Localization accuracy of single current dipoles from tangential components of auditory evoked fields

Towards Biomarkers for Autism Spectrum Disorder: Contributions of Magnetoencephalography (MEG)

There is no simple blood test for autism. Consequently, much attention has been paid to identifying noninvasive biomarkers using imaging (e.g., Magnetic resonance imaging, MRI) and electrophysiological (e.g., electroencephalography, EEG and magnetoencephalography, MEG) methods. While, in general, these lack direct biological specificity, they can (in principle) provide a useful tool, or suite of tools, for diagnostic, prognostic, stratification, and response monitoring purposes.This chapter focuses on the pursuit of biomarkers using magnetoencephalography (MEG). While closely related to the more common electroencephalography (EEG), MEG offers some unique characteristics (such as improved spatial resolution, in combination with real-time temporal resolution and spectral discrimination), that might be considered impactful in the pursuit of biomarkers.Given the widely-acknowledged heterogeneity of ASD ("if you've seen one child with autism, then you've seen one child with autism"), the tide of research is perhaps shifting away from diagnostic biomarkers toward biomarkers that can help stratify patients according to some similarity in biological basis, etiology, or pathway. This approach, somewhat pragmatic, may be of benefit when designing and conducting clinical trials of putative therapeutics, or when optimally designing behavioral supports (when "therapy" may not be indicated).Ultimately, MEG-derived biomarkers, however advantageous in themselves, may likely find a place as reference in the prioritization and roll-out of candidate biomarkers established using other modalities, more accessible and available to the global community.

Temporal hierarchy of cortical responses reflects core-belt-parabelt organization of auditory cortex in musicians

Human auditory cortex (AC) organization resembles the core-belt-parabelt organization in nonhuman primates. Previous studies assessed mostly spatial characteristics; however, temporal aspects were little considered so far. We employed co-registration of functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) in musicians with and without absolute pitch (AP) to achieve spatial and temporal segregation of human auditory responses. First, individual fMRI activations induced by complex harmonic tones were consistently identified in four distinct regions-of-interest within AC, namely in medial Heschl's gyrus (HG), lateral HG, anterior superior temporal gyrus (STG), and planum temporale (PT). Second, we analyzed the temporal dynamics of individual MEG responses at the location of corresponding fMRI activations. In the AP group, the auditory evoked P2 onset occurred ~25 ms earlier in the right as compared with the left PT and ~15 ms earlier in the right as compared with the left anterior STG. This effect was consistent at the individual level and correlated with AP proficiency. Based on the combined application of MEG and fMRI measurements, we were able for the first time to demonstrate a characteristic temporal hierarchy ("chronotopy") of human auditory regions in relation to specific auditory abilities, reflecting the prediction for serial processing from nonhuman studies.

Estimating functional connectivity using 2D tangential components in MEG sensor space

BACKGROUND

Connectivity analysis in magnetoencephalography (MEG) sensor space is commonly used for the preliminary estimation of the functional relationship between cortical areas, but the results are difficult to interpret due to the field spread effect. To improve the interpretability of sensor-level connectivity analysis, we introduce and test a new connectivity measure based on imaginary coherence in this study.

NEW METHOD

MEG signals from axial gradiometers are subjected to a wavelet transform at a frequency of interest, and are used to reconstruct 2D tangential magnetometer signals. The Euclidean norm of imaginary coherence values obtained from four available pairs between 2D tangential components at two locations is then used to estimate functional connectivity between sensor locations. The use of this new connectivity measure can be extended to 2D planar gradiometer signals or 3D source signals, where the functional relationship between multi-dimensional signals at different locations needs to be quantified as scalar variables.

RESULTS

The proposed method was applied to measured and simulated auditory evoked MEG data. The Euclidean norm of imaginary coherence reliably eliminated the field spread effect and showed increased inter-hemispheric coherence between sensors above the left and the right auditory cortex. The significance of the results was tested by introducing variability in spontaneous brain activities in multi-trial evoked data simulations.

COMPARISON WITH EXISTING METHOD

The distribution of imaginary coherence among axial gradiometer signals shows peaks not at the sensors directly above the neuronal current sources, but at sensors with field extreme.

