Assessing normal brain function with magnetoencephalography

Journal of Neurosurgery, 1999

Object. In this paper the authors demonstrate the concordance between magnetic source (MS) imaging and direct cortical stimulation for mapping receptive language cortex.Methods. In 13 consecutive surgical patients, cortex specialized for receptive language functions was identified noninvasively by obtaining activation maps aided by MS imaging in the context of visual and auditory word-recognition tasks. Surgery was then performed for treatment of medically intractable seizure disorder (eight patients), and for resection of tumor (four), or angioma (one). Mapping of language areas with cortical stimulation was performed intraoperatively in 10 patients and extraoperatively in three. Cortical stimulation mapping verified the accuracy of the MS imaging—based localization in all cases.Conclusions. Information provided by MS imaging can be especially helpful in cases of atypical language representation, including bihemispheric representation, and location of language in areas other than t...

Cerebral Cortex

The purpose of the present investigation was to describe spatiotemporal brain activation profiles during word reading using magnetic source imaging (MSI). Ten right-handed dyslexic children with severe phonological decoding problems ...

Psychology, 2014

This study investigates the functional connectivity of neuronal networks critical for working memory in individuals with dyslexia by means of magnetoenchephalographic (MEG) coherence imaging. Individuals with dyslexia showed an early onset of activation in anterior cortical regions (precentral gyrus and the superior frontal gyrus), which differed from controls where activation initiated in posterior cortical regions (supramarginal gyrus and superior temporal gyrus). Further, individuals with dyslexia showed lower brain activity in the right superior temporal gyrus and right middle temporal gyrus than controls during a spatial working memory (SWM) task. In contrast, during a verbal working memory (VWM) task, individuals with dyslexia showed lower activity in the right insular cortex and right superior temporal gyrus and higher, likely compensatory, activity in the right fusiform gyrus, left parahippocampal gyrus, and left precentral gyrus. When performing a SWM task, individuals with dyslexia showed significantly lower coherent activity and synchronization in 1) right frontal connectivity, 2) right fronto-temporal connectivity, 3) left and right frontal connectivity, 4) left temporal and right frontal connectivity, and 5) left occipital and right frontal connectivity. MEG coherence source imaging (CSI) by frequency bands showed lower * Corresponding author.

The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry, 2006

Magnetoencephalography (MEG) is a noninvasive neuroimaging method for detecting, analyzing, and interpreting the magnetic field generated by the electrical activity in the brain. Modern hardware can capture the MEG signal at hundreds of points around the head in a snapshot lasting only a fraction of a millisecond. The sensitivity of modern hardware is high enough to permit the extraction of a clean signal generated by the brain well above the noise level of the MEG hardware. It is possible to identify signatures of superficial and often deep generators in the raw MEG signal, even in snapshots of data. In a more quantitative way, tomographic images of the electrical current density in the brain can be extracted from each snapshot of MEG signal, providing a direct correlate of coherent collective neuronal activity. A number of recent studies have scrutinized brain function in the new spatiotemporal window that real-time tomographic analysis of MEG signals has opened. The results have ...

Cerebral Cortex, 2000

The purpose of the present investigation was to describe spatiotemporal brain activation profiles during word reading using magnetic source imaging (MSI). Ten right-handed dyslexic children with severe phonological decoding problems and eight age-matched non-impaired readers were tested in two recognition tasks, one involving spoken and the other printed words. Dyslexic children's activation profiles during the printed word recognition task consistently featured activation of the left basal temporal cortices followed by activation of the right temporoparietal areas (including the angular gyrus). Non-impaired readers showed predominant activation of left basal followed by left temporoparietal activation. In addition, we were able to rule out the hypothesis that hypoactivation of left temporoparietal areas in dyslexics was due to a more general cerebral dysfunction in these areas. Rather, it seems likely that reading difficulties in developmental dyslexia are associated with an aberrant pattern of functional connectivity between brain areas normally involved in reading, namely ventral visual association cortex and temporoparietal areas in the left hemisphere. The interindividual consistency of activation profiles characteristic of children with dyslexia underlines the potential utility of this technique for examining neurophysiological changes in response to specific educational intervention approaches.

Neuroscience Letters

The purpose of the study was to identify spatiotemporal brain activation profiles associated with phonological decoding in dyslexic children using magnetic source imaging. For this purpose maps of regional cerebral ...

European Journal of Neuroscience, 1995

The technique of functional magnetic resonance imaging (FMRI) allows the measurement of functional cerebral blood flow changes occurring with specific tasks. However, the spatial relationship between neuronal activity and functional cerebral blood flow changes is not known yet. This study compares the centre of neuronal activation (measured by magnetoencephalography) with that of the blood flow response (measured by FMRI) to unilateral motor stimulation in eight subjects. The results show a mean localization difference of 1.6 cm and demand application of methodological improvements as recently suggested.

