Experimental analysis of distortion of magnetoencephalography signals by the skull

Magnetoencephalography for Epilepsy Presurgical Evaluation

PURPOSE OF THE REVIEW

Magnetoencephalography (MEG) is a functional neuroimaging technique that records neurophysiology data with millisecond temporal resolution and localizes it with subcentimeter accuracy. Its capability to provide high resolution in both of these domains makes it a powerful tool both in basic neuroscience as well as clinical applications. In neurology, it has proven useful in its ability to record and localize epileptiform activity. Epilepsy workup typically begins with scalp electroencephalography (EEG), but in many situations, EEG-based localization of the epileptogenic zone is inadequate. The complementary sensitivity of MEG can be crucial in such cases, and MEG has been adopted at many centers as an important resource in building a surgical hypothesis. In this paper, we review recent work evaluating the extent of MEG influence of presurgical evaluations, novel analyses of MEG data employed in surgical workup, and new MEG instrumentation that will likely affect the field of clinical MEG.

RECENT FINDINGS

MEG consistently contributes to presurgical evaluation and these contributions often change the plan for epilepsy surgery. Extensive work has been done to develop new analytic methods for localizing the source of epileptiform activity with MEG. Systems using optically pumped magnetometry (OPM) have been successfully deployed to record and localize epileptiform activity. MEG remains an important noninvasive tool for epilepsy presurgical evaluation. Continued improvements in analytic methodology will likely increase the diagnostic yield of the test. Novel instrumentation with OPM may contribute to this as well, and may increase accessibility of MEG by decreasing cost.

Microscopic-scale magnetic recording of brain neuronal electrical activity using a diamond quantum sensor

Quantum sensors using solid state qubits have demonstrated outstanding sensitivity, beyond that possible using classical devices. In particular, those based on colour centres in diamond have demonstrated high sensitivity to magnetic field through exploiting the field-dependent emission of fluorescence under coherent control using microwaves. Given the highly biocompatible nature of diamond, sensing from biological samples is a key interdisciplinary application. In particular, the microscopic-scale study of living systems can be possible through recording of temperature and biomagnetic field. In this work, we use such a quantum sensor to demonstrate such microscopic-scale recording of electrical activity from neurons in fragile living brain tissue. By recording weak magnetic field induced by ionic currents in mouse corpus callosum axons, we accurately recover signals from neuronal action potential propagation while demonstrating in situ pharmacology. Our sensor allows recording of the electrical activity in neural circuits, disruption of which can shed light on the mechanisms of disease emergence. Unlike existing techniques for recording activity, which can require potentially damaging direct interaction, our sensing is entirely passive and remote from the sample. Our results open a promising new avenue for the microscopic recording of neuronal signals, offering the eventual prospect of microscopic imaging of electrical activity in the living mammalian brain.

Biomagnetism: The First Sixty Years

Biomagnetism is the measurement of the weak magnetic fields produced by nerves and muscle. The magnetic field of the heart-the magnetocardiogram (MCG)-is the largest biomagnetic signal generated by the body and was the first measured. Magnetic fields have been detected from isolated tissue, such as a peripheral nerve or cardiac muscle, and these studies have provided insights into the fundamental properties of biomagnetism. The magnetic field of the brain-the magnetoencephalogram (MEG)-has generated much interest and has potential clinical applications to epilepsy, migraine, and psychiatric disorders. The biomagnetic inverse problem, calculating the electrical sources inside the brain from magnetic field recordings made outside the head, is difficult, but several techniques have been introduced to solve it. Traditionally, biomagnetic fields are recorded using superconducting quantum interference device (SQUID) magnetometers, but recently, new sensors have been developed that allow magnetic measurements without the cryogenic technology required for SQUIDs.

Insular Involvement in Cases of Epilepsy Surgery Failure

Background: Epilepsy surgery failure is not uncommon, with several explanations having been proposed. In this series, we detail cases of epilepsy surgery failure subsequently attributed to insular involvement. Methods: We retrospectively identified patients investigated at the epilepsy monitoring units of two Canadian tertiary care centers (2004–2020). Included patients were adults who had undergone epilepsy surgeries with recurrence of seizures post-operatively and who were subsequently determined to have an insular epileptogenic focus. Clinical, electrophysiological, neuroimaging, and surgical data were synthesized. Results: We present 14 patients who demonstrated insular epileptic activity post-surgery-failure as detected by intracranial EEG, MEG, or seizure improvement after insular resection. Seven patients had manifestations evoking possible insular involvement prior to their first surgery. Most patients (8/14) had initial surgeries targeting the temporal lobe. Seizure recurrence ranged from the immediate post-operative period to one year. The main modality used to determine insular involvement was MEG (8/14). Nine patients underwent re-operations that included insular resection; seven achieved a favorable post-operative outcome (Engel I or II). Conclusions: Our series suggests that lowering the threshold for suspecting insular epilepsy may be necessary to improve epilepsy surgery outcomes. Detecting insular epilepsy post-surgery-failure may allow for re-operations which may lead to good outcomes.

New avenues for functional neuroimaging: ultra-high field MRI and OPM-MEG

Functional brain imaging technology has developed rapidly in recent years. On the one hand, high-field 7-Tesla magnetic resonance imaging (MRI) has excelled the limited spatial resolution of 3-Tesla MRI, allowing us to enter a new world of mesoscopic imaging from the macroscopic imaging of human brain functions. On the other hand, novel optical pumping magnetometer-magnetoencephalography (OPM-MEG) has broken down the technical barriers of traditional superconducting MEG, which brings imaging of neuronal electromagnetic signals from cortical imaging to whole-brain imaging. This article aims to present a brief introduction regarding the development of conventional MRI and MEG technology, and, more importantly, to delineate that high-field MRI and OPM-MEG complement each other and together will lead us into a new era of functional brain imaging.

Mapping the human auditory cortex using spectrotemporal receptive fields generated with magnetoencephalography

We present a novel method to map the functional organization of the human auditory cortex noninvasively using magnetoencephalography (MEG). More specifically, this method estimates via reverse correlation the spectrotemporal receptive fields (STRF) in response to a temporally dense pure tone stimulus, from which important spectrotemporal characteristics of neuronal processing can be extracted and mapped back onto the cortex surface. We show that several neuronal populations can be found examining the spectrotemporal characteristics of their STRFs, and demonstrate how these can be used to generate tonotopic gradient maps. In doing so, we show that the spatial resolution of MEG is sufficient to reliably extract important information about the spatial organization of the auditory cortex, while enabling the analysis of complex temporal dynamics of auditory processing such as best temporal modulation rate and response latency given its excellent temporal resolution. Furthermore, because spectrotemporally dense auditory stimuli can be used with MEG, the time required to acquire the necessary data to generate tonotopic maps is significantly less for MEG than for other neuroimaging tools that acquire BOLD-like signals.

