Determination of language dominance with synthetic aperture magnetometry: comparison with the Wada test

Combining Temporal and Spectral Information with Spatial Mapping to Identify Differences between Phonological and Semantic Networks: A Magnetoencephalographic Approach

Early, lesion-based models of language processing suggested that semantic and phonological processes are associated with distinct temporal and parietal regions respectively, with frontal areas more indirectly involved. Contemporary spatial brain mapping techniques have not supported such clear-cut segregation, with strong evidence of activation in left temporal areas by both processes and disputed evidence of involvement of frontal areas in both processes. We suggest that combining spatial information with temporal and spectral data may allow a closer scrutiny of the differential involvement of closely overlapping cortical areas in language processing. Using beamforming techniques to analyze magnetoencephalography data, we localized the neuronal substrates underlying primed responses to nouns requiring either phonological or semantic processing, and examined the associated measures of time and frequency in those areas where activation was common to both tasks. Power changes in the beta (14–30 Hz) and gamma (30–50 Hz) frequency bands were analyzed in pre-selected time windows of 350–550 and 500–700 ms In left temporal regions, both tasks elicited power changes in the same time window (350–550 ms), but with different spectral characteristics, low beta (14–20 Hz) for the phonological task and high beta (20–30 Hz) for the semantic task. In frontal areas (BA10), both tasks elicited power changes in the gamma band (30–50 Hz), but in different time windows, 500–700 ms for the phonological task and 350–550 ms for the semantic task. In the left inferior parietal area (BA40), both tasks elicited changes in the 20–30 Hz beta frequency band but in different time windows, 350–550 ms for the phonological task and 500–700 ms for the semantic task. Our findings suggest that, where spatial measures may indicate overlapping areas of involvement, additional beamforming techniques can demonstrate differential activation in time and frequency domains.

An MEG study of the spatiotemporal dynamics of bilingual verb generation

Studies of first (L1) and second (L2) language representation in the brain have not identified the timing and locations of neural regions involved in L1 and L2 processing. Magnetoencephalography offers high spatial and temporal resolution and can be employed to disentangle subtle timing and neural control differences between L1 and L2 use. We tested bilingual adults in the MEG as they completed a picture verb generation task in L1 and L2. We found the expected progression of activation from occipital to temporal to inferior frontal areas. We also observed the following differences. A sustained insula and early cingulate event-related desynchrony was observed only with L2; the fMRI literature suggests that the former reflects an activation, and the latter an inhibition, sub-process for language selection. L2 processes exhibited a lag and were bilateral compared to L1 processes. Finally, L1 and L2 activated adjacent language control in dorsolateral pre-frontal cortex.

Dynamics of hemispheric dominance for language assessed by magnetoencephalographic imaging

OBJECTIVE

The goal of the current study was to examine the dynamics of language lateralization using magnetoencephalographic (MEG) imaging, to determine the sensitivity and specificity of MEG imaging, and to determine whether MEG imaging can become a viable alternative to the intracarotid amobarbital procedure (IAP), the current gold standard for preoperative language lateralization in neurosurgical candidates.

METHODS

MEG was recorded during an auditory verb generation task and imaging analysis of oscillatory activity was initially performed in 21 subjects with epilepsy, brain tumor, or arteriovenous malformation who had undergone IAP and MEG. Time windows and brain regions of interest that best discriminated between IAP-determined left or right dominance for language were identified. Parameters derived in the retrospective analysis were applied to a prospective cohort of 14 patients and healthy controls.

RESULTS

Power decreases in the beta frequency band were consistently observed following auditory stimulation in inferior frontal, superior temporal, and parietal cortices; similar power decreases were also seen in inferior frontal cortex prior to and during overt verb generation. Language lateralization was clearly observed to be a dynamic process that is bilateral for several hundred milliseconds during periods of auditory perception and overt speech production. Correlation with the IAP was seen in 13 of 14 (93%) prospective patients, with the test demonstrating a sensitivity of 100% and specificity of 92%.

INTERPRETATION

Our results demonstrate excellent correlation between MEG imaging findings and the IAP for language lateralization, and provide new insights into the spatiotemporal dynamics of cortical speech processing.

Beta oscillations relate to the N400m during language comprehension

The relationship between the evoked responses (ERPs/ERFs) and the event-related changes in EEG/MEG power that can be observed during sentence-level language comprehension is as yet unclear. This study addresses a possible relationship between MEG power changes and the N400m component of the event-related field. Whole-head MEG was recorded while subjects listened to spoken sentences with incongruent (IC) or congruent (C) sentence endings. A clear N400m was observed over the left hemisphere, and was larger for the IC sentences than for the C sentences. A time-frequency analysis of power revealed a decrease in alpha and beta power over the left hemisphere in roughly the same time range as the N400m for the IC relative to the C condition. A linear regression analysis revealed a positive linear relationship between N400m and beta power for the IC condition, not for the C condition. No such linear relation was found between N400m and alpha power for either condition. The sources of the beta decrease were estimated in the LIFG, a region known to be involved in semantic unification operations. One source of the N400m was estimated in the left superior temporal region, which has been related to lexical retrieval. We interpret our data within a framework in which beta oscillations are inversely related to the engagement of task-relevant brain networks. The source reconstructions of the beta power suppression and the N400m effect support the notion of a dynamic communication between the LIFG and the left superior temporal region during language comprehension.

EEG correlates of haptic feedback in a visuomotor tracking task

This study investigates the temporal brain dynamics associated with haptic feedback in a visuomotor tracking task. Haptic feedback with deviation-related forces was used throughout tracking experiments in which subjects' behavioral responses and electroencephalogram (EEG) data were simultaneously measured. Independent component analysis was employed to decompose the acquired EEG signals into temporally independent time courses arising from distinct brain sources. Clustering analysis was used to extract independent components that were comparable across participants. The resultant independent brain processes were further analyzed via time-frequency analysis (event-related spectral perturbation) and event-related coherence (ERCOH) to contrast brain activity during tracking experiments with or without haptic feedback. Across subjects, in epochs with haptic feedback, components with equivalent dipoles in or near the right motor region exhibited greater alpha band power suppression. Components with equivalent dipoles in or near the left frontal, central, left motor, right motor, and parietal regions exhibited greater beta-band power suppression, while components with equivalent dipoles in or near the left frontal, left motor, and right motor regions showed greater gamma-band power suppression relative to non-haptic conditions. In contrast, the right occipital component cluster exhibited less beta-band power suppression in epochs with haptic feedback compared to non-haptic conditions. The results of ERCOH analysis of the six component clusters showed that there were significant increases in coherence between different brain networks in response to haptic feedback relative to the coherence observed when haptic feedback was not present. The results of this study provide novel insight into the effects of haptic feedback on the brain and may aid the development of new tools to facilitate the learning of motor skills.

