of various spinal neurostimulation paradigms on the supraspinal somatosensory evoked response (SER).

MATERIALS AND METHODS

Multiple scientific data bases were searched for studies that assessed the effect of spinal neurostimulation on the supraspinal SER, evoked by painful or nonpainful peripheral stimuli in patients with chronic pain. We found 205 studies, of which 24 were included. Demographic data, study design, and study outcome were extracted.

RESULTS

Of the 24 included studies, 23 used electroencephalography to assess the SER; one study used magnetoencephalography. Fifteen studies evaluated tonic SCS; six studies (also) evaluated paresthesia-free paradigms; three studies evaluated the effect of tonic DRGS or DREZS. Sixteen studies used nonpainful stimuli to elicit the SER, 14 observed a decreased SER amplitude. Ten studies used painful stimuli to elicit the SER, yielding mixed results.

DISCUSSION

The included studies suggest that both paresthesia-based and paresthesia-free spinal neurostimulation paradigms can decrease (part of) the SER elicited by a nonpainful peripheral stimulus. The observed SER amplitude reduction likely is the effect of various spinal and supraspinal mechanisms of spinal neurostimulation that also contribute to pain relief.

CONCLUSIONS

Spinal neurostimulation modulates the processing of a peripherally applied nonpainful stimulus. For painful stimuli, the results are not conclusive. It is not yet clear whether paresthesia-free neurostimulation affects the SER differently from paresthesia-based neurostimulation.

Somatosensory omissions reveal action-related predictive processing

The intricate relation between action and somatosensory perception has been studied extensively in the past decades. Generally, a forward model is thought to predict the somatosensory consequences of an action. These models propose that when an action is reliably coupled to a tactile stimulus, unexpected absence of the stimulus should elicit prediction error. Although such omission responses have been demonstrated in the auditory modality, it remains unknown whether this mechanism generalizes across modalities. This study therefore aimed to record action-induced somatosensory omission responses using EEG in humans. Self-paced button presses were coupled to somatosensory stimuli in 88% of trials, allowing a prediction, or in 50% of trials, not allowing a prediction. In the 88% condition, stimulus omission resulted in a neural response consisting of multiple components, as revealed by temporal principal component analysis. The oN1 response suggests similar sensory sources as stimulus-evoked activity, but an origin outside primary cortex. Subsequent oN2 and oP3 responses, as previously observed in the auditory domain, likely reflect modality-unspecific higher order processes. Together, findings straightforwardly demonstrate somatosensory predictions during action and provide evidence for a partially amodal mechanism of prediction error generation.

Mechanism of Action of Tibial Nerve Stimulation in the Treatment of Lower Urinary Tract Dysfunction

BACKGROUND AND OBJECTIVE

Tibial nerve stimulation (TNS) has long been used to effectively treat lower urinary tract dysfunction (LUTD). Although numerous studies have concentrated on TNS, its mechanism of action remains elusive. This review aimed to concentrate on the mechanism of action of TNS against LUTD.

MATERIALS AND METHODS

A literature search was performed in PubMed on October 31, 2022. In this study, we introduced the application of TNS for LUTD, summarized different methods used in exploring the mechanism of TNS, and discussed the next direction to investigate the mechanism of TNS.

RESULTS AND CONCLUSIONS

In this review, 97 studies, including clinical studies, animal experiments, and reviews, were used. TNS is an effective treatment for LUTD. The study of its mechanisms primarily concentrated on the central nervous system, tibial nerve pathway, receptors, and TNS frequency. More advanced equipment will be used in human experiments to investigate the central mechanism, and diverse animal experiments will be performed to explore the peripheral mechanism and parameters of TNS in the future.

Changes in somatosensory evoked potentials elicited by lateral cerebellar nucleus deep brain stimulation in the naïve rodent

Deep brain stimulation (DBS) of the deep cerebellar nuclei has been shown to enhance perilesional cortical excitability and promote motor rehabilitation in preclinical models of cortical ischemia and is currently being evaluated in patients with chronic, post-stroke deficits. Understanding the effects of cerebellar DBS on contralateral sensorimotor cortex may be key to developing approaches to optimize stimulation delivery and treatment outcomes. Using the naïve rat model, we characterized the effects of DBS of the lateral cerebellar nucleus (LCN) on somatosensory evoked potentials (SSEPs) and evaluated their potential use as a surrogate index of cortical excitability. SSEPs were recorded concurrently with continuous 30 Hz or 100 Hz LCN DBS and compared to the DBS OFF condition. Ratios of SSEP peak to peak amplitude during 100 Hz LCN DBS to DBS OFF at longer latency peaks were significantly>1, suggesting that cortical excitability was enhanced as a result of LCN DBS. Although changes in SSEP peak to peak amplitudes were observed, they were modest in relation to previously reported effects on motor cortical excitability. Overall, our findings suggest that LCN output influences thalamocortical somatosensory pathways, however further work is need to better understand the potential role of SSEPs in optimizing therapy.

Corticospinal excitability and somatosensory information processing of the lower limb muscle during upper limb voluntary or electrically induced muscle contractions

Neural interactions between upper and lower limbs underlie motor coordination in humans. Specifically, upper limb voluntary muscle contraction can facilitate spinal and corticospinal excitability of the lower limb muscles. However, little remains known on the involvement of somatosensory information in arm-leg neural interactions. Here, we investigated effects of voluntary and electrically induced wrist flexion on corticospinal excitability and somatosensory information processing of the lower limbs. In Experiment 1, we measured transcranial magnetic stimulation (TMS)-evoked motor evoked potentials (MEPs) of the resting soleus (SOL) muscle at rest or during voluntary or neuromuscular electrical stimulation (NMES)-induced wrist flexion. The wrist flexion force was matched to 10% of the maximum voluntary contraction (MVC). We found that SOL MEPs were significantly increased during voluntary, but not NMES-induced, wrist flexion, compared to the rest (P < 0.001). In Experiment 2, we examined somatosensory evoked potentials (SEPs) following tibial nerve stimulation under the same conditions. The results showed that SEPs were unchanged during both voluntary and NMES-induced wrist flexion. In Experiment 3, we examined the modulation of SEPs during 10%, 20%, and 30% MVC voluntary wrist flexion. During 30% MVC voluntary wrist flexion, P50-N70 SEP component was significantly attenuated compared to the rest (P = 0.003). Our results propose that the somatosensory information generated by NMES-induced upper limb muscle contractions may have a limited effect on corticospinal excitability and somatosensory information processing of the lower limbs. However, voluntary wrist flexion modulated corticospinal excitability and somatosensory information processing of the lower limbs via motor areas.

Somatosensory-Evoked Potentials as a Marker of Functional Neuroplasticity in Athletes: A Systematic Review

Background

Somatosensory-evoked potentials (SEP) represent a non-invasive tool to assess neural responses elicited by somatosensory stimuli acquired via electrophysiological recordings. To date, there is no comprehensive evaluation of SEPs for the diagnostic investigation of exercise-induced functional neuroplasticity. This systematic review aims at highlighting the potential of SEP measurements as a diagnostic tool to investigate exercise-induced functional neuroplasticity of the sensorimotor system by reviewing studies comparing SEP parameters between athletes and healthy controls who are not involved in organized sports as well as between athlete cohorts of different sport disciplines.

