Magnetoencephalography Localization Research Articles

Analysis and suppression of cross-axis modulation coupling errors in optically pumped magnetometers

We propose a method based on sensitive-axis high-frequency modulation (SHFM) method combined with PI closed-loop feedback to suppress the cross-axis modulation coupling error (CMCE) of the optically pumped magnetometer (OPM). Cross-axis modulation is a prevalent approach for three-axis measurement of OPM, but this scheme will introduce CMCE. These errors have the potential to pose undetected hazards in applications of multi-axis magnetic field measurement such as magnetoencephalography (MEG). The SHFM method suppressed the CMCE from 18.50% to 2.10% on average compared to the traditional method. Based on the static situation we also introduce PI closed-loop feedback, which suppressed the CMCE from 22.07% in open-loop mode to 2.10% in closed-loop mode. This approach is both straightforward and efficacious, offering seamless integration into an arrayed OPM-based system. Furthermore, SHFM has the potential to mitigate the challenges associated with MEG localization accuracy and other measurement errors introduced by CMCE, and further unleash the potential of applications of OPM–MEG system.

Mapping critical hubs of receptive and expressive language using MEG: A comparison against fMRI

The complexity of the widespread language network makes it challenging for accurate localization and lateralization. Using large-scale connectivity and graph-theoretical analyses of task-based magnetoencephalography (MEG), we aimed to provide robust representations of receptive and expressive language processes, comparable with spatial profiles of corresponding functional magnetic resonance imaging (fMRI). We examined MEG and fMRI data from 12 healthy young adults (age 20–37 years) completing covert auditory word-recognition task (WRT) and covert auditory verb-generation task (VGT). For MEG language mapping, broadband (3–30 Hz) beamformer sources were estimated, voxel-level connectivity was quantified using phase locking value, and highly connected hubs were characterized using eigenvector centrality graph measure. fMRI data were analyzed using a classic general linear model approach. A laterality index (LI) was computed for 20 language-specific frontotemporal regions for both MEG and fMRI. MEG network analysis showed bilateral and symmetrically distributed hubs within the left and right superior temporal gyrus (STG) during WRT and predominant hubs in left inferior prefrontal gyrus (IFG) during VGT. MEG and fMRI localization maps showed high correlation values within frontotemporal regions during WRT and VGT (r = 0.63, 0.74, q < 0.05, respectively). Despite good concordance in localization, notable discordances were observed in lateralization between MEG and fMRI. During WRT, MEG favored a left-hemispheric dominance of left STG (LI = 0.25 ± 0.22) whereas fMRI supported a bilateral representation of STG (LI = 0.08 ± 0.2). Laterality of MEG and fMRI during VGT consistently showed a strong asymmetry in left IFG regions (MEG-LI = 0.45 ± 0.35 and fMRI-LI = 0.46 ± 0.13). Our results demonstrate the utility of a large-scale connectivity and graph theoretical analyses for robust identification of language-specific regions. MEG hubs are in great agreement with the literature in revealing with canonical and extra-canonical language sites, thus providing additional support for the underlying topological organization of receptive and expressive language cortices. Discordances in lateralization may emphasize the need for multimodal integration of MEG and fMRI to obtain an excellent predictive value in a heterogeneous healthy population and patients with neurosurgical conditions.

Coregistrating magnetic source and magnetic resonance imaging for epilepsy surgery in focal cortical dysplasia.

BackgroundEpilepsy surgery for focal cortical dysplasia type II (FCD II) offers good chances for seizure freedom, but remains a challenge with respect to lesion detection, defining the epileptogenic zone and the optimal resection strategy. Integrating results from magnetic source imaging from magnetoencephalography (MEG) with magnetic resonance imaging (MRI) including MRI postprocessing may be useful for optimizing these goals.MethodsWe here present data from 21 adult FCD II patients, investigated during a 10 year period and evaluated including magnetic source imaging. 16 patients had epilepsy surgery, i.e. histopathologically verified FCD II, and a long follow up. We present our analysis of epileptogenic zones including MEG in relation to structural data according to MRI data and relate these results to surgical outcomes.ResultsFCD II in our cohort was characterized by high MEG yield and localization accuracy and MEG showed impact on surgical success-rates. MEG source localizations were detected in 95.2% of patients and were as close as 12.3 ± 8,1 mm to the MRI-lesion. After a mean follow up of >3 years, we saw >80% Engel I outcomes, with more favourable outcomes when the MEG source was completely resected (Fishers exact test 0,033).ConclusionWe argue for a high value of conducting a combined MEG-MRI approach in the presurgical workup and the resection strategy in patients with FCD II related epilepsy.