Characterizing the normal developmental trajectory of expressive language lateralization using magnetoencephalography

To characterize the developmental trajectory for expressive language representation and to test competing explanations for the relative neuroplasticity of language in childhood, we studied 28 healthy children and adolescents (aged 5-19 years) participating in a covert verb generation task in magnetoencephalography. Lateralization of neuromagnetic responses in the frontal lobe was quantified using a bootstrap statistical thresholding procedure for differential beamformer analyses. We observed a significant positive correlation between left hemisphere lateralization and age. Findings suggest that adult-typical left hemisphere lateralization emerges from an early bilateral language network, which may explain the pediatric advantage for interhemispheric plasticity of language.

Localization of Broca's area using verb generation tasks in the MEG: validation against fMRI

Functional MRI (fMRI) is routinely used to non-invasively localize language areas. Magnetoencephalography (MEG) is being explored as an alternative technique. MEG tasks to localize receptive language are well established although there are no standardized tasks to localize expressive language areas. We developed two expressive language tasks for MEG and validated their localizations against fMRI data. Ten right-handed adolescents (μ=17.5 years) were tested with fMRI and MEG on two tasks: verb generation to pictures and verb generation to words. MEG and fMRI data were normalized and overlaid. The number of overlapping voxels activated in fMRI and MEG were counted for each subject, for each task, at different thresholding levels. For picture verb generation, there was 100% concordance between MEG and fMRI lateralization, and for word verb generation, there was 75% concordance. A count showed 79.6% overlap of voxels activated by both MEG and fMRI for picture verb generation and 50.2% overlap for word verb generation. The percentage overlap decreased with increasingly stringent activation thresholds. Our novel MEG expressive language tasks successfully identified neural regions involved in language production and showed high concordance with fMRI laterality. Percentage overlap of activated voxels was also high when validated against fMRI, but showed task-specific and threshold-related effects. The high concordance and high percentage overlap between fMRI and MEG activations confirm the validity of our new MEG task. Furthermore, the higher concordance from the picture verb generation task suggests that this is a promising task for use in the young clinical population.

Using subdural electrodes to assess the safety of resections

Subdural electrodes are frequently used to aid in the neurophysiological assessment of patients with intractable seizures. We review their use for localizing cortical regions supporting movement, sensation, and language.

Assessment of language dominance by event-related oscillatory changes in an auditory language task: magnetoencephalography study

The authors investigated the oscillatory changes induced by auditory language task to assess hemispheric dominance of language. Magnetoencephalography studies were conducted during word listening in 6 normal right-handed volunteers and 13 epilepsy patients who underwent Wada test. We carried out a time-frequency analysis of event-related desynchronization (ERD)/event-related synchronization (ERS) and intertrial coherence. We localized ERD/ERS on each subject's magnetic resonance images using beamformer. We compared ERD/ERS values between the left and right side of regions of interest in inferior frontal and superior temporal areas. We assessed the target frequency range that correlated best with the Wada test results. In all normal subjects, gamma ERD was lateralized to the left side in both the inferior frontal and superior temporal areas. In epilepsy patients, the concordance rate of gamma ERD and the Wada test results was 76.9% for the inferior frontal area and 69.2% for the superior temporal area. Gamma ERD can be considered as an indicator of language function, although it was not sufficient to replace the Wada test in the evaluation of epilepsy patients. The gamma ERD value of the inferior frontal area was more reliable for the assessment of language dominance compared with that obtained in the superior temporal area.

Time is more than a sensory feature: Attending to duration triggers specific anticipatory activity

Time processing requires the estimation of events' duration per se, but also seems to trigger attentional and memory processes. To isolate attentional processes, we investigated neural correlates of anticipatory attention when estimating stimulus duration. Magneto-encephalographic (MEG) activity was recorded in fourteen healthy right-handed volunteers, who were cued to attend to either the duration or the intensity of a visual stimulus. We report an increase of gamma-band oscillations over right fronto-central and parietal regions when subjects are prompted to attend to duration, which is not present when subjects are cued to attend to intensity. Cue-related alpha power decreases over occipito-parietal regions were similar in the two conditions. Our results support the hypothesis that the right fronto-parietal network observed repeatedly in time estimation imaging studies is indeed involved in attentional control rather than stimulus processing. Moreover, they underline the supramodal property of time dimension that goes beyond purely perceptive features.

Image guidance and neuromonitoring in neurosurgery

INTRODUCTION

The localization of tumors and epileptogenic foci within the somatosensory or language cortex of the brain of a child poses unique neurosurgical challenges. In the past, lesions in these regions were not treated aggressively for fear of inducing neurological deficits. As a result, while function may have been preserved, the underlying disease may not have been optimally treated, and repeat neurosurgical procedures were frequently required. Today, with the advent of preoperative brain mapping, image guidance or neuronavigation, and intraoperative monitoring, peri-Rolandic and language cortex lesions can be approached directly and definitively with a high degree of confidence that neurosurgical function will be maintained.

METHODS AND RESULTS

The preoperative brain maps can now be achieved with magnetic resonance imaging (MRI), functional MRI, magnetoencephalography, and diffusion tensor imaging. Image guidance systems have improved significantly and include the use of the intraoperative MRI. Somatosensory, motor, and brainstem auditory-evoked potentials are used as standard neuromonitoring techniques in many centers around the world. Added to this now is the use of continuous train-of-five monitoring of the integrity of the corticospinal tract while operating in the peri-Rolandic region.

CONCLUSION

We are in an era where continued advancements can be expected in mapping additional pathways such as visual, memory, and hearing pathways. With these new advances, neurosurgeons can expect to significantly improve their surgical outcomes further.