Methods

A systematic literature search was conducted across three electronic databases (PubMed, Web of Science, and SPORTDiscus) by two independent researchers. Three hundred and ninety-seven records were identified, of which 10 cross-sectional studies were considered eligible.

Results

Differences in SEP amplitudes and latencies between athletes and healthy controls or between athletes of different cohorts as well as associations between SEP parameters and demographic/behavioral variables (years of training, hours of training per week & reaction time) were observed in seven out of 10 included studies. In particular, several studies highlight differences in short- and long-latency SEP parameters, as well as high-frequency oscillations (HFO) when comparing athletes and healthy controls. Neuroplastic differences in athletes appear to be modality-specific as well as dependent on training regimens and sport-specific requirements. This is exemplified by differences in SEP parameters of various athlete populations after stimulation of their primarily trained limb.

Conclusion

Taken together, the existing literature suggests that athletes show specific functional neuroplasticity in the somatosensory system. Therefore, this systematic review highlights the potential of SEP measurements as an easy-to-use and inexpensive diagnostic tool to investigate functional neuroplasticity in the sensorimotor system of athletes. However, there are limitations regarding the small sample sizes and inconsistent methodology of SEP measurements in the studies reviewed. Therefore, future intervention studies are needed to verify and extend the conclusions drawn here.

Modulation of the Somatosensory Evoked Potential by Attention and Spinal Cord Stimulation

Introduction: Spinal Cord Stimulation (SCS) is a last-resort treatment for patients with intractable chronic pain in whom pharmacological and other treatments have failed. Conventional tonic SCS is accompanied by tingling sensations. More recent stimulation protocols like burst SCS are not sensed by the patient while providing similar levels of pain relief. It has been previously reported that conventional tonic SCS can attenuate sensory-discriminative processing in several brain areas, but that burst SCS might have additional effects on the medial, motivational-affective pain system. In this explorative study we assessed the influence of attention on the somatosensory evoked brain responses under conventional tonic SCS as well as burst SCS regime.

Methods: Twelve chronic pain patients with an implanted SCS device had 2-weeks evaluation periods with three different SCS settings (conventional tonic SCS, burst SCS, and sham SCS). At the end of each period, an electro-encephalography (EEG) measurement was done, at which patients received transcutaneous electrical pulses at the tibial nerve to induce somatosensory evoked potentials (SEP). SEP data was acquired while patients were attending the applied pulses and while they were mind wandering. The effects of attention as well as SCS regimes on the SEP were analyzed by comparing amplitudes of early and late latencies at the vertex as well as brain activity at full cortical maps.

Results: Pain relief obtained by the various SCS settings varied largely among patients. Early SEP responses were not significantly affected by attention nor SCS settings (i.e., burst, tonic, and sham). However, late SEP responses (P300) were reduced with tonic and burst SCS: conventional tonic SCS reduced P300 brain activity in the unattended condition, while burst SCS reduced P300 brain activity in both attended and unattended conditions.

Conclusion: Burst spinal cord stimulation for the treatment of chronic pain seems to reduce cortical attention that is or can be directed to somatosensory stimuli to a larger extent than conventional spinal cord stimulation treatment. This is a first step in understanding why in selected chronic pain patients burst SCS is more effective than tonic SCS and how neuroimaging could assist in personalizing SCS treatment.

Transcutaneous spinal direct current stimulation shows no effect on paired stimulation suppression of the somatosensory cortex

Transcutaneous spinal direct current stimulation (tsDCS) is a safe and convenient method of neuromodulation. It has been proven to alter sensory processing at cervicomedullary level by amplitude changes of the P30 response of tibial nerve somatosensory evoked potentials (TN SEPs). With knowledge that tsDCS affects cortical circuits, we hypothesized that tsDCS may also affect intracortical excitability of the somatosensory cortex assessed by paired stimulation suppression (PSS). Fourteen healthy men were included in this prospective, single-blinded, placebo-controlled crossover study. Single (SS) and paired stimulation (PS) TN SEPs were recorded over the scalp before, immediately as well as 30 and 60 min after applying 15 min of tsDCS over the twelfth thoracic vertebra. Each volunteer underwent three independent and randomized sessions of either cathodal, anodal or sham stimulation. tsDCS showed no effect on peak-to-peak amplitudes or latencies of cortical P40-N50 response after SS. Furthermore, tsDCS failed to induce significant changes on amplitude ratios of PSS, thus showing no impact on intracortical excitability of the somatosensory cortex in healthy subjects. Further research is required to reveal the different mechanisms and to strengthen clinical use of this promising technique.

Imagined paralysis alters somatosensory evoked-potentials

Recent studies employing body illusions have shown that multisensory conflict can alter body representations and modulate low-level sensory processing. One defining feature of these body illusions is that they are sensory driven and thus passive on behalf of the participant. Thus, it remained to establish whether explicit alteration of own-body representations modulates low-level sensory processing. We investigated whether tibial nerve somatosensory-evoked potentials were modulated when participants imagined paralysis of their legs and arms. Imagined paralysis of the legs decreased P40 amplitude, but not imagined paralysis of the arms. These results show modulation of early somatosensory processing via explicit, top-down alteration to the internal representation of the body. Interestingly, P40 suppression positively correlated with bodily awareness scores whereas it negatively correlated with body dissociation scores. This suggests that the ability to actively alter own-body representation and its corresponding sensory processing depends upon dispositions to attend to and focus on bodily sensations.

Localized N20 Component of Somatosensory Evoked Magnetic Fields in Frontoparietal Brain Tumor Patients Using Noise-Normalized Approaches

PURPOSE

To localize sensorimotor cortical activation in 10 patients with frontoparietal tumors using quantitative magnetoencephalography (MEG) with noise-normalized approaches.

MATERIAL AND METHODS

Somatosensory evoked magnetic fields (SEFs) were elicited in 10 patients with somatosensory tumors and in 10 control participants using electrical stimulation of the median nerve via the right and left wrists. We localized the N20m component of the SEFs using dynamic statistical parametric mapping (dSPM) and standardized low-resolution brain electromagnetic tomography (sLORETA) combined with 3D magnetic resonance imaging (MRI). The obtained coordinates were compared between groups. Finally, we statistically evaluated the N20m parameters across hemispheres using non-parametric statistical tests.

RESULTS

The N20m sources were accurately localized to Brodmann area 3b in all members of the control group and in seven of the patients; however, the sources were shifted in three patients relative to locations outside the primary somatosensory cortex (SI). Compared with the affected (tumor) hemispheres in the patient group, N20m amplitudes and the strengths of the current sources were significantly lower in the unaffected hemispheres and in both hemispheres of the control group. These results were consistent for both dSPM and sLORETA approaches.