MEG biomarker of Alzheimer's disease: Absence of a prefrontal generator during auditory sensory gating.

Magnetoencephalography (MEG), a direct measure of neuronal activity, is an underexplored tool in the search for biomarkers of Alzheimer's disease (AD). In this study, we used MEG source estimates of auditory gating generators, nonlinear correlations with neuropsychological results, and multivariate analyses to examine the sensitivity and specificity of gating topology modulation to detect AD. Our results demonstrated the use of MEG localization of a medial prefrontal (mPFC) gating generator as a discrete (binary) detector of AD at the individual level and resulted in recategorizing the participant categories in: (1) controls with mPFC generator localized in response to both the standard and deviant tones; (2) a possible preclinical stage of AD participants (a lower functioning group of controls) in which mPFC activation was localized to the deviant tone only; and (3) symptomatic AD in which mPFC activation was not localized to either the deviant or standard tones. This approach showed a large effect size (0.9) and high accuracy, sensitivity, and specificity (100%) in identifying symptomatic AD patients within a limited research sample. The present results demonstrate high potential of mPFC activation as a noninvasive biomarker of AD pathology during putative preclinical and clinical stages. Hum Brain Mapp 38:5180-5194, 2017. © 2017 Wiley Periodicals, Inc.

Concordance between interictal MEG and stereo-EEG predicts seizure freedom after epilepsy surgery.

This scientific commentary refers to ‘Correlating magnetoencephalography to stereo-electroencephalography in patients undergoing epilepsy surgery’, by Murakami et al. (doi:10.1093/brain/aww215) . The potential of magnetoencephalography (MEG)—the ability to measure cerebral activity via the brain’s magnetic fields—was recognized in the early decades after Hans Berger developed the first electroencephalography (EEG) device (Berger, 1929). The technical progress necessary to measure the weak cerebral magnetic fields, however, was not achieved until the 1960s. In 1968, using a single induction-coil magnetometer made with two million turns of copper wire, the magnetic field created by the posterior ‘alpha’ rhythm described by Berger decades earlier was finally recorded (Cohen, 1968). The next 20 years saw significant technical improvements in MEG; however, practical clinical applications of the technique were slow to materialize. In this issue of Brain , Murakami and co-workers provide evidence to support the role of MEG in the presurgical evaluation of patients with epilepsy (Murakami et al. , 2016). Epileptogenic source localization using MEG was first described in 1987 (Rose et al. , 1987) in a study comparing EEG, MEG, and electrocorticography (ECoG) localization of epileptogenic tissue. The MEG 3D equivalent current dipoles were calculated and found to be well correlated with the discharges identified on ECoG and via depth electrode placement. Magnetic source imaging (MSI), the combination of MEG and anatomical imaging modalities, began to show particular promise in the 1990s with the development of dual magnetometer and subsequent whole-head sensor array systems, allowing more efficient and detailed bi-hemispheric recordings. An initial report of …

Magnetoencephalography-guided surgery in frontal lobe epilepsy using neuronavigation and intraoperative MR imaging

