4,026 research outputs found
Recommended from our members
The role of HG in the analysis of temporal iteration and interaural correlation
Magnetoencephalography in Stroke Recovery and Rehabilitation
Magnetoencephalography (MEG) is a non-invasive neurophysiological technique used to study the cerebral cortex. Currently, MEG is mainly used clinically to localize epileptic foci and eloquent brain areas in order to avoid damage during neurosurgery. MEG might, however, also be of help in monitoring stroke recovery and rehabilitation. This review focuses on experimental use of MEG in neurorehabilitation. MEG has been employed to detect early modifications in neuroplasticity and connectivity, but there is insufficient evidence as to whether these methods are sensitive enough to be used as a clinical diagnostic test. MEG has also been exploited to derive the relationship between brain activity and movement kinematics for a motor-based brain-computer interface. In the current body of experimental research, MEG appears to be a powerful tool in neurorehabilitation, but it is necessary to produce new data to confirm its clinical utility
Multimodal imaging of language perception
This Thesis draws together several lines of research by examining language perception in the same individuals using three neuroimaging methods: magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), and electroencephalography (EEG).
The MEG experiments conducted in this Thesis demonstrated that neural processing of written and spoken words converges to the superior temporal cortex following initial modality-specific analysis. In both reading and speech perception, the superior temporal cortex is involved in processing word meaning at ∼250-450 ms in the left hemisphere and after ∼450 ms bilaterally. The data thus support a view of a largely shared semantic system in auditory and visual language perception, in line with the assumption that reading acquisition makes use of the neural systems originally developed for speech perception during evolution and in individual language development.
The MEG experiments on reading morphologically complex words showed that the left superior temporal activation was enhanced for the morphologically complex words at ∼200-700 ms. The results suggest that the majority of inflected words in the highly inflected Finnish language are represented in a decomposed form and that the decomposition process requires additional neural resources. Only very high-frequency inflected words may acquire full-form representations.
The MEG results on parafoveal preview in reading indicated that neural processing of written words in the left hemisphere is affected by a preview of words in the right visual field. The underlying neural mechanism may facilitate reading of connected text in natural conditions.
In a direct comparison, MEG and fMRI showed diverging activation patterns in a reading task although the same individuals were performing the same task. Based on the similarity of the EEG responses recorded simultaneously with both MEG and fMRI, the participants were performing the task similarly during the two recordings. The divergent MEG and fMRI results cannot be attributed to differences in the experimental procedures or language since these factors were controlled. Rather, they are likely to reflect actual dissimilarities in the way neural activity in a high-level cognitive task is picked up by MEG evoked responses and fMRI signals
Language Mapping With Magnetoencephalography: An Update on the Current State of Clinical Research and Practice With Considerations for Clinical Practice Guidelines
Numerous studies have shown that language processing is not limited to a few brain areas. Visual or auditory stimuli activate corresponding cortical areas, then memory identifies the word or image, Wernicke\u27s and Broca\u27s areas support the processing for either reading/listening or speaking and many areas of the brain are recruited. Determining how a normal person processes language helps clinicians and scientist to understand how brain pathologies such as tumor or stroke can affect changes in language processing. Patients with epilepsy may develop atypical language organization. Over time, the chronic nature of epileptic activity, or changes from a tumor or stroke, can result in a shift of language processing area from the left to the right hemisphere, or re-routing of language pathways from traditional to non-traditional areas within the dominant left hemisphere. It is important to determine where these language areas are prior to brain surgery. MEG evoked responses reflecting cerebral activation of receptive and expressive language processing can be localized using several different techniques: Single equivalent current dipole, current distribution techniques or beamformer techniques. Over the past 20 years there have been at least 25 validated MEG studies that indicate MEG can be used to determine the dominant hemisphere for language processing. The use of MEG neuroimaging techniques is needed to reliably predict altered language networks in patients and to provide identification of language eloquent cortices for localization and lateralization necessary for clinical care
The relationship between MEG and fMRI
In recent years functional neuroimaging techniques such as fMRI, MEG, EEG and PET have provided researchers with a wealth of information on human brain function. However none of these modalities can measure directly either the neuro-electrical or neuro-chemical processes that mediate brain function. This means that metrics directly reflecting brain ‘activity’ must be inferred from other metrics (e.g. magnetic fields (MEG) or haemodynamics (fMRI)). To overcome this limitation, many studies seek to combine multiple complementary modalities and an excellent example of this is the combination of MEG (which has high temporal resolution) with fMRI (which has high spatial resolution). However, the full potential of multi-modal approaches can only be truly realised in cases where the relationship between metrics is known. In this paper, we explore the relationship between measurements made using fMRI and MEG. We describe the origins of the two signals as well as their relationship to electrophysiology. We review multiple studies that have attempted to characterise the spatial relationship between fMRI and MEG, and we also describe studies that exploit the rich information content of MEG to explore differing relationships between MEG and fMRI across neural oscillatory frequency bands. Monitoring the brain at “rest” has become of significant recent interest to the neuroimaging community and we review recent evidence comparing MEG and fMRI metrics of functional connectivity. A brief discussion of the use of magnetic resonance spectroscopy (MRS) to probe the relationship between MEG/fMRI and neurochemistry is also given. Finally, we highlight future areas of interest and offer some recommendations for the parallel use of fMRI and MEG