CONCLUSION

Tumors in the sensorimotor cortex lead to cortical functional reorganization and an increase in N20m amplitude and current-source strengths. Noise-normalized approaches for MEG analysis that are integrated with MRI show accurate and reliable localization of sensorimotor function.

Effects of aerobic exercise under different thermal conditions on human somatosensory processing

The present study aimed to investigate the effects of aerobic exercise on human somatosensory processing recorded by somatosensory evoked potentials (SEPs) under temperate [TEMP, 20°C and 40% relative humidity (RH)] and hot (HOT, 35°C and 30% RH) environments. Fifteen healthy subjects performed 4 × 15-min bouts of a moderate cycling exercise [mean power output: 156.5 ± 7.7 (SE) W], with a 10-min rest period and received a posterior tibial nerve stimulation at the left ankle before and after each exercise bout; SEPs were recorded in five sessions; 1st (pre), 2nd (post-1st exercise bout), 3rd (post-2nd exercise bout), 4th (post-3rd exercise bout), and 5th (post-4th exercise bout). The peak latencies and amplitudes of the P37, N50, P60, and N70 components at Cz were evaluated. The latencies of P37, N50, P60, and N70 were significantly shorter with the repetition of aerobic exercise, and these shortened latencies were significantly greater in the HOT condition than in the TEMP condition (P37: 3rd, P < 0.05, and 5th, P < 0.01; P60: 4th, P < 0.05, and 5th, P < 0.01; N70: 4th, P < 0.05, and 5th, P < 0.001). No significant differences were observed in the amplitudes of any SEP component under either thermal condition. These results suggest that the conduction velocity of the ascending somatosensory input was accelerated by increases in body temperature, and aerobic exercise did not alter the strength of neural activity in cortical somatosensory processing.

Magnetoencephalographic study of hand and foot sensorimotor organization in 325 consecutive patients evaluated for tumor or epilepsy surgery

Objectives

The presence of intracranial lesions or epilepsy may lead to functional reorganization and hemispheric lateralization. We applied a clinical magnetoencephalography (MEG) protocol for the localization of the contralateral and ipsilateral S1 and M1 of the foot and hand in patients with non-lesional epilepsy, stroke, developmental brain injury, traumatic brain injury and brain tumors. We investigated whether differences in activation patterns could be related to underlying pathology.

Methods

Using dipole fitting, we localized the sources underlying sensory and motor evoked magnetic fields (SEFs and MEFs) of both hands and feet following unilateral stimulation of the median nerve (MN) and posterior tibial nerve (PTN) in 325 consecutive patients. The primary motor cortex was localized using beamforming following a self-paced repetitive motor task for each hand and foot.

Results

The success rate for motor and sensory localization for the feet was significantly lower than for the hands (motor_hand 94.6% versus motor_feet 81.8%, p < 0.001; sensory_hand 95.3% versus sensory_feet 76.0%, p < 0.001). MN and PTN stimulation activated 86.6% in the contralateral S1, with ipsilateral activation < 0.5%. Motor cortex activation localized contralaterally in 76.1% (5.2% ipsilateral, 7.6% bilateral and 11.1% failures) of all motor MEG recordings. The ipsilateral motor responses were found in 43 (14%) out of 308 patients with motor recordings (range: 8.3–50%, depending on the underlying pathology), and had a higher occurrence in the foot than in the hand (motor_foot 44.8% versus motor_hand 29.6%, p = 0.031). Ipsilateral motor responses tended to be more frequent in patients with a history of stroke, traumatic brain injury (TBI) or developmental brain lesions (p = 0.063).

Conclusions

MEG localization of sensorimotor cortex activation was more successful for the hand compared to the foot. In patients with neural lesions, there were signs of brain reorganization as measured by more frequent ipsilateral motor cortical activation of the foot in addition to the traditional sensory and motor activation patterns in the contralateral hemisphere. The presence of ipsilateral neural reorganization, especially around the foot motor area, suggests that careful mapping of the hand and foot in both contralateral and ipsilateral hemispheres prior to surgery might minimize postoperative deficits.

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Using MEG, S1 and M1 responses of the hand and foot were mapped in patients with brain tumors or epilepsy.

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Localization of the hand was more successful than of the foot.

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Ipsilateral S1 responses were rarely seen but ipsilateral M1 responses differed by underlying pathology and limb.

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Results indicate that differential sensorimotor re-organization can occur in the presence of pathology.

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Ipsilateral and contralateral mapping of the hand and foot should be done to minimize postsurgical dysfunction.

Dipole orientation of receptive fields in the somatosensory cortex after stimulation of the posterior tibial nerve in humans

The origins of the earliest evoked potentials and magnetic fields after tibial nerve electrical stimulation are still controversial. We focused on the initial activity from the gyrus area and analyzed the components for the coronal and sagittal planes. In 12 healthy right-handed subjects, electrical stimuli were delivered to the left posterior tibial nerve at the ankle. The cortical somatosensory evoked fields were recorded, and the equivalent current dipoles were calculated and separated into the sagittal plane (y-components) and coronal plane (x-components) components. In nine subjects, we observed two deflections (y1 and y2) in the y-component waveform and two deflections (x1 and x2) in the x-component waveform over 60 milliseconds; y1 was directed anteriorly, y2 posteriorly, x1 to the left, and x2 to the right. The y1 was originated in the anterior wall of the central sulcus. By focusing on the y-component, we elucidated the existence of the posteroanterior component, which may originate from the area 3b (gyrus) in tibial nerve somatosensory evoked fields and has the same quality as N20m for median nerve somatosensory evoked fields. This is the first report to suggest that the posteroanterior component in the tibial nerve is analogous to N20m in the median nerve using magnetoencephalography.

Preoperative magnetoencephalographic sensory cortex mapping

The use of functional neuroimaging holds the promise of improving neurosurgical outcomes by providing preoperative localization of critical brain functions. The brain representation of somatosensory function can be effectively localized using magnetoencephalography (MEG) in both normal subjects and in patients with tumors, vascular malformation, and epilepsy. This study investigates the pattern of somatosensory localization in 45 patients. Thirty-two of these patients underwent subsequent resective surgery for brain pathologies. Electrical stimulation of the median nerve was conducted, and the most prominent somatosensory peak in the resultant averaged data was localized using the single equivalent current dipole technique. Results showed that this peak localized either to the central or postcentral sulcus of the somatosensory cortex. We found that neither age nor the presence of brain pathologies had significant effect on the recognition of the somatosensory cortex. Patients who underwent surgery after presurgical planning using MEG suffered no new somatosensory deficits, indicating the valuable role of pre-surgical mapping using MEG in the surgical planning.

Theta burst repetitive transcranial magnetic stimulation attenuates somatosensory evoked potentials from the lower limb

Background

Continuous theta burst stimulation (cTBS) is a form of repetitive transcranial magnetic stimulation which has been shown to alter cortical excitability in the upper limb representation of primary somatosensory cortex (SI). However, it is unknown whether cTBS modulates cortical excitability within the lower limb representation in SI. The present study investigates the effects of cTBS over the SI lower limb representation on cortical somatosensory evoked potentials (SEPs) and Hoffmann reflex (H-reflex) following tibial nerve stimulation at the knee. SEPs and H-reflex were recorded before and in four time blocks up to 30 minutes following cTBS targeting the lower limb representation within SI.