Influence of unfused cranial bones on magnetoencephalography signals in human infants

OBJECTIVE

To clarify the effects of unfused cranial bones on magnetoencephalography (MEG) signals during early development.

METHODS

In a simulation study, we compared the MEG signals over a spherical head model with a circular hole mimicking the anterior fontanel to those over the same head model without the fontanel for different head and fontanel sizes with varying skull thickness and conductivity.

RESULTS

The fontanel had small effects according to three indices. The sum of differences in signal over a sensor array due to a fontanel, for example, was < 6% of the sum without the fontanel. However, the fontanel effects were extensive for dipole sources deep in the brain or outside the fontanel for larger fontanels. The effects were comparable in magnitude for tangential and radial sources. Skull thickness significantly increased the effect, while skull conductivity had minor effects.

CONCLUSION

MEG signal is weakly affected by a fontanel. However, the effects can be extensive and significant for radial sources, thicker skull and large fontanels. The fontanel effects can be intuitively explained by the concept of secondary sources at the fontanel wall.

SIGNIFICANCE

The minor influence of unfused cranial bones simplifies MEG analysis, but it should be considered for quantitative analysis.

The Effects of Transcranial Electrical Stimulation on Human Motor Functions: A Comprehensive Review of Functional Neuroimaging Studies

Transcranial electrical stimulation (tES) is a promising tool to enhance human motor skills. However, the underlying physiological mechanisms are not fully understood. On the other hand, neuroimaging modalities provide powerful tools to map some of the neurophysiological biomarkers associated with tES. Here, a comprehensive review was undertaken to summarize the neuroimaging evidence of how tES affects human motor skills. A literature search has been done on the PubMed database, and 46 relative articles were selected. After reviewing these articles, we conclude that neuroimaging techniques are feasible to be coupled with tES and offer valuable information of cortical excitability, connectivity, and oscillations regarding the effects of tES on human motor behavior. The biomarkers derived from neuroimaging could also indicate the motor performance under tES conditions. This approach could advance the understanding of tES effects on motor skill and shed light on a new generation of adaptive stimulation models.

Variation in Reported Human Head Tissue Electrical Conductivity Values

Electromagnetic source characterisation requires accurate volume conductor models representing head geometry and the electrical conductivity field. Head tissue conductivity is often assumed from previous literature, however, despite extensive research, measurements are inconsistent. A meta-analysis of reported human head electrical conductivity values was therefore conducted to determine significant variation and subsequent influential factors. Of 3121 identified publications spanning three databases, 56 papers were included in data extraction. Conductivity values were categorised according to tissue type, and recorded alongside methodology, measurement condition, current frequency, tissue temperature, participant pathology and age. We found variation in electrical conductivity of the whole-skull, the spongiform layer of the skull, isotropic, perpendicularly- and parallelly-oriented white matter (WM) and the brain-to-skull-conductivity ratio (BSCR) could be significantly attributed to a combination of differences in methodology and demographics. This large variation should be acknowledged, and care should be taken when creating volume conductor models, ideally constructing them on an individual basis, rather than assuming them from the literature. When personalised models are unavailable, it is suggested weighted average means from the current meta-analysis are used. Assigning conductivity as: 0.41 S/m for the scalp, 0.02 S/m for the whole skull, or when better modelled as a three-layer skull 0.048 S/m for the spongiform layer, 0.007 S/m for the inner compact and 0.005 S/m for the outer compact, as well as 1.71 S/m for the CSF, 0.47 S/m for the grey matter, 0.22 S/m for WM and 50.4 for the BSCR.

Hypersynchrony in MEG spectral amplitude in prospectively-identified 6-month-old infants prenatally exposed to alcohol

Early identification of children who experience developmental delays due to prenatal alcohol exposure (PAE) remains a challenge for individuals who do not exhibit facial dysmorphia. It is well-established that children with PAE may still exhibit the cognitive and behavioral difficulties, and individuals without facial dysmorphia make up the majority of individuals affected by PAE. This study employed a prospective cohort design to capture alcohol consumption patterns during pregnancy and then followed the infants to 6 months of age. Infants were assessed using magnetoencephalography to capture neurophysiological indicators of brain development and the Bayley Scales of Infant Development-III to measure behavioral development. To account for socioeconomic and family environmental factors, we employed a two-by-two design with pregnant women who were or were not using opioid maintenance therapy (OMT) and did or did not consume alcohol during pregnancy. Based on prior studies, we hypothesized that infants with PAE would exhibit broad increased spectral amplitude relative to non-PAE infants. We also hypothesized that the developmental shift from low to high frequency spectral amplitude would be delayed in infants with PAE relative to controls. Our results demonstrated broadband increased spectral amplitude, interpreted as hypersynchrony, in PAE infants with no significant interaction with OMT. Unlike prior EEG studies in neonates, our results indicate that this hypersynchrony was highly lateralized to left hemisphere and primarily focused in temporal/lateral frontal regions. Furthermore, there was a significant positive correlation between estimated number of drinks consumed during pregnancy and spectral amplitude revealing a dose-response effect of increased hypersynchrony corresponding to greater alcohol consumption. Contrary to our second hypothesis, we did not see a significant group difference in the contribution of low frequency to high frequency amplitude at 6 months of age. These results provide new evidence that hypersynchrony, previously observed in neonates prenatally exposed to high levels of alcohol, persists until 6 months of age and this measure is detectable with low to moderate exposure of alcohol with a dose-response effect. These results indicate that hypersynchrony may provide a sensitive early marker of prenatal alcohol exposure in infants up to 6 months of age.