Language dominance and mapping based on neuromagnetic oscillatory changes: comparison with invasive procedures

Object

Event-related cerebral oscillatory changes reflect regional brain activation. In a previous study, the authors proposed a new method to determine language dominance: examine frontal oscillatory changes during silent reading by using synthetic aperture magnetometry (SAM). The authors' aims in the present study were to establish a normal template for this method, to confirm the results of their previous study with a larger patient population, and to evaluate their method with respect to language localization.

Methods

A statistical group analysis of 14 healthy volunteers was conducted to establish a normal control. Language dominance and localization were then evaluated in a larger population of 123 consecutive patients. Study participants were instructed to silently read 100 visually presented words. Using SAM, the spatial distribution of the oscillatory changes was obtained as the Student t statistic by comparing the current density for each voxel between 1 second before and 1 second after each word presentation. Group analyses of the healthy volunteers were performed using statistical nonparametric mapping. Language dominance in the patients was determined according to the laterality index (LI) calculated using peak t values of the left and right frontal desynchronizations. Language dominance was prospectively assessed, and the results were compared with those of the Wada test (63 patients). Language localization results were quantitatively compared with those of stimulation mapping (17 patients).

Results

Group analysis of the healthy volunteers indicated β to low γ band desynchronization in the left frontal area and α to β desynchronization in the left parietotemporal areas. In patients, the frontal language areas were detected in 118 persons (95.9%). Lateralization of β or low γ desynchronization in the inferior or middle frontal gyri corresponded well with language dominance. The introduction of the LI resulted in a quantitative evaluation of language dominance, whose results were concordant with those of the Wada test in 51 (85.0%) of 60 cases. The distance between the estimated frontal language areas and stimulation-positive sites was 6.0 ± 7.1 mm (mean ± SD).

Conclusions

This study is the first in which magnetoencephalography (MEG) was used to determine language dominance in a large population, and the results were compared with those of the Wada test. Moreover, language localization results obtained using MEG were compared with those obtained by invasive mapping. The authors' method, which is based on neuromagnetic oscillatory changes, is a new approach for noninvasively evaluating the frontal language areas, a procedure that has been problematic using MEG dipole methods. Synthetic aperture magnetometry is a noninvasive alternative to Wada testing for language dominance and helps to determine stimulation sites for invasive mapping.

Temporal lobe white matter asymmetry and language laterality in epilepsy patients

Recent studies using diffusion tensor imaging (DTI) have advanced our knowledge of the organization of white matter subserving language function. It remains unclear, however, how DTI may be used to predict accurately a key feature of language organization: its asymmetric representation in one cerebral hemisphere. In this study of epilepsy patients with unambiguous lateralization on Wada testing (19 left and 4 right lateralized subjects; no bilateral subjects), the predictive value of DTI for classifying the dominant hemisphere for language was assessed relative to the existing standard-the intra-carotid Amytal (Wada) procedure. Our specific hypothesis is that language laterality in both unilateral left- and right-hemisphere language dominant subjects may be predicted by hemispheric asymmetry in the relative density of three white matter pathways terminating in the temporal lobe implicated in different aspects of language function: the arcuate (AF), uncinate (UF), and inferior longitudinal fasciculi (ILF). Laterality indices computed from asymmetry of high anisotropy AF pathways, but not the other pathways, classified the majority (19 of 23) of patients using the Wada results as the standard. A logistic regression model incorporating information from DTI of the AF, fMRI activity in Broca's area, and handedness was able to classify 22 of 23 (95.6%) patients correctly according to their Wada score. We conclude that evaluation of highly anisotropic components of the AF alone has significant predictive power for determining language laterality, and that this markedly asymmetric distribution in the dominant hemisphere may reflect enhanced connectivity between frontal and temporal sites to support fluent language processes. Given the small sample reported in this preliminary study, future research should assess this method on a larger group of patients, including subjects with bi-hemispheric dominance.

Diffusion tensor-based tumor infiltration index cannot discriminate vasogenic edema from tumor-infiltrated edema

Diffusion tensor imaging (DTI) by magnetic resonance imaging (MRI) is now used not only for delineating white matter fiber tracts, but also for assessing the histological characteristics of pathological tissues. Among these uses, predicting the extent or existence of tumor cell invasion into white matter by DTI is under extensive investigation. The previously reported tumor infiltration index (TII) holds great potential for the discrimination of pure vasogenic edema from tumor-infiltrated edema. However, conflicting data are being reported questioning the clinical value of TII. The present investigation reevaluated the utility of TII in patients with meningioma or glioma. We found that TII was unable to discriminate vasogenic from tumor-infiltrated edema. Conversely, detailed voxel-by-voxel comparison of TII and (11)C-methionie PET in the T2-hyperintense area of gliomas showed that TII and (11)C-methionie PET has a positive correlation, suggesting that, although TII is unable to discriminate the cause of edema, the extent of tumor cell invasion into white matter is depicted in gliomas by TII. These data suggest that TII involves both vasogenic and tumor-infiltrated factors, rather than only a single factor. A more intensive investigation is required to reach a complete understanding of TII.

Distributed source modeling of language with magnetoencephalography: application to patients with intractable epilepsy

PURPOSE

To examine distributed patterns of language processing in healthy controls and patients with epilepsy using magnetoencephalography (MEG), and to evaluate the concordance between laterality of distributed MEG sources and language laterality as determined by the intracarotid amobarbital procedure (IAP).

METHODS

MEG was performed in 10 healthy controls using an anatomically constrained, noise-normalized distributed source solution (dynamic statistical parametric map, dSPM). Distributed source modeling of language was then applied to eight patients with intractable epilepsy. Average source strengths within temporoparietal and frontal lobe regions of interest (ROIs) were calculated, and the laterality of activity within ROIs during discrete time windows was compared to results from the IAP.

RESULTS

In healthy controls, dSPM revealed activity in visual cortex bilaterally from approximately 80 to 120 ms in response to novel words and sensory control stimuli (i.e., false fonts). Activity then spread to fusiform cortex approximately 160-200 ms, and was dominated by left hemisphere activity in response to novel words. From approximately 240 to 450 ms, novel words produced activity that was left-lateralized in frontal and temporal lobe regions, including anterior and inferior temporal, temporal pole, and pars opercularis, as well as bilaterally in posterior superior temporal cortex. Analysis of patient data with dSPM demonstrated that from 350 to 450 ms, laterality of temporoparietal sources agreed with the IAP 75% of the time, whereas laterality of frontal MEG sources agreed with the IAP in all eight patients.