Results

Following cTBS, the P1-N1 first cortical potential was significantly decreased at 12–16 minutes. CTBS also suppressed the P2-N2 second cortical potential for up to 30 minutes following stimulation. The H-reflex remained statistically unchanged following cTBS although there was a modest suppression observed.

Conclusion

We conclude that cTBS decreases cortical excitability of the lower limb representation of SI as evidenced by suppressed SEP amplitude. The duration and magnitude of the cTBS after effects are similar to those observed in upper limb studies.

MAGNETOENCEPHALOGRAPHIC STUDY OF POSTERIOR TIBIAL NERVE STIMULATION IN PATIENTS WITH INTRACRANIAL LESIONS AROUND THE CENTRAL SULCUS

OBJECTIVE

To study interhemispheric differences of somatosensory evoked field (SEF) characteristics and the spatial distribution of equivalent current dipole sources in patients with unilateral hemispheric lesions around the central sulcus region.

METHODS

In 17 patients with perirolandic lesions, averaged somatosensory responses after posterior tibial nerve stimulation at the ankle were recorded with magnetoencephalography. Dipole source solutions in the affected (AH) and unaffected (UH) hemispheres were analyzed and compared for latency, equivalent current dipole strength, root mean square, and spatial distribution in relation to clinical findings.

RESULTS

Three main SEF components, P45m, N60m, and P75m, were identified in the hemisphere contralateral to the stimulated nerve. Dipole strength for the P45m component was significantly higher in the AH compared with the UH. SEF characteristics in the AH and UH showed no significant differences with respect to component latency or dipole strength of the N60m and P75m components. Interdipole location asymmetries exceeded 1.0 cm in 71% of the patients. Comparison of the posterior tibial nerve evoked responses (P45m and N60m) in patients with motor deficits and patients without deficits showed that these responses are enlarged in the AH when perirolandic lesions are present. Patients with motor deficits also showed an increased response for P45m in the UH.

CONCLUSION

The results of posterior tibial nerve SEFs suggest spatial and functional changes in the somatosensory network as a result of perirolandic lesions with a possible relationship with clinical symptoms. The results can provide further basis for the evaluation of cortical changes in the presence of perirolandic lesions.

Evidence for early activation of primary motor cortex and SMA after electrical lower limb stimulation using EEG source reconstruction

Compared to median nerve somatosensory evoked potentials (SEP), less is known about activity evoked by nerve stimulation of the lower limb. To understand the mechanisms and the physiology of sensor- and motor control it is useful to investigate the sensorimotor functions as revealed by a standardized functional status. Therefore, we investigated SEPs of the lower limb in 6 healthy male volunteers. For each side, tibial and peroneal nerves were stimulated transcutaneously at the fossa poplitea. The tibial nerves were also stimulated further distally at the ankle joint. Source localization was applied to 64-EEG-channel data of the SEPs. In contrast to somatosensory areas, which are activated after median nerve stimulation, we found dipoles adjacent to motor areas near Brodmann area 4 (BA 4) for SEP components P 32/40 and P 54/60 and near the supplementary motor area (SMA) for the N 75/83 component. These sources could reliably be distinguished for each individual subject as well as for the grand mean data set. Our data show that afferent projections from the lower limb mainly reach primary motor areas (BA 4) and only subsequently, with a delay of 40 ms, higher order motor areas such as SMA. We conclude that a focused view on SEP of the lower limb could be a useful tool to investigate pathological states in motor control or peripheral deafferentiation.

Face representation in the human secondary somatosensory cortex

OBJECTIVE

To investigate the somatotopic organization of the facial skin area in the secondary somatosensory cortex (SII) in humans.

METHODS

Somatosensory evoked magnetic fields following air-puff stimulation of 5 body sites, the foot, the lip and 3 points of the facial skin (forehead, cheek and mandibular angle point), were recorded. We focused on activities in SII following stimulation of these 5 sites and compared dipole locations among them.

RESULTS

There was a clear somatotopic organization in SII with lip in the most lateral area, foot in the most medial area and face in an intermediate area close to the lip area. However, there was no significant difference of dipole localization in SII among the 3 areas of facial skin, similar to the overlapped somatotopic organization of facial skin areas in the primary somatosensory cortex in our previous study.

CONCLUSIONS

The facial skin areas are considered to occupy a small area in SII with insufficient spatial separation to differentiate each area of facial skin even using magnetoencephalography which has a high spatial resolution.

SIGNIFICANCE

This is the first systematic study of the activated regions in SII following stimulation of the facial skin.

Difference in somatosensory evoked fields elicited by mechanical and electrical stimulations: Elucidation of the human homunculus by a noninvasive method

We recently recorded somatosensory evoked fields (SEFs) elicited by compressing the glabrous skin of the finger and decompressing it by using a photosensor trigger. In that study, the equivalent current dipoles (ECDs) for these evoked fields appeared to be physiologically similar to the ECDs of P30m in median nerve stimulation. We sought to determine the relations of evoked fields elicited by mechanically stimulating the glabrous skin of the great toe and those of electrically produced P40m. We studied SEFs elicited by mechanical and electrical stimulations from the median and tibial nerves. The orientations of dipoles from the mechanical stimulations were from anterior-to-posterior, similar to the orientations of dipoles for P30m. The direction of the dipole around the peak of N20m from median nerve electrical stimulation was opposite to these directions. The orientations of dipoles around the peak of P40m by tibial nerve stimulation were transverse, whereas those by the compression and decompression stimulation of the toe were directed from anterior-to-posterior. The concordance of the orientations in ECDs for evoked fields elicited by mechanical and electrical stimulations suggests that the ECDs of P40m are physiologically similar to those of P30m but not to those of N20m. The discrepancy in orientations in ECDs for evoked field elicited by these stimulations in the lower extremity suggests that electrical and compression stimulations elicit evoked fields responding to fast surface rubbing stimuli and/or stimuli to the muscle and joint.

Gating of SEPs by contraction of the contralateral homologous muscle during the preparatory period of self-initiated plantar flexion

To investigate the centrifugal change in somatosensory information processing caused by contraction of the contralateral homologous muscle, we recorded the somatosensory-evoked potentials (SEPs) during the preparatory period of a self-initiated plantar flexion. The SEPs following stimulation of the right tibial nerve at the popliteal fossa were recorded in nine healthy subjects. Self-initiated plantar flexion of the left ankle was performed once every 5 to 7 s. The electrical stimulation was delivered continuously, and the subjects were instructed to concentrate on the movement and not to pay attention to the electrical stimulation. Based on the components of movement-related cortical potential, Bereitschaftspotential (BP) and Negative slope (NS), the preparatory period was divided into four sub-periods (NS, BP-1, BP-2, and Pre-BP). To obtain pre-movement SEPs, the signals following stimulation in each sub-period were averaged. SEPs were attenuated in the preparatory period, especially in the NS sub-period. The amplitude of N40 component was significantly attenuated compared with that in the stationary state and other sub-periods. The amplitude of P53 and N70 was smaller in the NS sub-period than other pre-movement sub-periods. Since there was no centripetal effect on SEPs in the preparatory period, these findings suggested that the activity of motor-related areas modulated the somatosensory information from the contralateral non-movement limb (centrifugal gating). It was assumed that an inhibition on the somatosensory inputs from contralateral limb was caused by the projection via either the corpus callosum or ipsilateral cortico-cortical projections.