Current and Emerging Potential of Magnetoencephalography in the Detection and Localization of High-Frequency Oscillations in Epilepsy

Up to one-third of patients with epilepsy are medically intractable and need resective surgery. To be successful, epilepsy surgery requires a comprehensive preoperative evaluation to define the epileptogenic zone (EZ), the brain area that should be resected to achieve seizure freedom. Due to lack of tools and methods that measure the EZ directly, this area is defined indirectly based on concordant data from a multitude of presurgical non-invasive tests and intracranial recordings. However, the results of these tests are often insufficiently concordant or inconclusive. Thus, the presurgical evaluation of surgical candidates is frequently challenging or unsuccessful. To improve the efficacy of the surgical treatment, there is an overriding need for reliable biomarkers that can delineate the EZ. High-frequency oscillations (HFOs) have emerged over the last decade as new potential biomarkers for the delineation of the EZ. Multiple studies have shown that HFOs are spatially associated with the EZ. Despite the encouraging findings, there are still significant challenges for the translation of HFOs as epileptogenic biomarkers to the clinical practice. One of the major barriers is the difficulty to detect and localize them with non-invasive techniques, such as magnetoencephalography (MEG) or scalp electroencephalography (EEG). Although most literature has studied HFOs using invasive recordings, recent studies have reported the detection and localization of HFOs using MEG or scalp EEG. MEG seems to be particularly advantageous compared to scalp EEG due to its inherent advantages of being less affected by skull conductivity and less susceptible to contamination from muscular activity. The detection and localization of HFOs with MEG would largely expand the clinical utility of these new promising biomarkers to an earlier stage in the diagnostic process and to a wider range of patients with epilepsy. Here, we conduct a thorough critical review of the recent MEG literature that investigates HFOs in patients with epilepsy, summarizing the different methodological approaches and the main findings. Our goal is to highlight the emerging potential of MEG in the non-invasive detection and localization of HFOs for the presurgical evaluation of patients with medically refractory epilepsy (MRE).

BabyMEG: A whole-head pediatric magnetoencephalography system for human brain development research

We developed a 375-channel, whole-head magnetoencephalography (MEG) system ("BabyMEG") for studying the electrophysiological development of human brain during the first years of life. The helmet accommodates heads up to 95% of 36-month old boys in the USA. The unique two-layer sensor array consists of: (1) 270 magnetometers (10 mm diameter, ∼15 mm coil-to-coil spacing) in the inner layer, (2) thirty-five three-axis magnetometers (20 mm × 20 mm) in the outer layer 4 cm away from the inner layer. Additionally, there are three three-axis reference magnetometers. With the help of a remotely operated position adjustment mechanism, the sensor array can be positioned to provide a uniform short spacing (mean 8.5 mm) between the sensor array and room temperature surface of the dewar. The sensors are connected to superconducting quantum interference devices (SQUIDs) operating at 4.2 K with median sensitivity levels of 7.5 fT/√Hz for the inner and 4 fT/√Hz for the outer layer sensors. SQUID outputs are digitized by a 24-bit acquisition system. A closed-cycle helium recycler provides maintenance-free continuous operation, eliminating the need for helium, with no interruption needed during MEG measurements. BabyMEG with the recycler has been fully operational from March, 2015. Ongoing spontaneous brain activity can be monitored in real time without interference from external magnetic noise sources including the recycler, using a combination of a lightly shielded two-layer magnetically shielded room, an external active shielding, a signal-space projection method, and a synthetic gradiometer approach. Evoked responses in the cortex can be clearly detected without averaging. These new design features and capabilities represent several advances in MEG, increasing the utility of this technique in basic neuroscience as well as in clinical research and patient studies.

Current Source Mapping by Spontaneous MEG and ECoG in Piglets Model

The previous research reveals the presence of relatively strong spatial correlations from spontaneous activity over cortex in Electroencephalography (EEG) and Magnetoencephalography (MEG) measurement. A critical obstacle in MEG current source mapping is that strong background activity masks the relatively weak local information. In this paper, the hypothesis is that the dominant components of this background activity can be captured by the first Principal Component (PC) after employing Principal Component Analysis (PCA), thus discarding the first PC before the back projection would enhance the exposure of the information carried by a subset of sensors that reflects the local neuronal activity. By detecting MEG signals densely (one measurement per 2×2 mm²) in three piglets neocortical models over an area of 18×26 mm² with a special shape of lesion by means of a μSQUID, this basic idea was demonstrated by the fact that a strong activity could be imaged in the lesion region after removing the first PC in Delta, Theta and Alpha band, while the original recordings did not show such activity clearly. Thus, the PCA decomposition can be employed to expose the local activity, which is around the lesion in the piglets' neocortical models, by removing the dominant components of the background activity.

Combined Invasive Subcortical and Non-invasive Surface Neurophysiological Recordings for the Assessment of Cognitive and Emotional Functions in Humans

In spite of the success in applying non-invasive electroencephalography (EEG), magneto-encephalography (MEG) and functional magnetic resonance imaging (fMRI) for extracting crucial information about the mechanism of the human brain, such methods remain insufficient to provide information about physiological processes reflecting cognitive and emotional functions at the subcortical level. In this respect, modern invasive clinical approaches in humans, such as deep brain stimulation (DBS), offer a tremendous possibility to record subcortical brain activity, namely local field potentials (LFPs) representing coherent activity of neural assemblies from localized basal ganglia or thalamic regions. Notwithstanding the fact that invasive approaches in humans are applied only after medical indication and thus recorded data correspond to altered brain circuits, valuable insight can be gained regarding the presence of intact brain functions in relation to brain oscillatory activity and the pathophysiology of disorders in response to experimental cognitive paradigms. In this direction, a growing number of DBS studies in patients with Parkinson's disease (PD) target not only motor functions but also higher level processes such as emotions, decision-making, attention, memory and sensory perception. Recent clinical trials also emphasize the role of DBS as an alternative treatment in neuropsychiatric disorders ranging from obsessive compulsive disorder (OCD) to chronic disorders of consciousness (DOC). Consequently, we focus on the use of combined invasive (LFP) and non-invasive (EEG) human brain recordings in assessing the role of cortical-subcortical structures in cognitive and emotional processing trough experimental paradigms (e.g. speech stimuli with emotional connotation or paradigms of cognitive control such as the Flanker task), for patients undergoing DBS treatment.

Skull Defects in Finite Element Head Models for Source Reconstruction from Magnetoencephalography Signals

Magnetoencephalography (MEG) signals are influenced by skull defects. However, there is a lack of evidence of this influence during source reconstruction. Our objectives are to characterize errors in source reconstruction from MEG signals due to ignoring skull defects and to assess the ability of an exact finite element head model to eliminate such errors. A detailed finite element model of the head of a rabbit used in a physical experiment was constructed from magnetic resonance and co-registered computer tomography imaging that differentiated nine tissue types. Sources of the MEG measurements above intact skull and above skull defects respectively were reconstructed using a finite element model with the intact skull and one incorporating the skull defects. The forward simulation of the MEG signals reproduced the experimentally observed characteristic magnitude and topography changes due to skull defects. Sources reconstructed from measured MEG signals above intact skull matched the known physical locations and orientations. Ignoring skull defects in the head model during reconstruction displaced sources under a skull defect away from that defect. Sources next to a defect were reoriented. When skull defects, with their physical conductivity, were incorporated in the head model, the location and orientation errors were mostly eliminated. The conductivity of the skull defect material non-uniformly modulated the influence on MEG signals. We propose concrete guidelines for taking into account conducting skull defects during MEG coil placement and modeling. Exact finite element head models can improve localization of brain function, specifically after surgery.

