Expanding the applicability of magnetoencephalography

Functional Brain Imaging, 1995

This chapter focuses on magnetoencephalography (MEG) used in brain imaging and its use in localizing the brain sources of externally recorded spontaneous activity and stimulus and task-induced activation. The chapter first describes the instruments used for recording the magnetoencephalographic signals and the neurogenesis of these signals. It then considers proposed solutions for the "inverse" problem and describes approaches for MEG source estimation, including a method that specifies only one or many equivalent current dipoles. It also explains the signal source-localizing technique known as beamforming and concluding with a discussion of practical issues in MEG/ MSI, with emphasis on those arising in clinical applications of the method.

Functional Neurology, 2014

To progress toward understanding of the mechanisms underlying the functional organization of the human brain, either a bottom-up or a top-down approach may be adopted. The former starts from the study of the detailed functioning of a small number of neuronal assemblies, while the latter tries to decode brain functioning by considering the brain as a whole. This review discusses the top-down approach and the use of magnetoencephalography (MEG) to describe global brain properties. The main idea behind this approach is that the concurrence of several areas is required for the brain to instantiate a specific behavior/functioning. A central issue is therefore the study of brain functional connectivity and the concept of brain networks as ensembles of distant brain areas that preferentially exchange information. Importantly, the human brain is a dynamic device, and MEG is ideally suited to investigate phenomena on behaviorally relevant timescales, also offering the possibility of capturing behaviorally-related brain connectivity dynamics.

Journal of Clinical Neurophysiology, 1991

is a new, noninvasive functional test equivalent to EEG. It has been used to localize the sources of evoked responses and interictal and ictal epileptiform discharges and to study patients with psychiatric illnesses, cerebrovascular accidents, and migraine. In epilepsy research, it is hoped that MEG will provide information similar to that yielded by depth or subdural electrode recording, or that the combination ofthese methods will provide more information than either one alone. The application of MEG appears to be widening, although it is not yet a routine clinical diagnostic tool. The utility ofMEG is limited by technological problems, but new and more efficient systems are becoming available. Within several years, advances in the technology and understanding ofMEG may modify the course ofits application.

Epilepsy & Behavior, 2004

Magnetoencephalography (MEG) is a relatively novel noninvasive technique, with a much shorter history than EEG, that conveys neurophysiological information complementary to that provided by EEG, with high temporal and spatial resolution. Despite its a priori, highly competitive profile, the role of MEG in the clinical setting is still controversial. We briefly review the major obstacles MEG faces in becoming a routine clinical test and the different strategies needed to bypass them. The high cost and complexity associated with MEG equipment are powerful hindrances to wide acceptance of this relatively new technique in clinical practice. The most straightforward advantage is based on the relative facility of MEG recordings in the process of source localization, which also carries some degree of uncertainty, thus partly explaining why the development of clinical applications of MEG has been so slow. Obviously, a decrease in the cost and the elaboration of semiautomatic protocols that could reduce the complexity of the studies and favor the development of consensual strategies, as well as a major effort on the part of clinicians to identify clinical issues where MEG could be decisive, would be most welcome.

Brain Sciences

Magnetoencephalography (MEG) is a neurophysiological technique that detects the magnetic fields associated with brain activity. Synthetic aperture magnetometry (SAM), a MEG magnetic source imaging technique, can be used to construct both detailed maps of global brain activity as well as virtual electrode signals, which provide information that is similar to invasive electrode recordings. This innovative approach has demonstrated utility in both clinical and research settings. For individuals with epilepsy, MEG provides valuable, nonredundant information. MEG accurately localizes the irritative zone associated with interictal spikes, often detecting epileptiform activity other methods cannot, and may give localizing information when other methods fail. These capabilities potentially greatly increase the population eligible for epilepsy surgery and improve planning for those undergoing surgery. MEG methods can be readily adapted to research settings, allowing noninvasive assessment of...

Nature Communications, 2019

The hippocampus and amygdala are key brain structures of the medial temporal lobe, involved in cognitive and emotional processes as well as pathological states such as epilepsy. Despite their importance, it is still unclear whether their neural activity can be recorded noninvasively. Here, using simultaneous intracerebral and magnetoencephalography (MEG) recordings in patients with focal drug-resistant epilepsy, we demonstrate a direct contribution of amygdala and hippocampal activity to surface MEG recordings. In particular, a method of blind source separation, independent component analysis, enabled activity arising from large neocortical networks to be disentangled from that of deeper structures, whose amplitude at the surface was small but significant. This finding is highly relevant for our understanding of hippocampal and amygdala brain activity as it implies that their activity could potentially be measured non-invasively.

Magnetoencephalography (MEG) and electroencephalography (EEG) were the Cinderellas of neuroimaging. On the one hand they are endowed with unparallel temporal resolution, while on the other they are in theory unable to uniquely determine the generators, even when a complete and exact set of measurements is available. Yet, study after study from our laboratories and others demonstrate that with modern hardware and software a very accurate estimate for the generators can be derived, at least from the MEG data. In this work we first review briefly theoretical arguments and the methods of source reconstruction. We then list experimental evidence for localization accuracy of a few millimeters from real MEG data using magnetic field tomography and a recent phantom study where a number of these techniques have been compared. We then put in context the accepted view of the electrophysiological basis of the EEG and MEG signal generation, adding caveats that must be considered given our incomplete knowledge of the anatomy and electrophysiology. We finally present results for processing of facial information that link the localization measures derived from MEG to the fMRI data at one end and invasive electrophysiology at the other and put them in the proper neurophysiological context.

Physica Scripta, 1989

Transfer of information from one neuron to another in the brain involves electric currents; when thousands of neighboring nerve cells act in concert they produce weak (about 100fT) magnetic fields that can be measured noninvasively outside the skull. The magnetic field detector used is the Superconducting Quantum Interference Device (SQUID).

Human Brain Mapping, 2005

We discuss the application of beamforming techniques to the field of magnetoencephalography (MEG). We argue that beamformers have given us an insight into the dynamics of oscillatory changes across the cortex not explored previously with traditional analysis techniques that rely on averaged evoked responses. We review several experiments that have used beamformers, with special emphasis on those in which the results have been compared to those observed in functional magnetic resonance imaging (fMRI) and on those studying induced phenomena. We suggest that the success of the beamformer technique, despite the assumption that there are no linear interactions between the mesoscopic local field potentials across distinct cortical areas, may tell us something of the balance between functional integration and segregation in the human brain. What is more, MEG beamformer analysis facilitates the study of these complex interactions within cortical networks that are involved in both sensory-motor and cognitive processes. Hum. Brain Mapp 25:199–211, 2005. © 2005 Wiley-Liss, Inc.

Computer Methods and Programs in Biomedicine, 1994

Magnetoencephalography (MEG) non-invasively infers the distribution of electric currents in the brain by measuring the magnetic fields they induce. Its superb spatial and temporal resolution provides a solid basis for the Tunctional imaging' of the brain provided it is integrated with other brain imaging techniques. MAGNOBRAIN is an applied research project that developed tools to integrate MEG with MRI and EEG. These include: (1) software for MEG oriented MRI feature extraction: (2) the Brain Data Base (BDB) which is a reference library of information on the brain used for more realistic and biologically meaningful functional localisations through MEG and EEG: and (3) a database of normative data (age and sex matched) for the interpretation of MEG. It is expected that these tools will evolve into a medical informatics environment that will aid the planning of neurosurgical operations as well as contribute to the exploration of mental function including the study of perception and cognition,