Changes in the centrifugal gating effect on somatosensory evoked potentials depending on the level of contractile force

In this study, we investigated the somatosensory evoked potentials (SEPs) during the preparatory period of self-initiated plantar flexion at different force levels of muscle contraction and elucidated the mechanism behind the centrifugal gating effect on somatosensory information processing. We recorded SEPs following stimulation of the tibial nerve at the popliteal fossa during the preparatory period of a 20% maximal voluntary contraction (MVC) and 50% MVC. The preparatory period was divided into two sub-periods based on the components of movement-related cortical potentials, the negative slope (NS sub-period) and the Bereitschaftspotential (BP sub-period). The subjects were instructed to concentrate on the movement and not to pay attention to the continuous electrical stimulation. Pre-movement SEPs were averaged separately during the two sub-periods under each MVC condition. The mean amplitudes of BP and NS were larger during the 50% MVC than the 20% MVC. As for the components of SEPs, during the NS sub-period the amplitude of P30 under the 50% MVC and N40 under both conditions were significantly smaller than that in the stationary sequence, and N40 amplitude was significantly smaller during the 50% MVC than the 20% MVC. During the BP sub-period, the amplitude of P30 and N40 during the 50% MVC was significantly smaller than during the stationary sequence, while it was not significantly different between the 20% and 50% MVCs. In conclusion, the extent of the centrifugal gating effect on SEPs was dependent on the activities of motor-related areas, which generated the NS and BP.

Effect of tactile interference stimulation of the ear in human primary somatosensory cortex: a magnetoencephalographic study

OBJECTIVE

To confirm the somatotopic representation of the ear in the primary somatosensory cortex (SI), we studied the tactile interference effects on somatosensory evoked magnetic fields (SEFs) following stimulation of the ear (Helix, Lobulus and Tragus).

METHODS

We applied tactile interference stimulation to the neck or face area continuously and concurrently while a time-locked electrical stimulation was applied to the ear. If the amplitude would be reduced by the interference, this would indicate that the cortical representation for both the time-locked electrical stimulation and the continuous interference stimulation overlapped. A two or 3-source model, Source 1 in the neck area of SI, Source 2 in the face area of SI, and Source 3 in the secondary somatosensory cortex (SII), was found to be the most appropriate by brain electric source analysis (BESA).

RESULTS

Amplitudes of Sources 1 and 2 in most interference conditions were decreased. Source 1 following stimulation of all 3 sites was significantly reduced when the interference was applied to the neck area. Source 2 following stimulation of all 3 sites was significantly reduced when the interference was applied to the face area.

CONCLUSIONS

These findings showed that the interference effect was found in both the neck and face areas of SI following the ear stimulation.

SIGNIFICANCE

The representation of the ear in SI might be located in both the neck and face areas.

Establishment of standard values for the latency, interval and amplitude parameters of tibial nerve somatosensory evoked potentials (SEPs)

OBJECTIVE

To establish standard values for tibial nerve somatosensory evoked potentials (SEPs).

METHODS

We examined SEPs following left tibial nerve stimulation in 65 normal subjects of various ages, and performed multiple regression analysis using height, age, (age-20)(2) and gender as predictor variables. We objectively selected the latency or interval parameters with less intersubject variability as the standard parameters for evaluation.

RESULTS

Among 3 cortical bipolar derivations investigated, the Cz'-Cc lead gave a more constant and stable P38 component than the Cz'-Fz or Ci-Cc lead. The latencies of the N8o (N8 onset) of the popliteal potential, P15 (P15 peak) in the contralateral iliac crest-ipsilateral greater trochanter lead, N21, N30 and P38o/P38 in the Cz'-Cc lead, as well as the intervals between these components were selected as standard parameters. P15 was easily identified in all of the subjects and is expected to be a new parameter to evaluate the proximal segment of the tibial nerve. The amplitudes of P15 and the other components were also evaluated. We present nomograms for the normal limit values of each parameter.

CONCLUSIONS

We present a thorough set of standard values for tibial SEPs where the subject factors were fully considered, and which is easily applicable to clinical practice.

Functional magnetic resonance imaging evidence for a representation of the ear in human primary somatosensory cortex: comparison with magnetoencephalography study

Our previous study (T. Nihashi et al., 2001, Neuro- Image 13: 295-304), using magnetoencephalography (MEG), revealed somatotopy of the ear in the human primary somatosensory cortex (SI); that is, the signals following stimulation of the ear reach both the neck and face areas of the SI. However, since this was the first report on somatotopy of the ear in humans, we performed an fMRI activation study to confirm the somatotopic representation, and compared the electrical activity by MEG and the cerebral blood flow change by fMRI. We studied eight healthy subjects using 3-T MRI. We stimulated three parts of the left ear: the helix, the lobulus, and the tragus. First, we identified the location of the ear area in the SI based on our previous MEG study, in which equivalent current dipoles (ECDs) were located in the neck and/or face areas of the SI. Then, we determined the search volume as a sphere with a 15-mm radius, which was placed in the neck and/or face area. We analyzed whether or not fMRI activation occurred inside such spheres. Stimulation of the helix activated the neck area of the SI in four of eight subjects, and both the neck and face areas in two. No activation was observed in two subjects. Stimulation of the lobulus activated the neck area in one subject, the face area in two, both in four, and neither in one. Stimulation of the tragus activated the face in four, both in three, and neither in one. These fMRI findings confirm the result of MEG that the representation of the ear in the SI is separated into neck and face areas.

Brain activation sequences following electrical limb stimulation of normal and paraplegic subjects

In current clinical practice the degree of paraplegia or quadriplegia is objectively determined with transcranial magnetic stimulation (TMS) and somatosensory-evoked potentials (SSEP). We measured the MEG signal following electrical stimulation of upper and lower limbs in two normal and three clinically complete paraplegic subjects. From the MEG signal we computed distributed estimates of brain activity and identified foci just behind the central sulcus consistent in location with primary somatosensory (SI) for arm and foot and secondary somatosensory (SII) areas. Activation curves were computed from regions of interest defined around these areas. Activation of the SI foot area was observed in normal and paraplegic subjects when the upper limb was stimulated. Surprisingly, for each paraplegic subject, stimulation below the lesion was followed by cortical activations. These activations were weak, only loosely time-locked to the stimulus and were seen intermittently behind the central sulcus and nearby cortical areas. Statistical analysis of tomographic solutions and activation curves showed consistent responses following foot stimulation in one paraplegic (PS1) and intermittently in another paraplegic subject. We repeated the same experiment for PS1 in a different laboratory and the results from the analysis of foot stimulation from both laboratories revealed statistically significant focal cortical response only in the contralateral SI foot area.