BackgroundEspecially in hidden lesions causing drug-resistant frontal lobe epilepsy (FLE), the localization of the epileptic zone EZ can be a challenge. Magnetoencephalography (MEG) can raise the chances for localization of the (EZ) in combination with electroencephalography (EEG). We investigated the impact of MEG-guided epilepsy surgery with the aid of neuronavigation and intraoperative MR imaging (iopMRI) on seizure outcome of FLE patients. MethodsTwenty-eight patients (15 females, 13 males; mean age 31.0±11.1 years) underwent surgery in our department. All patients underwent presurgical MEG monitoring (two-sensor Magnes II or whole head WH3600 MEG system; 4-D Neuroimaging, San Diego, CA, USA). Of those, six patients (group 1) with MRI-negative FLE were operated on before 2002 with intraoperative electrocorticography (ECoG) and invasive EEG mapping only. Eleven patients with MRI-negative FLE (group 2) and eleven with lesional FLE (group 3) underwent surgery using 1.5T-iopMRI and neuronavigation, including intraoperative visualization of the MEG localizations in 22 and functional MR imaging (for motor and speech areas) as well as DTI fiber tracking (for language and pyramidal tracts) in 13 patients. ResultsIn the first group, complete resection of the defined EZ including the MEG localization according to the latest postoperative MRI was achieved in four out of six patients. Groups two and three had complete removal of the MEG localizations in 20/22 (91%, 10 of 11 each). Intraoperative MRI revealed incomplete resection of the MEG localizations of four patients (12%; two in both groups), leading to successful re-resection. Transient and permanent neurological deficits alike occurred in 7.1%, surgery-associated complications in 11% of all patients. In the first group, excellent seizure outcome (Engel Class IA) was achieved in three (50%), in the second in 7 patients (61%) and third group in 8 patients (64%, two iopMRI-based re-resections). Mean follow-up was 70.3 months (from 12 to 284 months). ConclusionIn our series, MEG-guided resection using neuronavigation and iopMR imaging led to promising seizure control rates. Even in non-lesional FLE, seizure control rates and the probability of complete resection of the MEG localizations was similar to lesional FLE using multimodal navigation.

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

ObjectivesThe 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.MethodsUsing 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.ResultsThe 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).ConclusionsMEG 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.

Reliability for non-invasive somatosensory cortex localization: Implications for pre-surgical mapping

ObjectivesIn patients with epilepsy or space occupying tumors in cortical regions, surgical resection is often considered as the primary treatment. Pre-surgical neuroimaging can provide a detailed map of pathological and functional cortex, leading to safer surgery. Mapping can be achieved non-invasively using magnetoencephalography (MEG), and is concordant with invasive findings. However, the reliability of MEG mapping between sessions is not well established. The inter-session reliability is an important property in pre-surgical mapping to establish resection margins, but repeated scans are impracticable. The present study sought to quantify the intersession reliability of MEG localization of somatosensory cortex (S1). Patients and methodsEighteen healthy individuals underwent MEG sessions on 3 consecutive days. Five participants were excluded due to technical issues during one of the three days. Each session included clinical-style S1 localization using electrical stimuli to each median nerve at sub-motor thresholds. The 35ms peak of the somatosensory evoked field was used for localizing S1 in each session using a single equivalent current dipole model. Intersession reliability was quantified using two methods. Average Euclidean Distance (AED) quantified the difference in localization between each session and the inter-session mean localization. Session Euclidean Distance (SED) quantified the difference in localization between each pair of sessions. Results and discussionResults showed the AED was 4.8±1.9mm, whereas the SED was 8.3±3.4mm. While the AED values obtained parallel those reported previously in smaller samples, the SED values were substantially larger. ConclusionClinicians should consider up to an 8mm confidence interval around the estimated location of S1 based on MEG pre-surgical mapping.

Improving spatial localization in MEG inverse imaging by leveraging intersubject anatomical differences.