DISCUSSION

Our results reveal that dSPM can unveil the timing and spatial extent of language processes in patients with epilepsy and may enhance knowledge of language lateralization and localization for use in preoperative planning.

Use of fractional anisotropy for determination of the cut-off value in 11C-methionine positron emission tomography for glioma

Multimodal imaging is one of the necessary steps in the treatment of malignant brain tumors, and use of magnetic resonance imaging (MRI) and positron emission tomography (PET) are the current gold standard technique for the morphological and biological assessment of malignant brain tumors. In addition, fractional anisotropy (FA) obtained from diffusion tensor imaging (DTI) and 11C-methionine PET are useful to determine the tumor border at the tumor and white matter interface. Although there is no question of their value, a universally accepted cut-off value to discriminate normal and abnormal tissue has not been established. In this study we attempted to calculate and determine the cut-off values in FA and 11C-methionine PET that will allow delineation of the tumor border at the tumor and white matter interface by combining these two modalities. We were able to determine individual cut-off values for 11 patients, and then found an average cut-off value in the T/N ratio of 11C-methionine PET of 1.27 and in FA of 0.26, values similar to those previously confirmed by histological study. Moreover, reconstructing images delineating the tumor border was possible combining these two imaging modalities. We propose that the combined analysis of DTI and 11C-methionine PET has the potential to improve tumor border imaging in glioma patients, providing important information for establishing neurosurgical strategies.

The role of magnetoencephalography in epilepsy surgery

Epilepsy surgery requires the precise localization of the epileptogenic zone and the anatomical localization of eloquent cortex so that these areas can be preserved during cortical resection. Magnetoencephalography (MEG) is a technique that maps interictal magnetic dipole sources onto MR imaging to produce a magnetic source image. Magneto-encephalographic spike sources can be used to localize the epileptogenic zone and be part of the workup of the patient for epilepsy surgery in conjunction with data derived from an analysis of seizure semiology, scalp video electroencephalography, PET, functional MR imaging, and neuropsychological testing. In addition, magnetoencephalographic spike sources can be linked to neuronavigation platforms for use in the neurosurgical field. Finally, paradigms have been developed so that MEG can be used to identify functional areas of the cerebral cortex including the somatosensory, motor, language, and visual evoked fields.

The authors review the basic principles of MEG and the utility of MEG for presurgical planning as well as intra-operative mapping and discuss future applications of MEG technology.

Language lateralization using MEG beta frequency desynchronization during auditory oddball stimulation with one-syllable words

Some patients with epilepsy have difficulty performing complex language tasks due to the long duration of the disease and cognitive side effects of antiepileptic drugs. Therefore, a simple passive paradigm would be useful for determining the language dominance lateralization in epilepsy patients. The goal of this study was to develop an efficient and non-invasive analysis method for determining language dominance in epilepsy patients. To this end, magnetoencephalography was performed while an auditory stimulus sequence comprised of two one-syllable spoken words was presented to 17 subjects in an oddball paradigm without subject response. The time-frequency difference between deviant and standard sounds was then analyzed in the source space using a spatial filtering method that was based on minimum-norm estimation. The laterality index was estimated in language-related regions of interest (ROI). The results were compared to the traditional lateralization method using the Wada test. Beta band oscillation activity decreased during deviant stimulation, and the lateralization of the decrease was in good agreement with the Wada test, in the posterior part of the inferior frontal gyrus in 94% of the subjects and in the posterior part of the superior temporal gyrus in 71% of the subjects. In conclusion, the ROI-based time-frequency difference between deviant and standard sounds can be used to assess language lateralization in accordance with the Wada test.

The use of neuroimaging to study behavior in patients with epilepsy

Structural and functional neuroimaging continues to play an increasing role in the presurgical evaluation of patients with epilepsy. In addition to its value in localizing the epileptogenic zone and eloquent cortex, neuroimaging is contributing to our understanding of mood comorbidity in epilepsy. Although the vast majority of research has focused on patients with temporal lobe epilepsy (TLE), neuroimaging studies of patients with extratemporal epilepsy and primary generalized epilepsy are increasing in number. In this review, structural and functional imaging modalities that have received considerable research attention in recent years are reviewed, and their strengths and limitations for understanding behavior in epilepsy are assessed. In addition, advances in multimodal imaging are discussed along with their potential application to the presurgical evaluation of patients with seizure disorders.

Magnetoencephalography for pediatric epilepsy: how we do it

Magnetoencephalography (MEG) is increasingly being used in the preoperative evaluation of pediatric patients with epilepsy. The ability to noninvasively localize ictal onset zones (IOZ) and their relationships to eloquent functional cortex allows the pediatric epilepsy team to more accurately assess the likelihood of postoperative seizure freedom, while more precisely prognosticating the potential functional deficits that may be expected from resective surgery. Confirmation of clinically suggested multifocality may result in a recommendation against resective surgery because the probability of seizure freedom will be low. Current paradigms for motor and somatosensory testing are robust. Paradigms allowing localization of those regions necessary for competent language function, though promising, are under continuous optimization. MR imaging white matter trajectory data, created from diffusion tensor imaging obtained in the same setting as the localization brain MR imaging, provide ancillary information regarding connectivity of the IOZ to sites of rapid secondary spread and the spatial relationship of the IOZ to functionally important white matter bundles, such as the corticospinal tracts. A collaborative effort between neuroradiology, neurology, neurosurgery, neuropsychology, technology, and physics ensures successful implementation of MEG within a pediatric epilepsy program.