Dynamic activation of distinct cytoarchitectonic areas of the human SI cortex after median nerve stimulation

MEG recordings visualized non-invasively a serial mediolateral activation of the human somatosensory 3b area followed by a stationary activation of area 1 after median nerve stimulation. Somatosensory evoked fields (SEFs) were recorded over the hand area contralateral to the right median nerve stimulation at the wrist in six normal subjects. A newly developed MEG vector beamformer technique applied to the SEFs revealed two distinct sources (areas 3b and 1) in the primary somatosensory cortex (SI) during the primary N20m-P22m response in all subjects. The first source was located in area 3b, which started to move sequentially toward mediolateral direction 0.7 ms prior to the peak of N20m and ended its movement 1.4 ms after the peak with a total distance of 11.2 mm. We speculate that the movement reflects a sequential mediolateral activation of the pyramidal cells in area 3b, which is mediated by horizontal connections running parallel to the cortical surface. The second source in area 1, located 5.6 mm medial and 4.2 mm posterior to the first source, was active 1.0 ms after the N20m peak. Then, the first source became inactive and the second source was dominant. In sharp contrast with the first source, the second source was stationary. The different behavior of these two components (moving vs stationary) indicates independent parallel inputs to area 3b and area 1 from the thalamus.

Serial activation of distinct cytoarchitectonic areas of the human S1 cortex after posterior tibial nerve stimulation

MEG recordings visualized non-invasively a dynamic anterior-posterior activation in the pyramidal cell population of the human primary somatosensory cortex (S1) after posterior tibial nerve stimulation. Somatosensory evoked fields (SEFs) were recorded over the foot area in response to right posterior tibial nerve stimulation at the ankle in six normal subjects. A newly developed MEG vector beamformer technique applied to the SEFs revealed two distinct sources in the mesial wall of the left hemisphere around the primary P37m response typically separated by 1.3 cm. The first source was located in area 3b and oriented toward the contralateral hemisphere. The second source was assumed to be in an area near the marginal sulcus and the source orientation was directed posteriorly. The first source began to be active during the initial slope of the P37m. The second source was active after the P37m peak and the signal intensities of the first and second sources were equal at a mean latency of 2.6 ms after the peak of P37m. Then the first source became inactive and the second source was dominant after about 5 ms post-P37m peak. These findings suggest that a single peaked posterior tibial nerve P37m consists of partially overlapping two subcomponents generated in area 3b and an area near the marginal sulcus.

Analysis of pain-related somatosensory evoked magnetic fields using the MUSIC (multiple signal classification) algorithm for magnetoencephalography

We evaluated the effectiveness of the Multiple Signal Classification (MUSIC) algorithm by analysing pain-related somatosensory-evoked magnetic fields (SEFs) by 148-channel whole-head-type magnetoencephalography. MUSIC peaks of middle latency components were located around the primary somatosensory cortex (SI), contralateral to the stimulated finger. Long latency components were located around the bilateral secondary somatosensory cortices (SII) and cingulate gyri. Peaks at the SII and cingulate gyri were more prominent on very painful and moderately painful stimulation than on weak stimulation. The results were in very good agreement with results from single dipole estimation. These findings suggest that the MUSIC algorithm could be a useful tool for analysis of pain-related SEFs.

Representation of the ear in human primary somatosensory cortex

We studied 13 healthy subjects with a multichannel magnetoencephalography (MEG) system to investigate the somatotopic representation of the ear in the primary somatosensory cortex (SI). We stimulated three parts of the left ear: the helix, the lobulus, and the tragus. The somatosensory-evoked magnetic fields (SEFs) were successfully measured in 7 of 13 subjects. Short-latency responses were analyzed using both single dipole and multidipole models (brain electric source analysis, BESA). From the single dipole model, the equivalent current dipole (ECD) following the helix stimulation was estimated to be near the neck area of SI in all the subjects. In the lobulus stimulation, the ECDs were estimated around the neck area of SI in four subjects, in the face area in one subject, and in the deep white matter in two subjects. In the tragus stimulation, the ECDs were estimated around the neck area of SI in three subjects, in the hand area of SI in two subjects, and in the deep white matter in two subjects. When the ECDs were estimated to be located in unlikely sites (hand area and deep white matter), a two-dipole model, (1) the neck area of SI and (2) face area of SI, was found to be the most appropriate. Although this might be a preliminary study due to a relatively small number of subjects, it revealed that receptive fields of some part of the ear, such as the lobulus and tragus, might be present in both the neck and face areas of SI. These findings suggested that the "ear area" of SI has variability between subjects, unlike the other areas of SI, possibly because the ear is located on the border between the neck and face.

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.

Effects of check size on pattern reversal visual evoked magnetic field and potential

The effects of different check sizes on the 100m component of pattern reversal visual evoked magnetic fields (VEF) and the P100 component of visual evoked potentials (VEP) in terms of latency, amplitude and source localization were analyzed. Half field stimuli with or without central occlusion with check sizes of 15', 30', 60', 90' and 180' of visual arc were given to 7 healthy subjects. VEF and VEP were recorded simultaneously. The effect of the check size on the peak latency of both 100m and P100 was significant (P<0.01, ANOVA). The latencies for the smaller checks were significantly longer than those for the larger checks. The effect of the check size on the amplitude of the 100m to the stimulation with central occlusion was significant (P<0.05, ANOVA), but was not to the stimulation without central occlusion. That is, the amplitudes for the smaller checks were significantly smaller than those for the larger checks when using the stimulation with central occlusion, but not the stimulation without central occlusion. The effect of the check size on the P100 amplitude was not significant to the stimulation with and without central occlusion. The equivalent current dipoles were located around the calcarine fissure and did not differ significantly in location with check size. In conclusion, check size significantly affects the latency and amplitude of the 100m and/or P100, but not the receptive areas for the stimulation.