Brain connectivity in autism spectrum disorder

PURPOSE OF REVIEW

Many studies have reported that individuals with autism spectrum disorder (ASD) have different brain connectivity patterns compared with typically developing individuals. However, the results of more recent studies do not unanimously support the traditional view in which individuals with ASD have lower connectivity between distant brain regions and increased connectivity within local brain regions. In this review, we discuss different methods for measuring brain connectivity and how the use of different metrics may contribute to the lack of convergence of investigations of connectivity in ASD.

RECENT FINDINGS

The discrepancy in brain connectivity results across studies may be due to important methodological factors, such as the connectivity measure applied, the age of patients studied, the brain region(s) examined, and the time interval and frequency band(s) in which connectivity was analyzed.

SUMMARY

We conclude that more sophisticated electroencephalography analytic approaches should be utilized to more accurately infer causation and directionality of information transfer between brain regions, which may show dynamic changes of functional connectivity in the brain. Moreover, further investigations of connectivity with respect to behavior and clinical phenotype are needed to probe underlying brain networks implicated in core deficits of ASD.

Invariance in current dipole moment density across brain structures and species: physiological constraint for neuroimaging

Although anatomical constraints have been shown to be effective for MEG and EEG inverse solutions, there are still no effective physiological constraints. Strength of the current generator is normally described by the moment of an equivalent current dipole Q. This value is quite variable since it depends on size of active tissue. In contrast, the current dipole moment density q, defined as Q per surface area of active cortex, is independent of size of active tissue. Here we studied whether the value of q has a maximum in physiological conditions across brain structures and species. We determined the value due to the primary neuronal current (q primary) alone, correcting for distortions due to measurement conditions and secondary current sources at boundaries separating regions of differing electrical conductivities. The values were in the same range for turtle cerebellum (0.56-1.48 nAm/mm(2)), guinea pig hippocampus (0.30-1.34 nAm/mm(2)), and swine neocortex (0.18-1.63 nAm/mm(2)), rat neocortex (~2.2 nAm/mm(2)), monkey neocortex (~0.40 nAm/mm(2)) and human neocortex (0.16-0.77 nAm/mm(2)). Thus, there appears to be a maximum value across the brain structures and species (1-2 nAm/mm(2)). The empirical values closely matched the theoretical values obtained with our independently validated neural network model (1.6-2.8 nAm/mm(2) for initial spike and 0.7-3.1 nAm/mm(2) for burst), indicating that the apparent invariance is not coincidental. Our model study shows that a single maximum value may exist across a wide range of brain structures and species, varying in neuron density, due to fundamental electrical properties of neurons. The maximum value of q primary may serve as an effective physiological constraint for MEG/EEG inverse solutions.

Auditory evoked fields measured noninvasively with small-animal MEG reveal rapid repetition suppression in the guinea pig

In animal models, single-neuron response properties such as stimulus-specific adaptation have been described as possible precursors to mismatch negativity, a human brain response to stimulus change. In the present study, we attempted to bridge the gap between human and animal studies by characterising responses to changes in the frequency of repeated tone series in the anesthetised guinea pig using small-animal magnetoencephalography (MEG). We showed that 1) auditory evoked fields (AEFs) qualitatively similar to those observed in human MEG studies can be detected noninvasively in rodents using small-animal MEG; 2) guinea pig AEF amplitudes reduce rapidly with tone repetition, and this AEF reduction is largely complete by the second tone in a repeated series; and 3) differences between responses to the first (deviant) and later (standard) tones after a frequency transition resemble those previously observed in awake humans using a similar stimulus paradigm.

The use of magnetoencephalography in the study of psychopharmacology (pharmaco-MEG)

Magnetoencephalography (MEG) is a neuroimaging technique that allows direct measurement of the magnetic fields generated by synchronised ionic neural currents in the brain with moderately good spatial resolution and high temporal resolution. Because chemical neuromodulation can cause changes in neuronal processing on the millisecond time-scale, the combination of MEG with pharmacological interventions (pharmaco-MEG) is a powerful tool for measuring the effects of experimental modulations of neurotransmission in the living human brain. Importantly, pharmaco-MEG can be used in both healthy humans to understand normal brain function and in patients to understand brain pathologies and drug-treatment effects. In this paper, the physiological and technical basis of pharmaco-MEG is introduced and contrasted with other pharmacological neuroimaging techniques. Ongoing developments in MEG analysis techniques such as source-localisation, functional and effective connectivity analyses, which have allowed for more powerful inferences to be made with recent pharmaco-MEG data, are described. Studies which have utilised pharmaco-MEG across a range of neurotransmitter systems (GABA, glutamate, acetylcholine, dopamine and serotonin) are reviewed.

In vivo assessment of human brain oscillations during application of transcranial electric currents

Brain oscillations reflect pattern formation of cell assemblies’ activity, which is often disturbed in neurological and psychiatric diseases like depression, schizophrenia and stroke. In the neurobiological analysis and treatment of these conditions, transcranial electric currents applied to the brain proved beneficial. However, the direct effects of these currents on brain oscillations have remained an enigma because of the inability to record them simultaneously. Here we report a novel strategy that resolves this problem. We describe accurate reconstructed localization of dipolar sources and changes of brain oscillatory activity associated with motor actions in primary cortical brain regions undergoing transcranial electric stimulation. This new method allows for the first time direct measurement of the effects of non-invasive electrical brain stimulation on brain oscillatory activity and behavior.

Magnetoencephalography signals are influenced by skull defects

OBJECTIVE

Magnetoencephalography (MEG) signals had previously been hypothesized to have negligible sensitivity to skull defects. The objective is to experimentally investigate the influence of conducting skull defects on MEG and EEG signals.

METHODS

A miniaturized electric dipole was implanted in vivo into rabbit brains. Simultaneous recording using 64-channel EEG and 16-channel MEG was conducted, first above the intact skull and then above a skull defect. Skull defects were filled with agar gels, which had been formulated to have tissue-like homogeneous conductivities. The dipole was moved beneath the skull defects, and measurements were taken at regularly spaced points.

RESULTS

The EEG signal amplitude increased 2-10 times, whereas the MEG signal amplitude reduced by as much as 20%. The EEG signal amplitude deviated more when the source was under the edge of the defect, whereas the MEG signal amplitude deviated more when the source was central under the defect. The change in MEG field-map topography (relative difference measure, RDM(∗)=0.15) was geometrically related to the skull defect edge.