In vivo animation of auditory-language-induced gamma-oscillations in children with intractable focal epilepsy

We determined if high-frequency gamma-oscillations (50- to 150-Hz) were induced by simple auditory communication over the language network areas in children with focal epilepsy. Four children (aged 7, 9, 10 and 16 years) with intractable left-hemispheric focal epilepsy underwent extraoperative electrocorticography (ECoG) as well as language mapping using neurostimulation and auditory-language-induced gamma-oscillations on ECoG. The audible communication was recorded concurrently and integrated with ECoG recording to allow for accurate time lock on ECoG analysis. In three children, who successfully completed the auditory-language task, high-frequency gamma-augmentation sequentially involved: i) the posterior superior temporal gyrus when listening to the question, ii) the posterior lateral temporal region and the posterior frontal region in the time interval between question completion and the patient's vocalization, and iii) the pre- and post-central gyri immediately preceding and during the patient's vocalization. The youngest child, with attention deficits, failed to cooperate during the auditory-language task, and high-frequency gamma-augmentation was noted only in the posterior superior temporal gyrus when audible questions were given. The size of language areas suggested by statistically significant high-frequency gamma-augmentation was larger than that defined by neurostimulation. The present method can provide in vivo imaging of electrophysiological activities over the language network areas during language processes. Further studies are warranted to determine whether recording of language-induced gamma-oscillations can supplement language mapping using neurostimulation in presurgical evaluation of children with focal epilepsy.

Interhemispheric differences of spectral power in expressive language: a MEG study with clinical applications

In the last decade we have seen an exponential growth of functional imaging studies investigating multiple aspects of language processing. These studies have sparked an interest in applying some of the paradigms to various clinically relevant questions, such as the identification of the cortical regions mediating language function in surgical candidates for refractory epilepsy. Here we present data from a group of adult control participants in order to investigate the potential of using frequency specific spectral power changes in MEG activation patterns to establish lateralisation of language function using expressive language tasks. In addition, we report on a paediatric patient whose language function was assessed before and after a left hemisphere amygdalo-hippocampectomy. Our verb generation task produced left hemisphere decreases in beta-band power accompanied by right hemisphere increases in low beta-band power in the majority of the control group, a previously unreported phenomenon. This pattern of spectral power was also found in the patient's post-surgery data, though not her pre-surgery data. Comparison of pre and post-operative results also provided some evidence of reorganisation in language related cortex both inter- and intra-hemispherically following surgery. The differences were not limited to changes in localisation of language specific cortex but also changes in the spectral and temporal profile of frontal brain regions during verb generation. While further investigation is required to establish concordance with invasive measures, our data suggest that the methods described may serve as a reliable lateralisation marker for clinical assessment. Furthermore, our findings highlight the potential utility of MEG for the investigation of cortical language functioning in both healthy development and pathology.

Spatiotemporal patterns of oscillatory brain activity during auditory word recognition in children: a synthetic aperture magnetometry study

OBJECTIVE

We studied the task-induced spatiotemporal evolution and characteristics of cortical neural oscillations in children during an auditory word recognition task.

METHODS

We presented abstract nouns binaurally and recorded the MEG response in eight healthy right-handed children (6-12 years). We calculated the event-related changes in cortical oscillations using a beamformer spatial filter analysis technique (SAM), then transformed each subject's statistical maps into standard space and used these to make group statistical inferences.

RESULTS

Across subjects, the cortical response to words could be divided into at least two phases: an initial event-related synchronization in both the right temporal (100-300 ms, 15-25 Hz; 200-400 ms, 5-15 Hz) and left frontal regions (200-400 ms; 15-25 Hz); followed by a strong left-lateralized event-related desynchronization in the left temporal region (500-700 ms; 5-15 Hz).

CONCLUSIONS

We found bilateral event-related synchronization followed by later left lateralized event-related desynchronization in language-related cortical areas. These data demonstrate the spatiotemporal time course of neural activation during an auditory word recognition task in a group of children. As well, this demonstrates the utility of SAM analyses to detect subtle sequential task-related neural activations.

Silence is golden: transient neural deactivation in the prefrontal cortex during attentive reading

It is becoming increasingly clear that attention-demanding tasks engage not only activation of specific cortical regions but also deactivation of other regions that could interfere with the task at hand. At the same time, electrophysiological studies in animals and humans have found that the participation of cortical regions to cognitive processes translates into local synchronization of rhythmic neural activity at frequencies above 40 Hz (so-called gamma-band synchronization). Such synchronization is seen as a potential facilitator of neural communication and synaptic plasticity. We found evidence that cognitive processes can also involve the disruption of gamma-band activity in high-order brain regions. Intracerebral electroencephalograms were recorded in 3 epileptic patients during 2 reading tasks. Visual presentation of words induced a strong deactivation in a broad (20-150 Hz) frequency range in the left ventral lateral prefrontal cortex, in parallel with gamma-band activations within the reading network, including Broca's area. The observed energy decrease in neural signals was reproducible across patients. It peaked around 500 ms after stimulus onset and appeared subject to attention-modulated amplification. Our results suggest that cognition might be mediated by a coordinated interaction between regional gamma-band synchronizations and desynchronizations, possibly reflecting enhanced versus reduced local neural communication.

Use of magnetoencephalography in the presurgical evaluation of epilepsy patients

Magnetoencephalography (MEG) is used twofold for presurgical evaluation of patients with medically intractable partial epilepsy; to identify epileptogenic focus and to investigate functions of cortical areas at or near the epileptogenic focus or structural lesion. For the precise localization of the current source of epileptic discharge, the question as to whether MEG is superior to electroencephalography (EEG) is often addressed. To answer this question, so many factors, both biologically and technically related, have to be taken into consideration. The biological factors include the magnitude of epileptic discharge, its distribution over the cortex, depth of its source from the head surface, and the proportion of large pyramidal neurons tangentially oriented with respect to the head surface within the cortical area. The technical factors include the quality of the recording instrument such as the number of sensors and the use of gradiometer vs. magnetometer, the employed method of source analysis, and availability of experts in each institute. As far as the importance of ictal recording is emphasized, long-term video/EEG monitoring is of utmost importance. Thus, it is concluded that, once the epileptogenic focus is identified by the video/EEG monitoring, then MEG is superior to EEG in order to precisely localize the current source of the interictal epileptic discharge. Another question often addressed is whether MEG can replace the invasive intracranial EEG recording or not. In addition to the above-described factors, different coverage of the cortical areas by MEG vs. invasive intracranial EEG recording has to be taken into account to explain some of the recent reports related to this question. MEG can be effectively applied to the investigation of cortical functions near the epileptogenic focus. It is especially so when combined with other non-invasive studies like functional magnetic resonance imaging (fMRI). In addition to the source analysis of magnetic fields related to various events or tasks, analysis of the task-related change of rhythmic cortical oscillations is a useful tool for studying higher cortical functions such as language in the presurgical evaluation.