Research Square (Research Square), 2021

Verbal uency (VF) is a heterogeneous test that requires executive functions as well as language abilities. The purpose of this study was to elucidate the speci city of the resting state MEG correlates of the executive and language components. To this end, we administered a VFtest, another verbal test (Vocabulary), and another executive test (Trail Making Test), and we recorded 5-min eyes-open restingstate MEG data in 28 healthy participants. We used source-reconstructed spectral power estimates to compute correlation/anticorrelation MEG clusters with the performance at each test, as well as with the advantage in performance between tests, across individuals using cluster-level statisticsin the standard frequency bands. By obtaining conjunction clusters between verbal uency scores and factor loading obtained for verbal uency and each of the two other tests, we showed a core of slow clusters (delta to beta) localized in the right hemisphere, in adjacent parts of the premotor, pre-central and post-central cortex in the mid-lateral regions related to executive monitoring. We also found slow parietal clusters bilaterally and a cluster in the gamma 2 and 3 bandsin the left inferior frontal gyrus likely associated with phonological processinginvolved in verbal uency. 4, participants have to draw lines alternating between letters and numbers printed on a page, but in their alphabetical or numerical order as quickly and accurately as they can. The response is scored as completed within the time limit. In all the analyses, we only used the scaled scores, corrected for age. The three test scores did not show any signi cant correlation between them: VFL and TMT (r=.-013; p = .947), VFL and VOC (r = .211; p = .282), and TMT with VOC (r = .350; p = .068). MEG and Anatomical MRI Data Acquisition All 28 subjects were comfortably seated with eyes open, xating a back-illuminated screen located 75 cm in front of them. Two 5-minute periods of resting state were recorded at a sampling rate of 1200 Hz, using a CTF-VSM whole head 275-sensor MEG system (MEG core facility, Psychology Department, University of Montreal, QC, Canada). Following standard procedures, third-order gradiometer noise reduction was computed based on twenty-nine reference channels. Bipolar EOG (Vertical EOG and Horizontal EOG) was recorded in order to monitor eye blinks and ocular movements. ECG was also recorded to monitor heartbeats. Three head coils xed at the nasion and the bilateral preauricular points were used for head localization and were monitored at the beginning and the end of each session. Particular care was taken to ensure that head displacement across sessions remained below 5 mm. The neuropsychological assessments were done in the morning at the Ste-Justine Hospital (Montreal, QC, Canada). Later in the afternoon, the participants went to the MEG facility, located in the Psychology Department of the University of Montreal, for the MEG recordings.Structural MRI images were obtained for each subject with a 3-T General Electric (GE) scanner (Saint-Justine Hospital, Montreal, QC, Canada). The individual surfaces were used to carry out the co-registration between the MEG ducial markers (LAP, NAS, RAP) and the MRI structural image. The exact position of the head was re ned based on head shape position les obtained using a 3D-localization Polhemus system (Oswald et al., 2017). Data Pre-processing MEG data pre-processing was performed using the Matlab-based Brainstorm open-source software. The data was rst notch ltered at 60 Hz, and then between 0.5 Hz and 120 Hz. Cardiac artefacts, eye blinks, and eye movements were corrected using the Signal-Space-Projection method (SSP). Fifty signal epochs, centred on each artefact, were selected, and a singular value decomposition was applied to each artefact using built-in Matlab functions. Eigenvectors explaining at least 10 % of the variance of the artefacts were discarded and the remaining eigenvectors were used to de ne the SSP. The SSP method relies on a signal space decomposition procedure, where the statistical characteristics of the measured signals are used to determine the two subspaces spanned by the MEG brain signals and the unwanted artifacts, respectively. Projecting the continuous MEG data onto the signal subspace effectively removes the components belonging to the artifact subspace (Oswald et al., 2017).

Journal of Neurolinguistics, 2003

We propose five separate criteria for establishing the clinical and research utility of a functional neuroimaging method for the study of language. These criteria are the functional specificity, reliability, validity, spatial and temporal resolution, and logistical ease of the technique for studying language processes in patient and control populations. We describe the newest of the functional neuroimaging techniques, magnetic source imaging, and its advantages in satisfying these criteria. Finally, we review a series of language studies that have revealed dissociations between the relative contributions of superior temporal and middle and mesial temporal cortices in phonological versus semantic processing. q

NeuroImage, 1998

Visual and auditory tasks involving episodicmemory for verbalmaterials were used to elicit stimulusand taskspecific activation in normal subjectsand in candidates for brain surgery. That activity was recorded by a whole-head neuromagnetometer and its' sourceswere identified with specific brain structuresin each normal subject and patient. Task-related activity sources were found consistently in the perisylvian cortex and the hippocampus. In 90% of the subjects and patients, twice as much activity was found in the left as compared to the right hemisphere structures. Moreover, in two cases, intraoperative elcctrophysiology was used to verify the accuracy of MEG identification of language-specific cortex. The implications of these findings for non-invasive functional mappingof the brain through MEG,and its medical applications willbe discussed.