Modern neuroimaging techniques enable non-invasive observation of ongoing neural processing, with magnetoencephalography (MEG) in particular providing direct measurement of neural activity with millisecond time resolution. However, accurately mapping measured MEG sensor readings onto the underlying source neural structures remains an active area of research. This so-called “inverse problem” is ill posed, and poses a challenge for source estimation that is often cited as a drawback limiting MEG data interpretation. However, anatomically constrained MEG localization estimates may be more accurate than commonly believed. Here we hypothesize that, by combining anatomically constrained inverse estimates across subjects, the spatial uncertainty of MEG source localization can be mitigated. Specifically, we argue that differences in subject brain geometry yield differences in point-spread functions, resulting in improved spatial localization across subjects. To test this, we use standard methods to combine subject anatomical MRI scans with coregistration information to obtain an accurate forward (physical) solution, modeling the MEG sensor data resulting from brain activity originating from different cortical locations. Using a linear minimum-norm inverse to localize this brain activity, we demonstrate that a substantial increase in the spatial accuracy of MEG source localization can result from combining data from subjects with differing brain geometry. This improvement may be enabled by an increase in the amount of available spatial information in MEG data as measurements from different subjects are combined. This approach becomes more important in the face of practical issues of coregistration errors and potential noise sources, where we observe even larger improvements in localization when combining data across subjects. Finally, we use a simple auditory N100(m) localization task to show how this effect can influence localization using a recorded neural dataset.

Spatiotemporal Signatures of Lexical–Semantic Prediction

Although there is broad agreement that top-down expectations can facilitate lexical-semantic processing, the mechanisms driving these effects are still unclear. In particular, while previous electroencephalography (EEG) research has demonstrated a reduction in the N400 response to words in a supportive context, it is often challenging to dissociate facilitation due to bottom-up spreading activation from facilitation due to top-down expectations. The goal of the current study was to specifically determine the cortical areas associated with facilitation due to top-down prediction, using magnetoencephalography (MEG) recordings supplemented by EEG and functional magnetic resonance imaging (fMRI) in a semantic priming paradigm. In order to modulate expectation processes while holding context constant, we manipulated the proportion of related pairs across 2 blocks (10 and 50% related). Event-related potential results demonstrated a larger N400 reduction when a related word was predicted, and MEG source localization of activity in this time-window (350-450 ms) localized the differential responses to left anterior temporal cortex. fMRI data from the same participants support the MEG localization, showing contextual facilitation in left anterior superior temporal gyrus for the high expectation block only. Together, these results provide strong evidence that facilitatory effects of lexical-semantic prediction on the electrophysiological response 350-450 ms postonset reflect modulation of activity in left anterior temporal cortex.

MEG localization of spike sources in human patients with brain tumors.

The purpose of this study was to use magnetoencephalography (MEG) to identify epileptic zones in patients with brain tumors before undergoing tumor surgery. The MEG data were recorded with a 122-channel biomagnetometer. Equivalent current dipoles (ECD) were calculated for epileptic spikes on MEG recordings according to the single dipole model. Eight patients (five males and three females) within the age range (43-73 years; mean ± SD = 55.12 ± 9.77) were examined by MEG before neurosurgery operation. Four patients had meningioma grade I, three had glioblastoma grade IV and one had astrocytoma grade II. All the patients showed ECD at their MEG's before surgical operation except a female one with meningioma who showed no ECD. Tumors observed in the frontal areas show posteriorly located ECD. We conclude that the MEG is a valuable clinical tool for the localization of epileptic foci in patients with brain tumors before surgical tumor operation.

The utility of magnetoencephalography in the presurgical evaluation of refractory insular epilepsy

To study the utility of magnetoencephalography (MEG) in patients with refractory insular epilepsy. Covered by highly functional temporal, frontal, and parietal opercula, insular-onset seizures can manifest a variety of ictal symptoms falsely leading to a diagnosis of temporal, frontal, or parietal lobe seizures. Lack of recognition of insular seizures may be responsible for some epilepsy surgery failures. We retrospectively reviewed and analyzed MEG data in 14 patients with refractory insular seizures defined through intracranial electroencephalography (EEG) or by the presence of an epileptogenic lesion in the insula with compatible seizure semiology. MEG was performed as part of the noninvasive presurgical evaluation, using a 275-channel whole head MEG system. MEG data were analyzed using a single equivalent current dipole model. MEG localization was compared to interictal positron emission tomography (PET) and ictal single photon emission computed tomography (SPECT) results and to the resection margin. Three patterns of MEG spike sources were observed. Seven patients showed an anterior operculoinsular clusters and two patients had a posterior operculoinsular cluster. No spikes were detected in one patient, and the remaining four patients showed a diffuse perisylvian distribution. Spike sources showed uniform orientation perpendicular to the sylvian fissure. Nine patients proceeded to insular epilepsy surgery with favorable surgical outcome. Among patients with anterior operculoinsular cluster who proceeded to have surgery, MEG provided superior information to ictal SPECT in four of six patients and to interictal PET in five of six patients. MEG is useful in identifying patients who are likely to benefit from epilepsy surgery targeting the insula, particularly if a tight dipole cluster is identified even if other noninvasive modalities fail to produce localizing results.