Neuroanatomic substrates of lower extremity somatosensory evoked potentials

After stimulation of the lower extremity nerve (tibial nerve), N21 and N23 are recorded from L4 and T12 spine respectively. The far-field potentials of P31 and N35 are registered from Fpz-C5s (fifth cervical spine) or CPi (ipsilateral with respect to the side of stimulation)-ear derivation. Additional far-field potentials of P17 and P24 may be recorded from the scalp when a noncephalic (knee) reference is used. The major positive peak, P40, is registered at the vertex and the CPi. Preceding P40, there is a small negative peak, N37, recorded at the contralateral (CPc) hemisphere. Neuroanatomic substrates of these somatosensory evoked potential (SSEP) components are less well clarified compared with those of upper extremity (median nerve) SSEPs, primarily because clinical application of lower extremity SSEPs is more difficult, and all of the aforementioned potentials but one (P40) are not obligatory components. The concept of "paradoxical lateralization" complicates the issue further. Accumulating evidence, however, suggests that the far-field potentials of P17 and P31 arise from the distal portion of the sacral plexus and brainstem respectively. These correspond to P9 and P14 of the median nerve SSEPs respectively. The spinal potential of N23 is equivalent to the N13 cervical potential of the median nerve SSEP. N35 recorded from the ipsilateral hemisphere is analogous to N18 of the median nerve. Paradoxically lateralized P40 has been thought to represent the positive end of a dipole field, reflected by the negativity at the mesial surface of the contralateral hemisphere, and has commonly been considered to be equivalent to the first cortical potentials (N20) of the median nerve SSEP. However, more recent evidence suggests that the primary positivity is at the mesial cortical surface, and it more likely corresponds to P26 of the median nerve SSEP. Thus the first cortical potential corresponding to N20 is probably a small and inconsistent N37 recorded on the contralateral hemisphere. These assumptions need to be verified further by more extensive clinical studies applied to various neurologic disorders.

Somatosensory, auditory, and visual evoked magnetic fields in patients with brain diseases

The features of somatosensory (SEFs), auditory (AEFs), and visual evoked fields (VEFs) in healthy subjects and patients with brain diseases provide the basis for clinical investigations using magnetoencephalography (MEG). The SEFs provide clinically useful information to identify the central sulcus and somatotopic organization of the primary somatosensory cortex. Localization accuracy of the SEFs can be tested by cortical stimulation during surgery. Functional reorganization suggested by SEF studies must be verified by other modalities. The AEFs can localize the auditory cortex in the bilateral temporal lobes. Separation of bilateral activities is much clearer in AEFs than in auditory evoked potentials. Modulation of the interhemispheric differences of latency, amplitude, and source localization of AEFs can be used to evaluate auditory function in patients with intracranial lesions. Pattern reversal VEFs provide stable localization of the primary visual function. Separation of bihemispherical activities is the advantage of VEFs over visual evoked potentials. Investigation of VEFs provides objective evaluation of visual field deficits such as homonymous or bitemporal hemianopsia in patients with intracranial lesions. Evoked magnetic fields can provide useful diagnostic information. Such clinical findings, in turn, provides the opportunity to test the source estimation accuracy of MEG.

Dermatome versus homunculus; detailed topography of the primary somatosensory cortex following trunk stimulation

OBJECTIVE

Identification of a detailed topography of the receptive area for each of the thoracic dermatomes in humans using somatosensory evoked magnetic fields (SEF).

METHODS

We analyzed the location of the equivalent current dipole (ECD) of SEF following electrical stimulation of the skin at Th4, Th6, Th8, Th10 and Th12 dermatomes in 14 normal subjects.

RESULTS

Three deflections, M18, M25 and M40, were obtained within 60 ms of stimulation of Th6, Th8 and Th10 dermatomes. No consistent deflection could be identified following Th4 and Th12 dermatomal stimulation, probably due to a poor signal-to-noise ratio and difficulty in fixing the stimulation electrodes. M18 was absent or small in amplitude. The latency of M25 ranged from short to long in the order Th6, Th8 and Th10 (P<0.05). ECDs of all components for each site stimulation were located in the truncal area of the primary somatosensory cortex. Although the locations of the ECDs tend to be arranged from lateral to medial in the sequence Th6, Th8 and Th10, the difference was not significant.

CONCLUSION

The representation area of the trunk is small, and the receptive areas for the stimulation of Th6, Th8 and Th10 dermatomes are considered to be very close or to overlap.

Antidromic corticospinal tract potential of the brain

OBJECTIVE

To describe a novel potential component (antidromic corticospinal tract potential, ACSP) of the brain after translaminar spinal stimulation of a relaxed patient during scoliosis surgery. To study the origin of this component and to compare its source to known sources of the somatosensory evoked potentials (SEPs).

METHODS

We studied 17 consecutive patients during posterior scoliosis surgery. SEPs and ACSPs were elicited by translaminar spinal stimulation at the Th 2 and L 1 levels. ACSPs and SEPs were recorded on the scalp midline. Neurogenic motor evoked potentials (NMEPs) were recorded on the popliteal spaces. Preoperative tibial SEPs were also recorded.

RESULTS

ACSP was distinctly separated from the corresponding spinally evoked cortical SEP that showed longer latency than the ACSP. ACSPs decreased and disappeared when stimulation was moved to the caudal direction in the conus region while SEP persisted. In addition, the hemispheric origin of ACSP was confirmed with multichannel midline recordings of the scalp and neck. Thus there was no confusion to the response of nucleus gracilis, corresponding the P 31 response of the tibial nerve SEP.

CONCLUSIONS

The origin of ACSP seemed to be in the rostral part of the corticospinal tract. ACSP diminished in the conus region when stimulation was moved caudally and it disappeared when the stimulus was given to the root level. This proves that ACSP is not a response of the somatosensory tract, instead ACSP represents antidromic response of the pyramidal tract. ACSP can be used in monitoring of the motor tracts during scoliosis surgery together with NMEPs.

Comparison of somatosensory evoked high-frequency oscillations after posterior tibial and median nerve stimulation

OBJECTIVE

We compared the high-frequency oscillations (HFOs) evoked by posterior tibial nerve (PTN) and median nerve (MN) stimulation.

METHODS

Somatosensory evoked potentials (SEPs) were recorded with a filter set at 10-2000 Hz to right PTN and to right MN stimulation in 10 healthy subjects. The HFOs were obtained by digitally filtering the wide-band SEPs with a band-pass of 300-900 Hz.

RESULTS

HFOs were recorded in 8 of the 10 subjects for PTN, and in all subjects for MN stimulation. The HFOs after both PTN and MN stimulation started approximately at or after the onset of the primary cortical response (P37 and N20) and ended around the middle of the second slope. HFO amplitudes and area after PTN stimulation were significantly smaller than those after MN stimulation. HFO duration after PTN stimulation was markedly longer than that after MN stimulation. However, HFO interpeak latencies did not differ between the two nerves.

CONCLUSIONS

The present findings suggest that the HFOs after PTN and MN stimulation reflect a neural mechanism common to the hand and foot somatosensory cortex.

Magnetoencephalography in the study of human somatosensory cortical processing

Magnetoencephalography (MEG) is a totally non-invasive research method which provides information about cortical dynamics on a millisecond time-scale. Whole-scalp magnetic field patterns following stimulation of different peripheral nerves indicate activation of an extensive cortical network. At the SI cortex, the responses reflect mainly the activity of area 3b, with clearly somatotopical representations of different body parts. The SII cortex is activated bilaterally and it also receives, besides tactile input, nociceptive afference. Somatically evoked MEG signals may also be detected from the posterior parietal cortex, central mesial cortex and the frontal lobe. The serial versus parallel processing in the cortical somatosensory network is still under debate.