CONCLUSIONS

MEG and EEG signals can be substantially affected by skull defects.

SIGNIFICANCE

MEG source modeling requires realistic volume conductor head models that incorporate skull defects.

Effects of sutures and fontanels on MEG and EEG source analysis in a realistic infant head model

In infants, the fontanels and sutures as well as conductivity of the skull influence the volume currents accompanying primary currents generated by active neurons and thus the associated electroencephalography (EEG) and magnetoencephalography (MEG) signals. We used a finite element method (FEM) to construct a realistic model of the head of an infant based on MRI images. Using this model, we investigated the effects of the fontanels, sutures and skull conductivity on forward and inverse EEG and MEG source analysis. Simulation results show that MEG is better suited than EEG to study early brain development because it is much less sensitive than EEG to distortions of the volume current caused by the fontanels and sutures and to inaccurate estimates of skull conductivity. Best results will be achieved when MEG and EEG are used in combination.

Magnetoencephalography: From SQUIDs to neuroscience. Neuroimage 20th anniversary special edition

Magnetoencephalography (MEG), with its direct view to the cortex through the magnetically transparent skull, has developed from its conception in physics laboratories to a powerful tool of basic and clinical neuroscience. MEG provides millisecond time resolution and allows real-time tracking of brain activation sequences during sensory processing, motor planning and action, cognition, language perception and production, social interaction, and various brain disorders. Current-day neuromagnetometers house hundreds of SQUIDs, superconducting quantum interference devices, to pick up signals generated by concerted action of cortical neurons. Complementary MEG measures of neuronal involvement include evoked responses, modulation of cortical rhythms, properties of the on-going neural activity, and interareal connectivity. Future MEG breakthroughs in understanding brain dynamics are expected through advanced signal analysis and combined use of MEG with hemodynamic imaging (fMRI). Methodological development progresses most efficiently when linked with insightful neuroscientific questions.

Simultaneous EEG and MEG source reconstruction in sparse electromagnetic source imaging

Electroencephalography (EEG) and magnetoencephalography (MEG) have different sensitivities to differently configured brain activations, making them complimentary in providing independent information for better detection and inverse reconstruction of brain sources. In the present study, we developed an integrative approach, which integrates a novel sparse electromagnetic source imaging method, i.e., variation-based cortical current density (VB-SCCD), together with the combined use of EEG and MEG data in reconstructing complex brain activity. To perform simultaneous analysis of multimodal data, we proposed to normalize EEG and MEG signals according to their individual noise levels to create unit-free measures. Our Monte Carlo simulations demonstrated that this integrative approach is capable of reconstructing complex cortical brain activations (up to 10 simultaneously activated and randomly located sources). Results from experimental data showed that complex brain activations evoked in a face recognition task were successfully reconstructed using the integrative approach, which were consistent with other research findings and validated by independent data from functional magnetic resonance imaging using the same stimulus protocol. Reconstructed cortical brain activations from both simulations and experimental data provided precise source localizations as well as accurate spatial extents of localized sources. In comparison with studies using EEG or MEG alone, the performance of cortical source reconstructions using combined EEG and MEG was significantly improved. We demonstrated that this new sparse ESI methodology with integrated analysis of EEG and MEG data could accurately probe spatiotemporal processes of complex human brain activations. This is promising for noninvasively studying large-scale brain networks of high clinical and scientific significance.

Genetic influence demonstrated for MEG-recorded somatosensory evoked responses

We tested for a genetic influence on magnetoencephalogram (MEG)-recorded somatosensory evoked fields (SEFs) in 20 monozygotic (MZ) and 14 dizygotic (DZ) twin pairs. Previous electroencephalogram (EEG) studies that demonstrated a genetic contribution to evoked responses generally focused on characteristics of representative brain potentials. Here we demonstrate significantly smaller amplitude differences within MZ compared to DZ twin pairs for the complete SEF time series (across left and right hand SEFs: 0.37 vs. 0.60 pT(2) and 0.28 vs. 0.39 pT(2) for primary [SI] and secondary [SII] sensory cortex activation) and higher MZ than DZ wave shape correlations (.71 vs. .44 and .52 vs. .35 for SI and SII activation). Our findings indicate a genetic influence on MEG-recorded evoked brain activity and also confirm our recent conclusion (van 't Ent, van Soelen, Stam, De Geus, & Boomsma, 2009) that higher MZ resemblance for EEG amplitudes is not trivially reflecting greater MZ concordance in intervening biological tissues.

Variable anisotropic brain electrical conductivities in epileptogenic foci

Source localization models assume brain electrical conductivities are isotropic at about 0.33 S/m. These assumptions have not been confirmed ex vivo in humans. This study determined bidirectional electrical conductivities from pediatric epilepsy surgery patients. Electrical conductivities perpendicular and parallel to the pial surface of neocortex and subcortical white matter (n = 15) were measured using the 4-electrode technique and compared with clinical variables. Mean (±SD) electrical conductivities were 0.10 ± 0.01 S/m, and varied by 243% from patient to patient. Perpendicular and parallel conductivities differed by 45%, and the larger values were perpendicular to the pial surface in 47% and parallel in 40% of patients. A perpendicular principal axis was associated with normal, while isotropy and parallel principal axes were linked with epileptogenic lesions by MRI. Electrical conductivities were decreased in patients with cortical dysplasia compared with non-dysplasia etiologies. The electrical conductivity values of freshly excised human brain tissues were approximately 30% of assumed values, varied by over 200% from patient to patient, and had erratic anisotropic and isotropic shapes if the MRI showed a lesion. Understanding brain electrical conductivity and ways to non-invasively measure them are probably necessary to enhance the ability to localize EEG sources from epilepsy surgery patients.

Magnetoencephalography

Although magnetoencephalography (MEG) may not be familiar to many pediatric radiologists, it is an increasingly available neuroimaging technique both for evaluating normal and abnormal intracranial neural activity and for functional mapping. By providing spatial, temporal, and time-frequency spectral information, MEG affords patients with epilepsy, intracranial neoplasia, and vascular malformations an opportunity for a sensitive and accurate non-invasive preoperative evaluation. This technique can optimize selection of surgical candidates as well as increase confidence in preoperative counseling and prognosis. Research applications that appear promising for near-future clinical translation include the evaluation of children with autism spectrum disorder, traumatic brain injury, and schizophrenia.

Strong resemblance in the amplitude of oscillatory brain activity in monozygotic twins is not caused by "trivial" similarities in the composition of the skull

Previous twin studies have shown strong heritability of electroencephalogram amplitude characteristics, such as power spectra. However, it has been suggested that these high heritabilities may reflect "trivial" twin resemblance in intervening tissues such as the skull. Here we demonstrate strong monozygotic twin correlation (0.79 < r < 0.88) of eyes-closed resting-state magnetoencephalogram power, which is insensitive to intervening tissues. These results confirm that brain activity itself is highly heritable.