Interictal MEG reveals focal cortical dysplasias: Special focus on patients with no visible MRI lesions

To investigate the value of interictal magnetoencephalography (MEG) in localizing epileptogenic cortex in epilepsy surgery patients with focal cortical dysplasias (FCD), particularly in patients having no visible MRI lesions. Thirty-four patients with FCD and preoperative MEG were retrospectively evaluated. Interictal MEG spike source localizations in respect to the resected area were studied using postoperative MR imaging. The possible predictive value of MEG-findings in respect to the clinical outcome was evaluated. Results from intracranial recordings were also compared with the MEG localizations. Interictal MEG spikes were observed in all but one patient. 17 of the 34 (50%) patients became seizure free (Engel class I). In patients with MEG dipole clusters (n=20) and Engel class I or II (n=15) 49% of the source clusters were removed on the average; the corresponding value in patients with Engel class III or IV (n=5) was 5.5% (p=0.02). Seven (54%) of the 13 patients with an MRI-negative lesion achieved Engel class I; the outcomes did not differ from patients having a visible MRI lesion (n=21; p=0.82). The concordance between MEG localizations and the invasive studies was good in nine of the 13 patients with no visible MRI lesions MEG is particularly useful in finding small FCDs not visible on MRI. A more complete removal of MEG source cluster area is associated with better clinical outcome These features make it a valuable tool in pre-surgical evaluation of patients with intractable focal-type epilepsy and normal MRI.

Functional Plasticity of the Motor Cortical Structures Demonstrated by Navigated TMS in Two Patients with Epilepsy

BackgroundRecently, navigated transcranial magnetic stimulation (nTMS) has been suggested to be useful in preoperative functional localization of motor cortex in patients having tumors close to the somatomotor cortex. Resection of tumors in anatomically predicted eloquent areas without adverse effects have emphasized functional plasticity elicited by intracranial pathology. ObjectiveTo describe functional plasticity of motor cortex indicated by nTMS in two patients with epilepsy. MethodsnTMS, functional MRI (fMRI), diffusion-tensor (DT)-tractography and magnetoencephalography (MEG) were utilized to preoperatively localize motor cortical areas in the workup for epilepsy surgery. The localizations were compared with each other, with the cortical anatomical landmarks, and in one patient with invasive electrical cortical stimulation (ECS). ResultsIn two out of 19 studied patients, nTMS identified motor cortical sites that differed from those indicated by anatomical landmarks. In one patient, nTMS activated preferentially premotor cortex rather than pathways originating from the precentral gyrus. MEG and fMRI localizations conformed with nTMS whereas ECS localized finger motor function into the precentral gyrus. Resection of the area producing motor responses in biphasic nTMS did not produce a motor deficit. In the other patient, nTMS indicated abnormal ipsilateral hand motor cortex localization and confirmed the functionality of aberrant motor cortical representations of the left foot also indicated by fMRI and DT-tractography. ConclusionnTMS may reveal the functional plasticity and shifts of motor cortical function. Epileptic foci may modify cortical inhibition and the nTMS results. Therefore, in some patients with epilepsy, the nTMS results need to be interpreted with caution with regard to surgical planning.