Effects of visual and auditory stimulation on somatosensory evoked magnetic fields

DESIGN AND METHODS

We investigated the effects of continuous visual (cartoon and random dot motion) and auditory (music) stimulation on somatosensory evoked magnetic fields (SEFs) following electrical stimulation of the median nerve on 12 normal subjects using paired t test and two way ANOVA for the statistics.

RESULTS

In the hemisphere contralateral to the stimulated nerve, the middle-latency components (35-60 ms in latency) were significantly enhanced by visual, but not by auditory stimulation. The dipoles of all components within 60-70 ms following stimulation were estimated to be very close each other, around the hand area of the primary sensory cortex (SI). In the ipsilateral hemisphere, the middle-latency components (70-100 ms in latency), the dipoles of which were estimated to be in the second sensory cortex (SII), were markedly decreased in amplitude by both the visual and auditory stimulation.

CONCLUSIONS

These changes in waveform by visual and auditory stimulation are thought to be due to the effects of the activation of polymodal neurons, which receive not only somatosensory but also visual and/or auditory inputs, in areas 5 and/or 7 as well as in the medial superior temporal region (MST) and superior temporal sulcus (STS), although a change of attention might also be a factor causing such findings.

Brain electrical source analysis of primary cortical components of the tibial nerve somatosensory evoked potential using regional sources

Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings revealed a source in the foot area of the contralateral hemisphere. We now investigated this paradoxical lateralization by performing a brain electrical source analysis in the P40 time window (34-46 ms). The tibial nerve was stimulated behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verified electrode positions (bandpass 0.5-500 Hz). In each case, the P40 amplitude was higher ipsilaterally (0.45 +/- 0.14 microV) than contralaterally (-0.49 +/- 0.16 microV). The best fitting regional source, however, was always located in the contralateral hemisphere with a mean distance of 8.2 +/- 4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a frontal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation compared to the tangential P37 dipole, it is systematically underestimated in MEG. Dipole orientations explained interindividual variability of scalp potential distribution. SEP amplitudes were smaller when generated in the dominant (left) hemisphere. This is explained by deeper located sources (5.4 +/- 1.6 mm) with a more tangential orientation (delta theta = 17.5 +/- 2.3 degrees) in the left hemisphere.

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.

Somatosensory homunculus as drawn by MEG

We studied a detailed somatosensory representation map of the human primary somatosensory cortex using magnetoencephalography. Somatosensory-evoked magnetic fields following tactile stimulation of multiple points in the right hemibody (including the tongue, lips, fingers, arm, trunk, leg, and foot) were analyzed in five normal subjects. We were able to estimate equivalent current dipoles (ECDs) following stimulation of the tongue, lips, fingers, palm, forearm, elbow, upper arm, and toes in most subjects and those following the stimulation of the chest, ankle, and thigh in one subject. The ECDs were located in the postcentral gyrus and generally arranged in order along the central sulcus, which is compatible with the somatosensory "homunculus." Linear distances, averaged in five subjects, from the receptive area of the thumb to that of the tongue, little finger, forearm, upper arm, and toes were estimated to be 2.42 +/- 0.28, 1.25 +/- 0.28, 2.21 +/- 0.72, 2.75 +/- 0.63, and 5.29 +/- 0.48 cm, respectively. The moment of each ECD, which suggested the size of the cortical areas responsive to the stimulation, was also compatible with the bizarre proportion of the homunculus with a large tongue, lips, and fingers. According to these results, we were able to reproduce a large part of the somatosensory homunculus quantitatively on an individual brain MRI.

Generators of short latency human somatosensory-evoked potentials recorded over the spine and scalp

Somatosensory evoked potentials (SEPs) are most commonly obtained after stimulation of the median nerve and the posterior tibial nerve. SEPs reflect conduction of the afferent volley along the peripheral nerve, dorsal columns, and medial lemniscal pathways to the primary somatosensory cortex. Short-latency SEPs are recorded over the spine and scalp. After posterior tibial nerve stimulation, the following waveforms are recorded: N22, W3, the dorsal column volley, N29, P31, N34, and P37. After median nerve stimulation, the brachial plexus volley, dorsal column volley (N11), N13, P14, N18, N20, and P22 potentials are recorded. We discuss the current state of knowledge about the generators of these SEPs. Such information is crucial for proper interpretation of SEP abnormalities.

Effects of tactile interference stimulation on somatosensory evoked magnetic fields following tibial nerve stimulation

We studied the effects of interfering tactile stimulation applied to the foot ipsilateral and contralateral to the stimulation on somatosensory evoked magnetic fields (SEFs) following tibial nerve stimulation at the ankle. Equivalent current dipoles (ECDs) of all 4 components, 1M-4M, in all sessions were estimated to be very close each other, around the foot area of the primary sensory cortex (SI). The 1M, 2M and 4M components were significantly reduced in amplitude by the ipsilateral-foot interference, and we consider that this phenomenon is due mainly to 'saturation' of the neurons in area 3b of the SI. In contrast, the 3M component was significantly enhanced in amplitude by the contralateral-foot interference. We suspect that this result was due to the effects of neuronal activities in areas 2, 5 and/or 7, which receive inputs from both sides of the body, i.e. to 'bilateral function'. Considering the various types of interference effects on SEFs and somatosensory evoked potentials (SEPs) observed in not only the present, but also in the previous studies, we conclude that both SEFs and SEPs following tibial nerve stimulation are generated mainly by ascending signals mediated by cutaneous fibers of the peripheral nerves rather than the muscle afferents.

Somatosensory evoked magnetic fields following passive finger movement

The somatosensory evoked magnetic field (SEF) following passive finger movement and electrical stimulation of finger was studied in 10 normal subjects. Four main components were identified in SEFs recorded at the hemisphere contralateral to the moved finger: 1M(P), 2M(P), 3M(P) and 4M(P). The 1M(P) was clearly identified only in three subjects and was smaller than other components. The equivalent current dipoles (ECDs) of 1M(P) were located around the finger area of the primary sensorimotor cortex and oriented either posteriorly or anteriorly. We speculate that it was generated in areas 3a or 2 of the primary sensory cortex. The 2M(P) and 3M(P) were usually combined as one large deflection with two peaks. Because the ECDs of 2M(P) and 3M(P) were located around the finger area of the sensorimotor cortex and both oriented posteriorly, they were considered to be generated in area 4 and/or 3b, and their activities have temporal overlapping. The 4M(P) has large inter-individual difference in terms of amplitude and latency. The ECD of 4M(P) was also located around the finger area of the primary sensorimotor cortex, and oriented anteriorly. The 4M(PI), the main component recorded from the hemisphere ipsilateral to the moved finger, was located in the upper bank of the sylvian fissure, probably the second sensory cortex (SII). Five components, 1M(E), 2M(E), 3M(E), 4M(E) and 4M(EI), corresponding to 1M(P), 2M(P), 3M(P), 4M(P) and 4M(PI), were identified following electrical stimulation of the same finger. However, SEFs following passive movement were clearly different from SEFs following electrical stimulation, in terms of waveforms and source locations, probably due to differences of ascending fibers and receptive fields.