Neuropsychology: traditional and new methods of investigation

The neuropsychological assessment is an integral part of the clinical investigation of patients suffering from epilepsy. The aim of the evaluation is to determine disease-related and treatment-related effects on cognition and behavior in order to orient therapeutic interventions, by taking into account the compensatory mechanisms that are available to the patient. Examples of the tests best illustrating the classical neuropsychological protocol are presented. Neuropsychology also plays an important role in the assessment of language lateralization in patients slated for epilepsy surgery. Traditionally, this has been achieved by means of the rather invasive Wada procedure. However, with the advent of new neuroimaging techniques, this procedure is gradually being replaced by minimally invasive or noninvasive methods, such as functional magnetic resonance imaging, positron emission tomography, and optical imaging. In the present paper, we discuss some of the newer techniques that are available to the neuropsychologist for the study of the impact of epilepsy on cerebral functioning.

Magnetoencephalographic analysis of cortical oscillatory activity in patients with brain tumors: Synthetic aperture magnetometry (SAM) functional imaging of delta band activity

Abnormal focal slow wave activity on electroencephalography and magnetoencephalography (MEG) is often seen in patients with various brain pathologies and MEG is capable of localizing cortical oscillatory activity with enhanced accuracy. In addition, MEG with synthetic aperture magnetometry (SAM) can depict changes in cortical oscillatory activity tomographically. Using SAM, we recorded cortical rhythms in patients with a brain tumor and evaluated the tomographic appearance of focal slow wave activity in relation to clinical signs and symptoms. Spontaneous MEG recordings were obtained in 15 patients with brain tumors. Statistically-determined power distributions in the delta-, theta-, and alpha-frequency bands were displayed tomographically and overlaid on individual magnetic resonance images. The location, strength and volume of enhanced activity were analyzed. Delta and theta band activities were significantly more intense in the cortex adjacent to tumors and in the surrounding edematous cortical areas than in other portions of the cortex. In 13 of the 15 patients, spatial distribution of enhanced focal delta activity coincided with the area responsible for the presenting signs and symptoms. Volumetric analysis revealed that emergence of tumor-related focal delta band activity in the cortex adjacent to a tumor, or with peritumoral edema, was greater for intra-axial tumors involving subcortical fibers than for extra-axial tumors. Patients with an increased volume of enhanced delta activity exhibited poor recovery of function in the early postoperative period. It is concluded that SAM imaging of focal delta activity can reveal functional alterations in cortical activity in patients with brain tumors and is useful for assessing cortical states associated with the existing pathology.

Clinical neurophysiology of language: The MEG approach

Clinical evaluation of language function and basic neuroscience research into the neurophysiology of language are tied together. Whole-head MEG systems readily facilitate detailed spatiotemporal characterization of language processes. A fair amount of information is available about the cortical sequence of word perception and comprehension in the auditory and visual domain, which can be applied for clinical use. Language production remains, at present, somewhat less well charted. In clinical practice, the most obvious needs are noninvasive evaluation of the language-dominant hemisphere and mapping of areas involved in language performance to assist surgery. Multiple experimental designs and analysis approaches have been proposed for estimation of language lateralization. Some of them have been compared with the invasive Wada test and need to be tested further. Development of approaches for more comprehensive pre-surgical characterization of language cortex should build on basic neuroscience research, making use of parametric designs that allow functional mapping. Studies of the neural basis of developmental and acquired language disorders, such as dyslexia, stuttering, and aphasia can currently be regarded more as clinical or basic neuroscience research rather than as clinical routine. Such investigations may eventually provide tools for development of individually targeted training procedures and their objective evaluation.

Effects of the emotional connotations in words on the frontal areas--a spatially filtered MEG study

The objective of this study was to elucidate how and where emotional connotations in words influenced linguistic processing in the language-related areas. We recorded neuromagnetic signals in nine right-handed and one left-handed healthy volunteers while they silently read emotional and emotionless words written in Japanese kanji characters, and investigated the distribution of the cerebral oscillatory changes using synthetic aperture magnetometry. Event-related desynchronizations (ERDs) in the beta-low gamma bands were observed in the anterior cingulate cortex (ACC) that were specific to the reading of emotional words in seven of nine right-handers. Beta-low gamma band ERDs were also detected in the left inferior or middle frontal gyrus (IFG or MFG) in nine right-handers regardless of the tasks. The magnitude of the ERDs in the IFG or MFG was significantly greater during emotional-word reading than during emotionless-word reading in nine right-handers. Left-dominant ERDs in the beta and gamma bands were observed in the prefrontal cortex (PFC) when negative emotional words were read (i.e. sadness), while right-dominant ERDs were observed for positive emotional words (i.e. happiness) in seven of nine right-handers (p=0.012, corrected). In one left-hander, the ERD in the ACC and the greater ERD that occurred in the IFG that was specific to emotional-word reading were also observed, but their lateralities in the IFG and PFC were reversed. These results suggest that emotional connotations in words facilitated the ERDs in the frontal language-related areas, and that these facilitations might be modulated by emotional processing in the ACC. Furthermore, negative and positive emotional words may be processed by different mechanisms.

Magnetoencephalography in Neurosurgery

OBJECTIVE:

To present applications of magnetoencephalography (MEG) in studies of neurosurgical patients.

METHODS:

MEG maps magnetic fields generated by electric currents in the brain, and allows the localization of brain areas producing evoked sensory responses and spontaneous electromagnetic activity. The identified sources can be integrated with other imaging modalities, e.g., with magnetic resonance imaging scans of individual patients with brain tumors or intractable epilepsy, or with other types of brain imaging data.

RESULTS:

MEG measurements using modern whole-scalp instruments assist in tailoring individual therapies for neurosurgical patients by producing maps of functionally irretrievable cortical areas and by identifying cortical sources of interictal and ictal epileptiform activity. The excellent time resolution of MEG enables tracking of complex spaciotemporal source patterns, helping, for example, with the separation of the epileptic pacemaker from propagated activity. The combination of noninvasive mapping of subcortical pathways by magnetic resonance imaging diffusion tensor imaging with MEG source localization will, in the near future, provide even more accurate navigational tools for preoperative planning. Other possible future applications of MEG include the noninvasive estimation of language lateralization and the follow-up of brain plasticity elicited by central or peripheral neural lesions or during the treatment of chronic pain.