A MEG investigation of somatosensory processing in the rhesus monkey

The use of minimally and non-invasive neuroimaging methods in animal models has sharply increased over the past decade. Such studies have enhanced understanding of the neural basis of the physical signals quantified by these tools, and have addressed an assortment of fundamental and otherwise intractable questions in neurobiology. To date, these studies have almost exclusively utilized positron-emission tomography or variants of magnetic resonance based imaging. These methods provide largely indirect measures of brain activity and are strongly reliant on intact vasculature and normal blood-flow, which is known to be compromised in many clinical conditions. The current study provides the first demonstration of whole-head magnetoencephalography (MEG), a non-invasive and direct measure of neuronal activity, in a rhesus monkey, and in the process supplies the initial data on systems-level dynamics in somatosensory cortices. An adult rhesus monkey underwent three separate studies of tactile stimulation on the pad of the right second or fifth digit as whole-head MEG data were acquired. The neural generators of the primary neuromagnetic components were localized using an equivalent-current-dipole model. Second digit stimulation produced an initial cortical response peaking approximately 16 ms after stimulus onset in the contralateral somatosensory cortices, with a later response at approximately 96 ms in an overlapping or nearby neural area with a roughly orthogonal orientation. Stimulation of the fifth digit produced similar results, the main exception being a substantially weaker later response. We believe the 16 ms response is likely the monkey homologue of the human M50 response, as both are the earliest cortical response and localize to the contralateral primary somatosensory area. Thus, these data suggest that mechanoreception in nonhuman primates operates substantially faster than that in adult humans. More broadly, these results demonstrate that it is feasible to use current human whole-head MEG instrumentation to record neuromagnetic responses in adult rhesus monkeys. Nonhuman primate models of human disease provide the closest phylogenetic link to humans. The present, non-invasive imaging study could promote exciting translational integration of invasive animal studies and non-invasive human studies, allowing experimentally induced deficits and pharmacological treatments to be interpreted in light of resulting brain network interactions.

Quantification of the benefit from integrating MEG and EEG data in minimum l2-norm estimation

Source current estimation from electromagnetic (MEG and EEG) signals is an ill-posed problem that often produces blurry or inaccurately positioned estimates. The two modalities have distinct factors limiting the resolution, e.g., MEG cannot detect radially oriented sources, while EEG is sensitive to accuracy of the head model. This makes combined EEG+MEG estimation techniques desirable, but different acquisition noise statistics, complexity of the head models, and lack of pertinent metrics all complicate the assessment of the resulting improvements. We investigated analytically the effect of including EEG recordings in MEG studies versus the addition of new MEG channels when computing noise-normalized minimum l(2)-norm estimates. Three-compartment boundary-element forward models were constructed using structural MRI scans for four subjects. Singular value analysis of the resulting forward models predicted better performance of the EEG+MEG case in the form of higher matrix rank. MNE inverse operators for EEG, MEG and EEG+MEG were constructed using the sensor noise covariance estimated from data. Metrics derived from the resolution matrices predicted higher spatial resolution in EEG+MEG as compared to MEG due to decreased spread (lower spatial dispersion, higher resolution index) with no reduction in dipole localization error. The effect was apparent in all source locations, with increased magnitude for deep areas such as the cingulate cortex. We were also able to corroborate the results for the somatosensory cortex using median nerve responses.

Utility of Magnetoencephalography in the Evaluation of Recurrent Seizures after Epilepsy Surgery

PURPOSE

To study the role of magnetoencephalography (MEG) in the surgical evaluation of children with recurrent seizures after epilepsy surgery.

METHODS

We studied 17 children with recurrent seizures after epilepsy surgery using interictal and ictal scalp EEG, intracranial video EEG (IVEEG), MRI, and MEG. We analyzed the location and distribution of MEG spike sources (MEGSSs) and the relationship of MEGSSs to the margins of previous resections and surgical outcome.

RESULTS

Clustered MEGSSs occurred at the margins of previous resections within two contiguous gyri in 10 patients (group A), extended spatially from a margin by < or =3 cm in three patients (group B), and were remote from a resection margin by >3 cm in six patients (group C). Two patients had concomitant group A and C clusters. Thirteen patients underwent second surgeries. IVEEG was used in four patients. Six of seven patients with group A MEGSS clusters did not require IVEEG for second surgeries. Follow-up periods ranged from 0.6 to 4.3 years (mean: 2.6 years). Eleven children, including eight who became seizure-free, achieved Engel class I or II.

CONCLUSION

Our data demonstrate the utility of MEG for evaluating patients with recurrent seizures after epilepsy surgery. Specific MEGSS cluster patterns delineate epileptogenic zones. Removing cluster regions adjacent to the margins of previous resections, in addition to removing recurrent lesions, achieves favorable surgical outcome. Cluster location and extent identify which patients require IVEEG, potentially eliminating IVEEG for some. Patients with remotely located clusters require IVEEG for accurate assessment and localization of the entire epileptogenic zone.

Magnetoencephalography: In search of neural processes for visual motion information

Magnetoencephalography (MEG) has become a standard approach to the investigation of human brain functions. This review starts with a brief review of the human visual system and studies on visual motion detection mechanisms is followed by the presentation of MEG studies that have contributed to the field. Emphasis is placed on the fact that because the neural activities measured in functional magnetic resonance imaging (fMRI) differ substantially from those measured in MEG--fMRI data cannot be used directly to estimate MEG signal sources. The basic ideas behind the methods of signal processing and analyses generally used in MEG studies are described and theoretical considerations of the neural mechanisms determining MEG response latency and amplitude changes are discussed. Here, scalar fields theory is proposed to explain MEG responses to incoherent motions, and the ways in which detection of complex motions such as transparency, rotation and expansion can be explained by this theory are also presented. Relationships between human behavioral reaction time and MEG response latency suggest a new concept underlying the reasons why humans are late in detecting slow motion.

Magnetoencephalographic spike sources associated with auditory auras in paediatric localisation-related epilepsy

OBJECTIVE

To characterise magnetoencephalographic spike sources in paediatric patients with auditory auras and recurrent localisation-related epilepsy.

METHODS

Six patients (four boys and two girls (ages 7-14 years) were retrospectively studied. All patients had auditory auras as part of their initial seizure manifestation, including four patients who underwent previous brain surgery. Scalp video electroencephalography and magnetoencephalography (MEG) were carried out in six patients, intraoperative electrocorticography in three patients and extraoperative intracranial video electroencephalography in one patient. MEG auditory-evoked fields (AEFs) were studied in four patients.