MEG in frontal lobe epilepsies: Localization and postoperative outcome

This study aimed to analyze magnetoencephalography (MEG) localizations of epileptic clusters in different cortical regions of the frontal lobe and relate these findings to postoperative outcomes associated with frontal lobe epilepsy (FLE). Thirty-nine patients from the Epilepsy Center of Erlangen-Nuremberg University with or without lesions on their magnetic resonance imaging (MRI) scans underwent MEG measurements and operation and were then analyzed retrospectively. MEG data were obtained using systems with either 74 or 248 channels. Single dipole analysis assuming a spherical head model was performed for localization. Epileptic clusters were detected by MEG in 30 patients, corresponding to a sensitivity of 76.9%; there was a sensitivity of 66.7% (20 of 30) in patients with monofocal activity (70% had an Engel class 1 outcome) and 33.3% (10 of 30) in patients with multifocal activity (20% had an Engel class 1 outcome). Of the patients who had isolated clusters, the distance between the MEG localizations and the respective lesions was equal to or <3 cm in 90% (18 of 20) of patients (13 of them had an Engel class 1 outcome) and >3 cm in 10% (2 of 20) of patients (one of them had an Engel class 1 outcome). A statistical difference was found between the outcomes of patients with a single focus and with multiple foci (p < 0.05). Patients with a single focus had better postoperative outcomes compared with patients with multiple foci. MEG localizations close to the lesion marked the lesion or its surrounding network as epileptogenic. Therefore, source localization can provide important information for the presurgical evaluation of patients with FLE.

The correlation between ictal semiology and magnetoencephalographic localization in frontal lobe epilepsy

ObjectiveThe aim of this study was to investigate the correlation of ictal semiology with localization and/or lateralization by magnetoencephalography (MEG). MethodsSeven patients from the Neurology Department of the University Hospital Erlangen who underwent resective surgery for frontal lobe epilepsy (FLE) with an Engel 1a outcome were investigated retrospectively. MEG localizations were classified according to five compartments (separate or combined) of the frontal lobe: frontal basal (FB), frontal lateral (FL), frontal polar (FP), frontal mesial (FM), and frontal precentral (FPr). On the basis of previous studies that investigated the value of ictal semiology in localization and lateralization, we compared the experiential localization and/or lateralization of the epileptogenic region deduced from ictal semiology, that is, both seizure history and ictal video/EEG monitoring, with MEG localization. ResultsIt is easier to determine lateralization than localization from ictal semiology because of the variety of signs and fast propagation in FLE. All of the patients had specific MEG localizations according to favorable postoperative outcome. Three patients had MEG foci associated with ictal semiology; in another four, the MEG localization was adjacent to the estimated area suggested by ictal semiology. Head version signs could be observed in all compartments of the frontal lobe: clonic in FB and FP areas; postural in FPr, FL, and FM areas; hypermotor in FB, FP, FPr, and FM areas; sensation aura in FB, FL, and FM areas; and automatisms in FP, FPr, and FL areas. All patients had concordant lateralizing and limited valuable locating information from ictal semiology, but no complete correlation with MEG foci. ConclusionIctal semiology may indicate the involvement of a symptomatogenic brain region during a seizure, but extent of seizure onset in central motor or sensorimotor area is not reliable enough to indicate the seizure onset zone and favorable postoperative outcome in FLE. MEG provided specific localization of epileptic activity in a FLE compartment, and indicated the relationship between epileptogenic region and lesion. MEG can complement ictal semiology in establishing a noninvasive focal localization hypothesis.

Combined use of non-invasive techniques for improved functional localization for a selected group of epilepsy surgery candidates

Invasive cortical mapping is conventionally required for preoperative identification of epileptogenic and eloquent cortical regions before epilepsy surgery. The decision on the extent and exact location of the resection is always demanding and multimodal approach is desired for added certainty. The present study describes two non-invasive preoperative protocols, used in addition to the normal preoperative work-up for localization of the epileptogenic and sensorimotor cortical regions, in two young patients with epilepsy. Magnetoencephalography (MEG) was used to determine the primary somatosensory cortex (S1) and the ictal onset zones. Navigated transcranial magnetic stimulation (nTMS) was used to determine the location and the extent of the primary motor representation areas. The localization results from these non-invasive methods were used for guiding the subdural grid deployment and later compared with the results from electrical cortical stimulation (ECS) via subdural grids, and validated by surgery outcome. The results from MEG and nTMS localizations were consistent with the ECS results and provided improved spatial precision. Consistent results of our study suggest that these non-invasive methods can be added to the standard preoperative work-up and may even hold a potential to replace the ECS in a subgroup of patients with epilepsy who have the suspected epileptogenic zone near the sensorimotor cortex and seizures frequent enough for ictal MEG.

