Magnetic field tomography of cortical and deep processes: examples of “real-time mapping” of averaged and single trial MEG signals (original) (raw)
Related papers
Magnetoencephalography as a research tool in neuroscience: state of the art
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 ...
Magnetoencephalography in studies of human cognitive brain function
Trends in Neurosciences, 1994
Magnetoencephalography provides a new dimension to the functional imaging of the brain. The cerebral magnetic fields recorded noninvasively enable the accurate determination of locations of cerebral activ@ with an uncompromized time resolution. The first whole-scalp sensor arrays have just recently come into operation, and significant advances are to be expected in both neurophysiological and cognitive studies, as well as in clinical practice. However, although the accuracy of locating isolated sources of brain activity has improved, identification of multiple simultaneous sources can still be a problem. Therefore, attempts are being made to combine magnetoencephalography with other brainimaging methods to improve spatial localization of multiple sources and, simultaneously, to achieve a more complete characterization of different aspects qf brain activi& during cognitive processing. Owing to its good time resolution and considerably better spatial accuracy than that provided by E E G, magnetoencephalography holds great promise as a tool for revealing informationprocessing sequences of the human brain.
A new approach to neuroimaging with magnetoencephalography
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.
Magnetic field tomography of coherent thalamocortical 40-Hz oscillations in humans
Proceedings of the National Academy of Sciences, 1991
This paper introduces the use of magnetic field tomography (MFT), a noninvasive technique based on distributed source analysis of magnetoencephalography data, which makes possible the three-dimensional reconstruction of dynamic brain activity in humans. MFE has a temporal resolution better than 1 msec and a spatial accuracy of 2-5 mm at the cortical level, which deteriorates to 1-3 cm at depths of 6 cm or more. MFT is used here to visualize the origin of a spatiotemporally organized pattern of coherent 40-Hz electrical activity. This coherence, initially observed during auditory input, was proposed to be generated by recurrent corticothalamic oscillation. In support of this hypothesis, we illustrate well-defined 40-Hz coherence between corticalsubcortical
Human Brain Mapping, 2009
Although magnetoencephalography (MEG) and electroencephalography (EEG) have been available for decades, their relative merits are still debated. We examined regional differences in signal-tonoise-ratios (SNRs) of cortical sources in MEG and EEG. Data from four subjects were used to simulate focal and extended sources located on the cortical surface reconstructed from highresolution magnetic resonance images. The SNR maps for MEG and EEG were found to be complementary. The SNR of deep sources was larger in EEG than in MEG, whereas the opposite was typically the case for superficial sources. Overall, the SNR maps were more uniform for EEG than for MEG. When using a noise model based on uniformly distributed random sources on the cortex, the SNR in MEG was found to be underestimated, compared with the maps obtained with noise estimated from actual recorded MEG and EEG data. With extended sources, the total area of cortex in which the SNR was higher in EEG than in MEG was larger than with focal sources. Clinically, SNR maps in a patient explained differential sensitivity of MEG and EEG in detecting epileptic activity. Our results emphasize the benefits of recording MEG and EEG simultaneously.
Assessing normal brain function with magnetoencephalography
International Congress Series, 2002
Magnetoencephalography (MEG) is a completely noninvasive method of functional imaging. MEG performs noninvasive functional imaging by recording the magnetic flux on the head surface associated with electrical currents in activated set of neurons, estimating the location of such sets, and projecting the location onto the MRI of the brain to identify and visualize the activated brain region. MEG has rapidly evolved in the last two decades due to the introduction of whole head systems and advances in computer technology. MEG is now the imaging modality of choice where a precise and high degree of localization is required. Ongoing studies show that it provides superior temporal and spatial resolution when compared to functional MRI. MEG is the only imaging technique that can reveal brain function over millisecond intervals. Magnetoencephalography was initially used to localize the primary sensory cortices, and depending on the nature of stimulus, this has been validated for visual, auditory or somatosensory areas. In order to localize brain networks involved during the engagement of cognitive tasks, both temporal and spatial resolution are critical. MEG is the only imaging technology capable of providing this information. We have successfully used magnetoencephalography to noninvasively localize brain areas involved with key language functions. These have been validated through the Wada procedure and with direct electrocortical stimulation. The utility of MEG in noninvasively localizing language function is reviewed. MEG also allows us to understand the differences in functional organization of the brain underlying the reading performances of dyslexic 0531-5131/02
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.
Human Brain Mapping, 2002
The central auditory system of the human brain uses a variety of mechanisms to analyze auditory scenes, among others, preattentive detection of sudden changes in the sound environment. Electroencephalography (EEG) and magnetoencephalography (MEG) provide a measure to monitor neuronal cortical currents. The mismatch negativity (MMN) or field (MMNm) reflect preattentive activation in response to deviants within a sequence of homogenous auditory stimuli. Functional magnetic resonance imaging (fMRI) allows for a higher spatial resolution as compared to the extracranial electrophysiological techniques. The image encoding gradients of echo planar imaging (EPI) sequences, however, elicit an interfering background noise. To circumvent this shortcoming, the present study applied multi-echo EPI mimicking an auditory oddball design. The gradient trains (SOA ϭ 800 msec, 94.5 dB SPL, stimulus duration ϭ 152 msec) comprised amplitude (Ϫ9 dB) and duration (76 msec) deviants in a randomized sequence. Moreover, the scanner noise was recorded and applied in a whole-head MEG device to validate the properties of this specific material. Robust fMRI activation patterns emerged in response to the deviant gradient switching. Changes in amplitude activated the entire auditory cortex, whereas the duration deviants elicited right-lateralized signal increase in secondary areas. The recorded scanner noise evoked reliably right-lateralized mismatch MEG responses. Source localization was in accordance with activation of secondary auditory cortex. The presented paradigm provides a robust and feasible tool to study the functional anatomy of early cognitive auditory processing in clinical populations such as schizophrenia. Hum. Brain Mapping 16:190-195, 2002.