RESULTS

Three patients had elementary auditory auras, one had complex auditory aura and two had both complex and elementary auras. All six patients had clustered MEG spike sources with coexisting scattered spike sources. MEG clusters were localised in the superior temporal gyrus with surrounding scatters in four patients (two left and two right); two patients had scattered spikes in the superior temporal gyrus in addition to clustered MEG spike sources in the left inferior and middle frontal gyri or parieto-occipital region. AEFs were located within an MEG cluster in one patient and within 3 cm of a cluster in two patients. Surgical resection, including the regions of MEG clusters, was carried out in four patients. Three of four patients who had previous surgeries were seizure free at 2 years after excision of the MEG cluster region.

CONCLUSIONS

MEG spike sources clustered in the superior temporal gyrus in six patients with auditory auras. These spike sources were in close proximity or seemed to engulf the magnetic AEF. Areas with MEG spike sources contained the residual or recurrent epileptogenic zone after incomplete cortical excision for lesional epilepsy.

Immaturity of somatosensory cortical processing in human newborns

The development of the early component of somatosensory evoked potentials (SEPs) from the neonatal N1 to adult N20 response has previously been described. The main emphasis has been on the change in the response latency during maturation. We used magnetoencephalography (MEG) to characterize the cortical generators of the N1 and the subsequent response in healthy human newborns. Furthermore, we studied the maturation of tactile processing according to responses evoked by tactile stimulation of the index finger in newborns, 6-month-old babies and adults. This study provides evidence of specific differences in the somatosensory processing in neonates compared to that in adults. Although the initial cortical response to electrical median nerve stimulation in the newborns was similar in field distribution to the corresponding N20m in adults, the subsequent major deflection in the response waveform had the opposite polarity. Similar immaturity in cortical processing was seen in the tactile evoked fields in both the newborns and the 6-month-old infants compared with the adults. Our results indicate that although the somatosensory pathway in full-term newborns is sufficiently developed to supply the brain with tactile information, the cortical neuronal networks for processing the input may not function in the same way as in adults.

Electrical conductivities of the freshly excised cerebral cortex in epilepsy surgery patients; correlation with pathology, seizure duration, and diffusion tensor imaging

The electrical conductivities (sigma) of freshly excised neocortex and subcortical white matter were studied in the frequency range of physiological relevance for EEG (5-1005 Hz) in 21 patients (ages 0.67 to 55 years) undergoing epilepsy neurosurgery. Surgical patients were classified as having cortical dysplasia (CD) or non-CD pathologies. Diffusion tensor imaging (DTI) for apparent diffusion coefficient (ADC) and fractional anisotropy (FA) was obtained in 9 patients. Results found that electrical conductivities in freshly excised neocortex vary significantly from patient to patient (sigma = 0.0660-0.156 S/m). Cerebral cortex from CD patients had increased conductivities compared with non-CD cases. In addition, longer seizure durations positively correlated with conductivities for CD tissue, while they negatively correlated for non-CD tissue. DTI ADC eigenvalues inversely correlated with electrical conductivity in CD and non-CD tissue. These results in a small initial cohort indicate that electrical conductivity of freshly excised neocortex from epilepsy surgery patients varies as a consequence of clinical variables, such as underlying pathology and seizure duration, and inversely correlates with DTI ADC values. Understanding how disease affects cortical electrical conductivity and ways to non-invasively measure it, perhaps through DTI, could enhance the ability to localize EEG dipoles and other relevant information in the treatment of epilepsy surgery patients.

Effects of temporary bilateral ligation of the internal carotid arteries on the low- and high-frequency somatic evoked potentials in the swine

OBJECTIVE

We studied effects of a temporary bilateral ligation of the internal carotid arteries on the subcortical and cortical structures of the somatosensory system by examining the thalamic input and postsynaptic cortical responses contained in the somatic evoked potentials (SEPs) recorded from the primary somatosensory cortex (SI) of the juvenile piglets in vivo. We predicted that the ligation should differentially affect these structures due to differences in blood supply.

METHODS

The SEPs between 1 and 3000 Hz were measured in the SI cortex with a multichannel electrode array before, during and after a 20 min bilateral ligation of the internal carotid arteries in the swine under a barbiturate anesthesia.

RESULTS

The ligation differentially affected the thalamic input and the cortical responses contained in the high-frequency signals (HFSs) between 400 and 2000 Hz. The amplitude of the thalamic input did not change, but the amplitudes of the cortical HFS postsynaptic to the thalamic inputs decreased immediately after start of ligation, recovering over the next 30-90 min. The latency showed a small, but significant increase for several minutes after the start of ligation for both the thalamic input and cortical responses. The ligation increased the latency and reduced the amplitude of the peak of the first cortical response in the wideband SEP corresponding to human N20.

CONCLUSIONS

The HFS is useful for distinguishing selective effects of the temporary ligation on the subcortical and cortical structures of the somatosensory system. Since the porcine N20 starts after the presynaptic HFS, it was not useful in differentiating thalamic and cortical effects.

SIGNIFICANCE

The HFS may open a new window in studying the cortical physiology in humans.

Effects of alcohol on spontaneous neuronal oscillations: a combined magnetoencephalography and electroencephalography study

Electroencephalography (EEG) and magnetoencephalography (MEG) can detect different aspects of alcohol effects on auditory processing measured with event-related potentials and magnetic fields. The present study aimed to detect alcohol-induced changes in spontaneous neuronal oscillations with combined EEG and MEG techniques. The effects of alcohol on spontaneous neuronal rhythms were studied in 12 healthy subjects after 0.8 g/kg alcohol or juice in a double-blind, placebo-controlled, cross-over design using simultaneous high-resolution MEG and EEG in eyes-open and eyes-closed conditions. The data were analyzed with a power spectral density analysis. MEG recording showed that alcohol significantly increased the relative power of alpha rhythm (8-10 Hz) and reduced the relative power of beta activity (17-25 Hz) in both left and right hemispheres, but only in the eyes-closed condition. These effects did not depend on gender. No analogous statistically significant changes were observed in EEG rhythms. However, the power of alpha and beta rhythms was positively correlated in MEG and EEG recordings, indicating that MEG and EEG reflect similar processes. A distinct sensitivity of MEG and EEG to the sources of cortical oscillations, a better signal-to-noise ratio of MEG, as well as strong spatial blurring of potentials in EEG are most likely the reasons for the observed differences in the effects of alcohol on spontaneous oscillations as detected with two methods.