What Is the Evidence for Use Dependent Learning After Stroke?

Our laboratory has been interested in the patterns of synaptic plasticity after cortical stroke in animal models.1 These protein expression studies suggest that regions both ipsilateral and contralateral to the stroke are involved during the recovery period. An emerging issue of interest is whether the recovered function is indeed normal or reflects some compensatory function.2 One approach to resolving these questions is to assess different patterns of functional recovery in patients who do and do not recover function with and without therapy and compare them to normal patterns. Investigation of the molecular mechanisms of brain plasticity after injury suggests that functional activity stimulates this recovery and guides the development of functional pathways.3,4 Several investigators postulate that recovery from stroke resembles “use-dependent” learning.5 For this report, we review pertinent literature related to this phenomenon, assess how recovery of language resembles normal language function, and provide examples from our own research on functional imaging that assess patterns of activation related to recovery. The primary treatment modality that we review is constraint-induced language therapy (CILT), in which stroke patients are restricted to using verbal communication.6 CILT is based on principles of constraint-induced therapy for other conditions.6 CILT is performed in chronic stroke …

Predicting the location of mouth motor cortex in patients with brain tumors by using somatosensory evoked field measurements

Before resective brain surgery, localization of the functional regions is necessary to minimize postoperative deficits. The face area has been relatively difficult to map noninvasively by using functional imaging techniques. Preoperative localization of face somatosensory cortex with magnetoencephalography (MEG) may allow the surgeon to predict the location of mouth motor areas. The authors compared the location of face somatosensory cortex obtained with somatosensory evoked fields during preoperative MEG with the mouth motor areas identified during intraoperative electrocortical stimulation (ECS) mapping in 13 patients undergoing resection of brain tumor. In this group of patients, ECS mouth motor sites were usually anterior and lateral to MEG localizations of lip somatosensory cortex. The consistent quantitative relationship between results of these two mapping procedures allows the practitioner to predict the location of mouth motor cortex based on noninvasive preoperative MEG measurements. Based on this result, the authors suggest that somatosensory mapping using MEG can be used to guide intraoperative mapping and neurosurgical planning.

Magnetoencephalography Is More Successful for Screening and Localizing Frontal Lobe Epilepsy than Electroencephalography

The diagnosis of frontal lobe epilepsy may be compounded by poor electroclinical localization, due to distributed or rapidly propagating epileptiform activity. This study aimed at developing optimal procedures for localizing interictal epileptiform discharges (IEDs) of patients with localization related epilepsy in the frontal lobe. To this end the localization results obtained for magnetoencephalography (MEG) and electroencephalography (EEG) were compared systematically using automated analysis procedures. Simultaneous recording of interictal EEG and MEG was successful for 18 out of the 24 patients studied. Visual inspection of these recordings revealed IEDs with varying morphology and topography. Cluster analysis was used to classify these discharges on the basis of their spatial distribution followed by equivalent dipole analysis of the cluster averages. The locations of the equivalent dipoles were compared with the location of the epileptogenic lesions of the patient or, if these were not visible at MRI with the location of the interictal onset zones identified by subdural electroencephalography. Generally IEDs were more abundantly in MEG than in the EEG recordings. Furthermore, the duration of the MEG spikes, measured from the onset till the spike maximum, was in most patients shorter than the EEG spikes. In most patients, distinct spike subpopulations were found with clearly different topographical field maps. Cluster analysis of MEG spikes followed by dipole localization was successful (n = 14) for twice as many patients as for EEG source analysis (n = 7), indicating that the localizability of interictal MEG is much better than of interictal EEG. The automated procedures developed in this study provide a fast screening method for identifying the distinct categories of spikes and the brain areas responsible for these spikes. The results show that MEG spike yield and localization is superior compared with EEG. This finding is of importance for the diagnosis and preoperative evaluation of patients with frontal lobe epilepsy.