Human Brain Mapping, 2001

We used a current localization by spatial filtering-technique to determine primary language areas with magnetoencephalography (MEG) using a silent reading and a silent naming task. In all cases we could localize the sensory speech area (Wernicke) in the posterior part of the left superior temporal gyrus (Brodmann area 22) and the motor speech area (Broca) in the left inferior frontal gyrus (Brodmann area 44). Left hemispheric speech dominance was determined in all cases by a laterality index comparing the current source strength of the activated left side speech areas to their right side homologous. In 12 cases we found early Wernicke and later Broca activation corresponding to the Wernicke-Geschwind model. In three cases, however, we also found early Broca activation indicating that speech-related brain areas need not necessarily be activated sequentially but can also be activated simultaneously. Magnetoencephalography can be a potent tool for functional mapping of speech-related brain areas in individuals, investigating the time-course of brain activation, and identifying the speech dominant hemisphere. This may have implications for presurgical planning in epilepsy and brain tumor patients.

Neuropsychology, 2007

Intervention-related changes in spatiotemporal profiles of regional brain activation were examined by whole-head magnetoencephalography in 15 children with severe reading difficulties who had failed to show adequate progress to quality reading instruction during Grade 1. Intensive intervention initially focused on phonological decoding skills (for 8 weeks) and, during the subsequent 8 weeks, on rapid word recognition ability. Clinically significant improvement in reading skills was noted in 8 children who showed "normalizing" changes in their spatiotemporal profiles of regional brain activity (increased duration of activity in the left temporoparietal region and a shift in the relative timing of activity in temporoparietal and inferior frontal regions). Seven children who demonstrated "compensatory" changes in brain activity (increased duration of activity in the right temporoparietal region and frontal areas, bilaterally) did not show adequate response to intervention. Nonimpaired readers did not show systematic changes in brain activity across visits. 496 SIMOS ET AL.

2009

Magnetoencephalography (MEG) offers a unique way to non-invasively monitor the neural activity in the human brain. MEG is based on measuring the very weak magnetic fields generated by the electric currents in the active neurons. Such measurements allow, with certain limitations, estimating the underlying current distribution and thus the locations and time courses of the neural generators with an excellent temporal resolution.

Magnetoencephalography (MEG) is a tool to investigate sources of brain activity with high spatial and temporal resolution. MEG also allows to evaluate higher cognitive functions like speech processing . In this study we tried to investigate the time course of cortex activation of speech processing and to localize the underlying sources in a first step. Secondly we wanted to make these localizations available for clinical application. Functional mapping of speech eloquent areas is of high value for patients with brain tumors in the speech dominant hemisphere adjacent to the sensory (Wernicke) or the motor speech area (Broca). Neurosurgical treatment around eloquent brain areas bears a high risk of postoperative neurological deficits. To date the invasive Wada test [14] is the clinical standard to investigate speech lateralisation and memory functions. Functional mapping of eloquent areas is performed by intraoperative electrocorticography in local anesthesia at a few specialized neurosurgical centers only [11]. In recent years first results of noninvasive methods based on different physiologic mechanisms like MEG [3,8,13], fMRI [1,15,16] and PET [9,16] for mapping speech relevant areas and determining the speech dominant hemisphere were presented. Our aim was to preoperatively identify the speech dominant hemisphere and localize the speech relevant brain areas noninvasively by means of MEG. Similar to surgery of tumors around the sensorimotor cortex by functional neuronavigation [2,5] as performed in our department we wanted to make the MEG localizations intraoperatively available for the surgeon by displaying the Broca or Wernicke area in the operation microscope. 2 Methods We investigated 8 healthy subjects (5 male, 3 female, age range between 20 and 35 years) and 44 patients (27 male, 17 female, between 10 and 69 years) with brain tumors adjacent to the Wernicke or Broca area. 30 patients suffered from permanent or incidental speech disorders during seizures. All subjects and 38 patients revealed right handedness, six patients were clearly left handed, tested by the Edinburgh Handedness Inventory [7]

Neuroscience Letters, 2000

The purpose of the study was to identify spatiotemporal brain activation pro®les associated with phonological decoding in dyslexic children using magnetic source imaging. For this purpose maps of regional cerebral activation were obtained from eleven children diagnosed with dyslexia and ten children without reading problems during engagement in a pseudoword rhyme-matching task. All dyslexic children showed aberrant activation maps consisting of reduced activity in temporoparietal areas in the left hemisphere (including the posterior part of the superior temporal, angular and supramarginal gyri) and increased activity in the right homotopic region. In contrast, the two groups of children did not differ in the degree of activity in basal temporal areas that typically precedes temporoparietal activation. This is the ®rst study to demonstrate the existence of distinct activation pro®les associated with phonological decoding in individual dyslexic children.