Lateralization of language and music: A whole-head meg study
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Magnetoencephalography
Magnetoencephalography (MEG) is a technique that enables the measurement of the magnetic fields produced by the brain. It is a noninvasive method that allows, similarly to electroencephalography (EEG), to follow the evolution of electrophysiological processes in the millisecond scale. It is used for localizing functional regions of the brain, with a better spatial resolution than EEG, and for assessing the health of sensory pathways. The most important clinical applications of MEG are the presurgical functional localization and the localization of epileptiform activity.
Magnetoencephalography
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Magnetoencephalography is a newly developed technology used for diagnostic and brain mapping imaging during the presurgical evaluation of patients with medically intractable epilepsy. It provides comprehensive localisation of an epileptogenic focus using simultaneous recordings from the entire brain surface. Magnetoencephalography and electroencephalography are considered complementary and confirmatory to one another. We present a patient with magnetic resonance imaging-negative, non-lesional, neocortical epilepsy. Magnetoencephalography was used for re-evaluation of the epileptogenic zone and this enabled subsequent surgical removal of the epileptic focus. The role of magnetoencephalography in epilepsy surgery is discussed in this report.
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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.
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Magnetoencephalography is the detection of the magnetic field distribution across the surface of the head, which is generated by a neuronal discharge within the brain. Magnetoencephalography is used in clinical epilepsy to localize the epileptogenic region prior to its surgical removal. A discussion of the instrumentation based on the superconducting quantum interference device that is used for detecting the magnetic field distribution, the analytical techniques, current research, and future directions of magnetoencephalography in epilepsy research is presented.
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Schizophrenia impairs many cognitive functions, and abnormalities in language processing have been proposed as one of the bases for this disorder. Previously, it was reported that different magnetoencephalography (MEG) patterns of the evoked oscillatory activity (eOA) of 20–45 Hz to speech and nonspeech sounds were evidence of a fast mechanism for the representation and identification of speech sounds in humans. The current study tested the hypothesis that the schizophrenics would show abnormal neural oscillatory activity, as measured by eOA, to speech and nonspeech sounds. Twenty patients and 23 control subjects participated in this study. MEG responses to speech and nonspeech sounds were recorded and eOA power and phase locking at 20–45 Hz were analyzed. Patients showed significantly delayed peak latencies of the eOA power and phase locking to speech sounds in the left hemisphere and to nonspeech sounds in the right hemisphere. Patients also showed a significantly reduced eOA power to speech sounds in the left hemisphere in 0–50 ms and a significantly larger eOA power to speech sounds in the left hemisphere in 100–150 ms. In addition, the analyses of the lateralization index revealed the pattern of hemispheric lateralization to be the opposite in patients. These results indicated that patients showed different characteristics of eOA compared with normal controls, probably related to deficits in a fast mechanism for identifying speech sounds. Moreover, the present study suggests that schizophrenia might be characterized by an opposite pattern of hemispheric lateralization in auditory evoked oscillations.
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The theory of magnetoencephalography (MEG) and its application to epilepsy research are reviewed briefly. The MEG prediction appears to agree in general with regions where epileptiform discharges are found on the electrocorticogram. MEG appears to have a somewhat better localizing capability than EEG, although MEG may well miss the tangential component of magnetic fields. Thus, the combination of MEG and EEG may be more fruitful than either one alone.
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Magnetoencephalography
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Brain Function
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Magnetoencephalography
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Summary: Normal variants, although not occurring frequently, may appear similar to epileptic activity. Misinterpretation may lead to false diagnoses. In the context of presurgical evaluation, normal variants may lead to mislocalizations with severe impact on the viability and success of surgical therapy. While the different variants are well known in EEG, little has been published in regard to their appearance in magnetoencephalography. Furthermore, there are some magnetoencephalography normal variants that have no counterparts in EEG. This article reviews benign epileptiform variants and provides examples in EEG and magnetoencephalography. In addition, the potential of oscillatory configurations in different frequency bands to appear as epileptic activity is discussed.
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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.
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