CONCLUSION:

MEG is a mature technique suitable for producing preoperative “road maps” of eloquent cortical areas and for localizing epileptiform activity.

The assessment of hemispheric lateralization in functional MRI--robustness and reproducibility

Various methods have been proposed to calculate a lateralization index (LI) on the basis of functional magnetic resonance imaging (fMRI) data. Most of them are either based on the extent of the activated brain region (i.e., the number of "active" voxels) or the magnitude of the fMRI signal change. The purpose of the present study was to investigate the characteristics of various variants of these approaches and to identify the one that yields the most robust and reproducible results. Robustness was assessed by evaluating the dependence on arbitrary external parameters, reproducibility was assessed by Pearson's correlation coefficient. LIs based on active voxels counts at one single fixed statistical threshold as well as LIs based on unthresholded signal intensity changes (i.e., based on all voxels in a region of interest) yielded neither robust nor reproducible laterality results. Instead, the lateralization of a cognitive function was best described by "thresholded" signal intensity changes where the activity measure was based on signal intensity changes in those voxels in a region of interest that exceeded a predefined activation level. However, not all other approaches should be discarded completely since they have their own specific application fields. First, LIs based on active voxel counts in the form of p-value-dependent lateralization plots (LI=LI(p)) can be used as a straightforward measure to describe hemispheric dominance. Second, LIs based on active voxel counts at variable thresholds (standardized by the total number of active voxels) are a good alternative for big regions of interest since LIs based on signal intensity changes are restricted to small ROIs.

Language lateralization in magnetoencephalography: two tasks to investigate hemispheric dominance

Hemispheric specialization for language has been the focus of many studies, mainly using functional magnetic resonance imaging. Here, we used magnetoencephalography to investigate hemispheric dominance and time-dependent aspects of cortical language processing. We implemented a verb generation task and a newly designed vowel identification task. Eleven healthy adults were investigated. By using oscillatory magnetoencephalography spectral analysis, significant hemispheric differences were found for both tasks in cerebral language areas. Robust left-lateralization in frontal brain regions was observed with the verb generation task, confirming previous functional magnetic resonance imaging and magnetoencephalography studies. Our new vowel identification task yields significant left-lateralization in posterior language regions, making this silent and child-friendly task a valuable alternative for non-invasive language assessment in difficult populations.

Post-movement beta rebound is generated in motor cortex: evidence from neuromagnetic recordings

Voluntary movements are accompanied by amplitude changes in cortical rhythms presumably as a result of functional activation of sensorimotor areas. Recently, the location of the neural generators involved in increasing power within the beta (15-30 Hz) frequency band following movement (post-movement beta rebound, PMBR) has come into question [Parkes, L.M, Bastiaansen, M.C.M, Norris, D.G., 2006. Combining EEG and fMRI to investigate the post-movement beta rebound. NeuroImage 29, 685-696.]. We used the synthetic aperture magnetometry (SAM) spatial filtering method to identify the time course and location of oscillatory changes within the beta and mu (8-14 Hz) frequency bands during the performance of voluntary movements. Neuromagnetic activity was recorded from 10 adult subjects during abduction of the right index finger. Changes in beta and mu source power were calculated for periods during and following movement, relative to pre-movement baseline activity. Decreases in beta band activity (event-related desynchronization, ERD) were observed during movement, with a strong increase (PMBR) beginning 230+/-170 ms following movement, lasting for 680+/-170 ms. Mu band ERD was observed both during and following movement, with little to no post-movement rebound. Beta and mu ERD were localized bilaterally to the hand region of postcentral gyrus whereas PMBR was localized bilaterally to the hand region of precentral gyrus (motor cortex). Both PMBR and beta ERD were strongest contralateral to the side of movement. These results provide further evidence that movement influences independent cortical rhythms in sensorimotor areas, and confirm previous reports of precentral generators of PMBR in the region of motor cortex, with postcentral generators of beta and mu ERD during movement.

Revisiting the role of magnetoencephalography in epilepsy

PURPOSE OF REVIEW

This review considers the current role of magnetoencephalography in clinical epileptology.

RECENT FINDINGS

While magnetoencephalography and electroencephalography complement each other for interictal spike detection, magnetoencephalography is more sensitive in neocortical epilepsy. In temporal lobe epilepsy, magnetoencephalography can attribute epileptic activity to subcompartments of the temporal lobe and differentiate between patients with mesial, lateral and diffuse seizure onsets. In extratemporal epilepsy, magnetoencephalography provides unique information in nonlesional cases and helps to define the relationship of epileptic activity with respect to lesions and eloquent cortex. Magnetoencephalography also contributes to the clinical decision process in patients with cortical dysplasias, Landau-Kleffner syndrome and recurrent seizures after prior epilepsy surgery. Magnetoencephalography-guided re-evaluation of magnetic resonance imaging helps to reveal previously unrecognized lesions. In a presurgical setting interictal magnetoencephalography was superior to scalp electroencephalography. Complete resection of the magnetoencephalography-defined irritative zone has prognostic implications on postoperative seizure control. Magnetoencephalography can reliably localize sensorimotor and language cortex. Disadvantages of this technique include the difficulties in obtaining ictal recordings and the considerable costs involved.

SUMMARY

Magnetoencephalography has been developed to a valuable noninvasive tool in clinical epileptology. The development of approaches which take into account both magnetoencephalography and electroencephalography simultaneously should provide more useful information in the future.

