Relative influence of model assumptions and measurement procedures in the analysis of the MEG

Meijs, J. W. H.; Peters, M.J.; Boom, H.B.K.; Silva, F.H. Lopes da

doi:10.1007/bf02442255

Cited by 24 publications

(8 citation statements)

References 11 publications

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“…The boundary element method (BEM) was used for calculating the signal expected at each magnetoencephalograph sensor, for each dipole location (deMunck, 1992;Oostendorp and Van Oosterom, 1992). The computation of the MEG forward solution has been shown to require only the inner skull boundary to achieve an accurate solution (Meijs et al, 1988;Hamalainen and Sarvas, 1989).…”

Section: Figmentioning

confidence: 99%

N400-like Magnetoencephalography Responses Modulated by Semantic Context, Word Frequency, and Lexical Class in Sentences

et al. 2002

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Words have been found to elicit a negative potential at the scalp peaking at ϳ400 ms that is strongly modulated by semantic context. The current study used whole-head magnetoencephalography (MEG) as male subjects read sentences ending with semantically congruous or incongruous words. Compared with congruous words, sentence-terminal incongruous words consistently evoked a large magnetic field over the left hemisphere, peaking at ϳ450 ms. Source modeling at this latency with conventional equivalent current dipoles (ECDs) placed the N400m generator in or near the left superior temporal sulcus. A distributed solution constrained to the cortical surface suggested a sequence of differential activation, beginning in Wernicke's area at ϳ250 ms, spreading to anterior temporal sites at ϳ270 ms, to Broca's area by ϳ300 ms, to dorsolateral prefrontal cortices by ϳ320 ms, and to anterior orbital and frontopolar cortices by ϳ370 ms. Differential activity was exclusively left-sided until >370 ms, and then involved right anterior temporal and orbital cortices. At the peak of the N400m, activation in the left hemisphere was estimated to be widespread in the anterior temporal, perisylvian, orbital, frontopolar, and dorsolateral prefrontal cortices. In the right hemisphere, the orbital, as well as, weakly, the right anterior temporal cortices were activated. Similar but weaker field patterns were evoked by intermediate words in the sentences, especially to low-frequency words occurring in early sentence positions where there is little preceding context. The locations of the N400m sources identified with the distributed solution correspond well with those previously demonstrated with direct intracranial recordings, and suggested by functional magnetic resonance imaging (fMRI). These results help identify a distributed cortical network that supports online semantic processing. © 2002 Elsevier Science (USA)

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Section: Figmentioning

confidence: 99%

N400-like Magnetoencephalography Responses Modulated by Semantic Context, Word Frequency, and Lexical Class in Sentences

et al. 2002

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“…The magnitude factor (MAG) is a measure of the difference in magnitude between the two and is defined as follows (Meijs et al, 1988):…”

Section: Quantitative Measuresmentioning

confidence: 99%

Patient-specific solution of the electrocorticography forward problem in deforming brain

Zwick¹,

Bourantas²,

Safdar³

et al. 2021

Preprint

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Invasive intracranial electroencephalography (iEEG) or electrocorticography (ECoG) measures electrical potential directly on the surface of the brain, and, combined with numerical modeling, can be used to inform treatment planning for epilepsy surgery. Accurate solution of the iEEG or ECoG forward problem, which is a crucial prerequisite for solving the inverse problem in epilepsy seizure onset localization, requires accurate representation of the patient's brain geometry and tissue electrical conductivity after implantation of electrodes. However, implantation of subdural grid electrodes causes the brain to deform, which invalidates preoperatively acquired image data. Moreover, postoperative magnetic resonance imaging (MRI) is incompatible with implanted electrodes and computed tomography (CT) has insufficient range of soft tissue contrast, which precludes both MRI and CT from being used to obtain the deformed postoperative geometry. In this paper, we present a biomechanics-based image warping procedure using preoperative MRI for tissue classification and postoperative CT for locating implanted electrodes to perform non-rigid registration of the preoperative image data to the postoperative configuration. We solve the iEEG forward problem on the predicted postoperative geometry using the finite element method (FEM) which accounts for patient-specific inhomogeneity and anisotropy of tissue conductivity. Results for the simulation of a current source in the brain show large differences in electrical potential (RDM ≥ 0.18 and MAG ≥ 0.84) predicted by the models based on the original images and the deformed images corresponding to the brain geometry deformed by placement of invasive electrodes. Computation of the leadfield matrix (useful for solution of the iEEG inverse problem) also showed significant differences between the different models. The results suggest that significant improvements in source localization accuracy may be realized by the application of the proposed modeling methodology.

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“…It has been frequently reported that just considering the inner skull boundary is sufficient for the MEG forward calculations [Meijs et al, 1988;Hämäläinen and Sarvas, 1989]. The boundary surface used for the BEM was generated by commercial source analysis software (ASA v. 2.1; ANT Software) and was composed of 1,016 elements and 510 nodes.…”

Section: Simulation Study Using Forward Data Simulation Set-ups and Rmentioning

confidence: 99%

fMRI‐constrained MEG source imaging and consideration of fMRI invisible sources

Jung

Fujimaki

2005

Human Brain Mapping

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Recent studies on multimodal brain source imaging have shown that the use of functional MRI (fMRI) prior information could enhance spatial resolution of magnetoencephalography (MEG), while MEG could compensate poor temporal resolution of fMRI. This article deals with a multimodal imaging method, which combines fMRI and MEG for enhancing both spatial and temporal resolutions. Recent studies on the combination of fMRI and MEG have suggested that the fMRI prior information could be very easily implemented by just giving different weighting factors to the diagonal terms of source covariance matrix in linear inverse operator. We applied the fMRI constrained imaging method to several simulation data and experimental data (Japanese language lexical judgment experiment), and found that some MEG sources may be eliminated by the introduction of the fMRI weighting and the eliminated sources may affect source estimation in fMRI activation regions. In this article, in order to check whether the eliminated sources were fMRI invisible ones or just spurious ones, we placed small numbers of regional sources (rotating dipoles) around all possible activation regions and investigated their temporal changes. By investigating the results carefully, we could evaluate whether the missed sources were real or not.

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Relative influence of model assumptions and measurement procedures in the analysis of the MEG

Cited by 24 publications

References 11 publications

N400-like Magnetoencephalography Responses Modulated by Semantic Context, Word Frequency, and Lexical Class in Sentences

N400-like Magnetoencephalography Responses Modulated by Semantic Context, Word Frequency, and Lexical Class in Sentences

Patient-specific solution of the electrocorticography forward problem in deforming brain

fMRI‐constrained MEG source imaging and consideration of fMRI invisible sources

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