Network perspectives on epilepsy using EEG/MEG source connectivity
The evolution of EEG/MEG source connectivity is both, a promising, and controversial advance in the characterization of epileptic brain activity. In this narrative review we elucidate the potential of this technology to provide an intuitive view of the epileptic network at its origin, the different brain regions involved in the epilepsy, without the limitation of electrodes at the scalp level. Several studies have confirmed the added value of using source connectivity to localize the seizure onset zone and irritative zone or to quantify the propagation of epileptic activity over time. It has been shown in pilot studies that source connectivity has the potential to obtain prognostic correlates, to assist in the diagnosis of the epilepsy type even in the absence of visually noticeable epileptic activity in the EEG/MEG, and to predict treatment outcome. Nevertheless, prospective validation studies in large and heterogeneous patient cohorts are still lacking and are needed to bring these techniques into clinical use. Moreover, the methodological approach is challenging, with several poorly examined parameters that most likely impact the resulting network patterns. These fundamental challenges affect all potential applications of EEG/MEG source connectivity analysis, be it in a resting, spiking, or ictal state, and also its application to cognitive activation of the eloquent area in presurgical evaluation. However, such method can allow unique insights into physiological and pathological brain functions and have great potential in (clinical) neuroscience
Spatiotemporal techniques in multimodal imaging for brain mapping and epilepsy
Thesis (Ph.D.)--Boston UniversityThis thesis explored multimodal brain imaging using advanced
spatiotemporal techniques. The first set of experiments were based on
simulations. Much controversy exists in the literature regarding the differences
between magnetoencephalography (MEG) and electroencephalography (EEG},
both practically and theoretically. The differences were explored using
simulations that evaluated the expected signal-to-noise ratios from reasonable brain sources. MEG and EEG were found to be complementary, with each
modality optimally suited to image activity from different areas of the cortical
surface. Consequently, evaluations of epileptic patients and general
neuroscience experiments will both benefit from simultaneously collected
MEG/EEG. The second set of experiments represent an example of MEG
combined with magnetic resonance imaging (MRI) and functional MRI (fMRI)
applied to healthy subjects. The study set out to resolve two questions relating to
shape perception. First, does the brain activate functional areas sequentially
during shape perception, as has been suggested in recent literature? Second,
which , if any, functional areas are active time-locked with reaction-time? The
study found that functional areas are non-sequentially activated, and that area IT
is active time-locked with reaction-time. These two points, coupled with the
method for multimodal integration , can help further develop our understanding of
shape perception in particular, and cortical dynamics in general for healthy
subjects. Broadly, these two studies represent practical guidelines for epilepsy
evaluations and brain mapping studies. For epilepsy studies, clinicians could
combine MEG and EEG to maximize the probability of finding the source of
seizures. For brain mapping in general, EEG, MEG, MRI and fMRI can be
combined in the methods outlined here to obtain more sophisticated views of
cortical dynamics
Sensorimotor Mapping With MEG: An Update on the Current State of Clinical Research and Practice With Considerations for Clinical Practice Guidelines
Published: November 2020In this article, we present the clinical indications and
advances in the use of magnetoencephalography to map the
primary sensorimotor (SM1) cortex in neurosurgical patients
noninvasively. We emphasize the advantages of
magnetoencephalography over sensorimotor mapping using
functional magnetic resonance imaging. Recommendations to the
referring physicians and the clinical magnetoencephalographers
to achieve appropriate sensorimotor cortex mapping using
magnetoencephalography are proposed. We finally provide some
practical advice for the use of corticomuscular coherence, corticokinematic
coherence, and mu rhythm suppression in this
indication. Magnetoencephalography should now be considered
as a method of reference for presurgical functional mapping of
the sensorimotor cortex.X. De Ti ege is Post-doctorate Clinical Master Specialist at
the Fonds de la Recherche Scientifique (FRS-FNRS, Brussels,
Belgium). M. Bourguignon has been supported by the program
Attract of Innoviris (Grant 2015-BB2B-10), by the Spanish
Ministry of Economy and Competitiveness (Grant PSI2016-
77175-P), and by the Marie Sk1odowska-Curie Action of the
European Commission (Grant 743562). H. Piitulainen has been
supported by the Academy of Finland (Grants #266133 and #296240), the Jane and Aatos Erkko Foundation, and the Emil
Aaltonen Foundation. The authors thank Professor Riitta Hari for
her support in most of the research works published by the
authors and presented in this article. The MEG project at the
CUB H^opital Erasme is financially supported by the Fonds
Erasme (Research convention “Les Voies du Savoir,” Fonds
Erasme, Brussels, Belgium)
- …