A verifiable solution to the MEG inverse problem

Magnetoencephalography (MEG) is a non-invasive brain imaging technique with the potential for very high temporal and spatial resolution of neuronal activity. The main stumbling block for the technique has been that the estimation of a neuronal current distribution, based on sensor data outside the head, is an inverse problem with an infinity of possible solutions. Many inversion techniques exist, all using different a-priori assumptions in order to reduce the number of possible solutions. Although all techniques can be thoroughly tested in simulation, implicit in the simulations are the experimenter's own assumptions about realistic brain function. To date, the only way to test the validity of inversions based on real MEG data has been through direct surgical validation, or through comparison with invasive primate data. In this work, we constructed a null hypothesis that the reconstruction of neuronal activity contains no information on the distribution of the cortical grey matter. To test this, we repeatedly compared rotated sections of grey matter with a beamformer estimate of neuronal activity to generate a distribution of mutual information values. The significance of the comparison between the un-rotated anatomical information and the electrical estimate was subsequently assessed against this distribution. We found that there was significant (P < 0.05) anatomical information contained in the beamformer images across a number of frequency bands. Based on the limited data presented here, we can say that the assumptions behind the beamformer algorithm are not unreasonable for the visual-motor task investigated.

Neuromagnetic Changes of Brain Rhythm Evoked by Intravenous Olfactory Stimulation in Humans

To identify the changes in the respective frequency band and brain areas related to olfactory perception, we measured magnetoencephalographic (MEG) signals before and after instilling intravenously thiamine propyl disulfide (TPD) and thiamine tetrahydrofurfuryl disulfide monohydrochloride (TTFD), which evoked a strong and weak sensation of odor, respectively. For the frequency analysis of MEG, a beamformer program, synthetic aperture magnetometry (SAM), was employed and event-related desynchronization (ERD) or synchronization (ERS) was statistically determined. Both strong and weak odors induced ERD in (1) beta band (13-30 Hz) in the right precentral gyrus, and the superior and middle frontal gyri in both hemispheres, (2) low gamma band (30-60 Hz) in the left superior frontal gyrus and superior parietal lobule, and the middle frontal gyrus in both hemispheres, and (3) high gamma band 2 (100-200 Hz) in the right inferior frontal gyrus. TPD induced ERD in the left temporal, parietal and occipital lobes, while TTFD induced ERD in the right temporal, parietal and occipital lobes. The results indicate that physiological functions in several regions in the frontal lobe may change and the strength of the odor may play a different role in each hemisphere during olfactory perception in humans.

Pediatric Magnetoencephalography and Magnetic Source Imaging

Magnetoencephalography (MEG) and magnetic source imaging (MSI) together represent a uniquely powerful functional imaging modality because of their capabilities of directly observing the electrophysiologic activity of neurons with exquisite temporal detail and accurately localizing corresponding neuromagnetic field sources onto high-resolution MR images. These features have and should continue to advance our understanding of the complex spatiotemporal basis of normal and abnormal brain function and development in children. By more clearly delineating and characterizing epileptogenic foci and their relation to eloquent cortex, MSI enables earlier and more effective neurosurgery to be performed, thus resulting in improved seizure outcomes. Although MEG and MSI cannot replace scalp electroencephalography, neuropsychologic testing, and the need for meticulous intraoperative cortical mapping in patients undergoing excision of epileptogenic lesions, their increasing availability should ultimately persuade many clinicians of their key, if not essential, role in the evaluation and treatment of children with epilepsy.

Neuromagnetic responses to vowels vs. tones reveal hemispheric lateralization

OBJECTIVE

To evaluate whether a simple auditory paradigm could demonstrate a difference in cortical lateralization between right- and left-handed subjects. Such information would be important for later development of clinical noninvasive tests of hemispheric language dominance in candidates for brain surgery.

METHODS

Healthy subjects (10 strongly right-handed, 10 strongly left-handed, 5 weakly right-handed, and two ambidextrous) listened to binaural pairs of tones and pairs of Finnish vowels and decided whether the items in the pair were the same (target probability 20%). Cortical responses were recorded with whole-scalp magnetoencephalography.

RESULTS

The laterality index for strengths of the auditory-cortex 100 ms responses (N100m) to vowels vs. tones suggested left-hemispheric dominance in 8 of the 10 strongly right-handed subjects, and right-hemispheric dominance in 7 of the 10 left-handed subjects.

CONCLUSIONS

Our results demonstrate difference in hemispheric dominance for processing of vowels between right-handed and left-handed subjects. This difference resembles language lateralization suggested by previous invasive studies as well as by anatomical and functional comparisons in left- and right-handed subjects.

SIGNIFICANCE

After comparison with the Wada test, this simple paradigm could prove useful as a noninvasive test for language lateralization in clinical settings.

Language lateralization development in children with autism: Insights from the late field magnetoencephalogram

Left hemisphere dominance represents the typical language lateralization profile for the majority of neurologically healthy, right-handed individuals. We investigated hemispheric dominance for language in language-impaired children with autism and typically developing controls to investigate the hypothesis that atypical functional specialization for language represents one component of developmental language impairment in autism. Late field magnetoencephalography (MEG) recordings were used to calculate a hemispheric Lateralization Index from the neuromagnetic activity evoked by passive auditory presentation of vowel stimuli. Results indicate that children with autism and typically developing children follow opposite maturational trajectories in language lateralization; while leftward lateralization (i.e. left hemisphere dominance) emerged from bilaterally symmetric neuronal activation as age increased in our sample of typically developing children, rightward lateralization emerged from bilaterally symmetric activity as age increased in our sample of children with autism.

Alternatives to the Wada test: a critical view of functional magnetic resonance imaging in preoperative use

PURPOSE OF REVIEW

The Wada or intracarotid amobarbital procedure is already in its fifties and, despite its invasive character, still routine for determining the lateralization of language and memory prior to epileptic surgery. Among the new techniques available, functional magnetic resonance imaging is one of the most promising alternatives. This non-invasive method has several advantages including the possibility of mapping relevant areas within the hemispheres and being able to prolong examination time in case of discordant results.

RECENT FINDINGS

Many functional magnetic resonance imaging studies have focused on correlations with the intracarotid amobarbital procedure as the gold standard and found an agreement of about 90%. More importantly, recent studies demonstrated a significant correlation between presurgical functional magnetic resonance imaging testing and postsurgical outcome for functional magnetic resonance imaging activations in frontal language areas. In some studies, prediction for outcome is already higher for functional magnetic resonance imaging than for the intracarotid amobarbital procedure.

SUMMARY

Current data support functional magnetic resonance imaging as a valid alternative to the intracarotid amobarbital procedure. Small sample sizes in outcome studies and restrictions to certain sites of operation, however, still call for caution. A standardized series of tasks to activate the whole language and memory system paired with good comparability between medical centres is needed.