Evaluation of the distortion of EEG signals caused by a hole in the skull mimicking the fontanel in the skull of human neonates

OBJECTIVE

Interpretation of Electroencephalography (EEG) signals from newborns is in some cases difficult because the fontanels and open sutures produce inhomogeneity in skull conductivity. We experimentally determined how EEG is influenced by a hole mimicking the anterior fontanel since distortion of EEG signals is important in neurological examinations during the perinatal period.

METHODS

Experiments were carried out on 10 anesthetized farm swine. The fontanel was mimicked by a hole (12 x 12 mm) in the skull. The hole was filled with 3 types of medium differing in conductivity (air, 0 S/m; sucrose-agar, 0.017 S/m; saline-agar, 1.28 S/m). Three positions of the snout were stimulated with a concentric bipolar electrode to activate cortical areas near the middle, the edge, and the outside of the hole. The somatic-evoked potential (SEP) was recorded by a 4 x 4 electrode array with a 4mm grid spacing. It was placed on the 4 quadrants of a 28 x 28 mm measurement area on a saline-soaked filter paper over the skull, which served as artificial scalp.

RESULTS

The SEP over the hole was clearly stronger when the hole was filled with sucrose- or saline-agar as compared to air, although paradoxically the leakage current was stronger for the sucrose- than saline-agar. The current leaking from the hole was strongly related to position of the active tissue. It was nearly negligible for sources 6-10 mm away from the border of the hole. The distortion was different for 3 components of the SEP elicited by each stimulus, probably indicating effects of source distance relative to the hole.

CONCLUSIONS

EEG is strongly distorted by the presence of a hole/fontanel with the distortion specifically dependent on both conductivity of the hole and source location.

SIGNIFICANCE

The distortion of the EEG is in contrast to the lack of distortion of magnetoencephalography (MEG) signals shown by previous studies. In studying brain development with EEG, the infant's head and sources should be modeled accurately in order to relate the signals to the underlying activity. MEG may be particularly advantageous over EEG for studying brain functions in infants since it is relatively insensitive to skull defects.

Origins of the somatic N20 and high-frequency oscillations evoked by trigeminal stimulation in the piglets

OBJECTIVE

In humans, the somatic evoked potentials (SEPs) and magnetic fields (SEFs) elicited by peripheral nerve stimulation contain high-frequency oscillations (HFOs) around 600 Hz superimposed on the initial cortical response N20. Responses elicited by snout stimulation in the swine also contain similar HFOs during the rising phase of the porcine N20. This study examined the generators of the N20 and HFOs in the swine.

METHODS

We recorded intracortical SEPs and multi-unit activities in the sulcal area of the primary somatosensory cortex (SI) simultaneously with SEFs. The laminar profiles of the potential and current-source-density (CSD) were analyzed.

RESULTS

The CSD analysis revealed that the N20 was produced by two dipolar generators, both directed toward the cortical surface. After the arrival of the initial thalamocortical volley in layer IV, the sink of the first generator shifted toward shallower layers II-III with a velocity of 0.109+/-0.038 m/s (mean+/-SD). The sink of the second generator moved to layer V. The initial thalamocortical axonal component of the HFO was produced by repolarizing current with the sink in layer IV. The CSD laminar profile of the postsynaptic component was very similar to the profile of intracortical N20. The current sink within each cycle of HFO propagated upward with a velocity of 0.633+/-0.189 m/s, indicating backpropagation.

CONCLUSIONS

We propose that the N20 is generated by two sets of excitatory neurons which also produce the HFOs. Although the loci of synaptic inputs are unknown, these neurons appear to fire initially in the soma and produce backpropagating spikes toward distal apical dendrites.

SIGNIFICANCE

These conclusions relate the N20 to the HFO and provide a new explanation of how the current underlying the N20 is invariantly directed toward superficial layers across species.

Visualization of conductive spinal cord activity using a biomagnetometer

STUDY DESIGN

The authors measured conductive cervical spinal cord evoked magnetic fields (SCEFs) after thoracic spinal cord stimulation in cats and visualized spinal cord activities.

OBJECTIVE

To evaluate the usefulness of magnetic field measurement.

SUMMARY OF BACKGROUND DATA

Magnetic field measurement has several theoretical advantages compared with electric potential measurement. Although biomagnetometers for the brain and heart are already on the market and are widely used, methods for magnetic field measurement of the spinal cord have not been established.

METHOD

Cervical laminectomy was performed on adult cats under anesthesia and the dural tube was exposed. Electrical stimuli were applied to the lower thoracic spinal cord by a catheter epidural electrode. SCEFs were recorded using a biomagnetometer specially designed for recording spinal cord action potentials. SCEFs were measured at 35 different points over the cervical spine and isomagnetic field maps of SCEFs were constructed. Thereafter, the spinal cord was transected completely at C5 and SCEFs were measured again.

RESULTS

The detected SCEFs showed a clear biphasic configuration. The first deflection of the magnetic fields from the left side was directed outward, but the right-side deflection was directed inward. The second deflection showed reversed polarity. The isomagnetic field maps of SCEFs clearly demonstrated the quadrupolar pattern and propagated at a conduction velocity of 80-120 m/s. After spinal cord transection, the propagation of SCEFs stopped at the transection site, and the SCEFs could not be obtained above the site.

CONCLUSIONS

The authors concluded that magnetic field measurement is useful for evaluation of spinal cord function. Moreover, it was apparent that SCEFs could indicate conduction block in the spinal cord.

Synchronized spikes of thalamocortical axonal terminals and cortical neurons are detectable outside the pig brain with MEG

We show that it is feasible to monitor the synchronized population spikes of the thalamocortical axonal terminals and cortical neurons outside the brain using high-resolution magnetoencephalography (MEG). Electrical stimulation of the snout elicited somatic-evoked magnetic fields (SEFs) above the primary somatosensory cortex (SI) of the piglet. The SEFs contained high-frequency oscillations (HFOs) around 600 Hz similar in many respects to the noninvasively measured HFOs from humans with MEG and electroencephalography (EEG). These HFOs were highly correlated with those in simultaneously measured intracortical somatic-evoked potentials (SEPs) in the snout projection area in SI. Both HFOs in SEFs and SEPs consisted of an initial component insensitive to cortically injected kynurenic acid (Kyna, 20 mM), a nonspecific antagonist of glutamatergic receptors, and a subsequent Kyna-sensitive component. The former was localized in cortical layer IV, indicating that it was due to spikes produced by the specific thalamocortical axonal terminals, whereas the latter was initially localized in layer IV and subsequently in the superficial and deeper layers. These results suggest that it may be possible to study properties of the thalamocortical and cortical spike activities in humans with MEG.