Reconstruction of quasi-radial dipolar activity using three-component magnetic field measurements

OBJECTIVE

While standard magnetoencephalographic systems record only one component of the biomagnetic field, novel vector-biomagnetometers enable measurement of all three components of the field at each sensing point. Because information content in standard one-component magnetoencephalography (MEG) is often not adequate to reconstruct quasi-radial dipolar activity, we tested the hypothesis that quasi-radial activity can be estimated using three-component MEG.

METHODS

We stimulated the right median nerve in 11 healthy volunteers and recorded the somatosensory evoked fields over the contralateral hemisphere using a novel vector-biomagnetometer system comprised of SQUID-based magnetometer triplets. Source reconstruction for the early cortical components N20m and P25m was subsequently performed.

RESULTS

Both tangential and quasi-radial dipolar activity could be reconstructed in 10 of the 11 participants. Dipole locations were found in the vicinity of the central sulcus, and dipole orientations were predominantly tangential for N20m and quasi-radial for P25m. The mean location difference between the tangential and quasi-radial dipoles was 11.9 mm and the mean orientation difference was 97.5°.

CONCLUSIONS

Quasi-radial dipolar activity can be reconstructed from three-component magnetoencephalographic measurements.

SIGNIFICANCE

Three-component MEG provides higher information content than does standard MEG.

Direct reconstruction algorithm of current dipoles for vector magnetoencephalography and electroencephalography

This paper presents a novel algorithm to reconstruct parameters of a sufficient number of current dipoles that describe data (equivalent current dipoles, ECDs, hereafter) from radial/vector magnetoencephalography (MEG) with and without electroencephalography (EEG). We assume a three-compartment head model and arbitrary surfaces on which the MEG sensors and EEG electrodes are placed. Via the multipole expansion of the magnetic field, we obtain algebraic equations relating the dipole parameters to the vector MEG/EEG data. By solving them directly, without providing initial parameter guesses and computing forward solutions iteratively, the dipole positions and moments projected onto the xy-plane (equatorial plane) are reconstructed from a single time shot of the data. In addition, when the head layers and the sensor surfaces are spherically symmetric, we show that the required data reduce to radial MEG only. This clarifies the advantage of vector MEG/EEG measurements and algorithms for a generally-shaped head and sensor surfaces. In the numerical simulations, the centroids of the patch sources are well localized using vector/radial MEG measured on the upper hemisphere. By assuming the model order to be larger than the actual dipole number, the resultant spurious dipole is shown to have a much smaller strength magnetic moment (about 0.05 times smaller when the SNR = 16 dB), so that the number of ECDs is reasonably estimated. We consider that our direct method with greatly reduced computational cost can also be used to provide a good initial guess for conventional dipolar/multipolar fitting algorithms.

Spatial resolution of EEG cortical source imaging revealed by localization of retinotopic organization in human primary visual cortex

The aim of the present study is to investigate the spatial resolution of electroencephalography (EEG) cortical source imaging by localizing the retinotopic organization in the human primary visual cortex (V1). Retinotopic characteristics in V1 obtained from functional magnetic resonance imaging (fMRI) study were used as reference to assess the spatial resolution of EEG since fMRI can discriminate small changes in activation in visual field. It is well known that the activation of the early C1 component in the visual evoked potential (VEP) elicited by pattern onset stimuli coincides well with the activation in the striate cortex localized by fMRI. In the present experiments, we moved small circular checkerboard stimuli along horizontal meridian and compared the activations localized by EEG cortical source imaging with those from fMRI. Both fMRI and EEG cortical source imaging identified spatially correlated activity within V1 in each subject studied. The mean location error, between the fMRI-determined activation centers in V1 and the EEG source imaging activation peak estimated at equivalent C1 components (peak latency: 74.8+/-10.6 ms), was 7 mm (25% and 75% percentiles are 6.45 mm and 8.4 mm, respectively), which is less than the change in fMRI activation map by a 3 degrees visual field change (7.8 mm). Moreover, the source estimates at the earliest major VEP component showed statistically good correlation with those obtained by fMRI. The present results suggest that the spatial resolution of the EEG cortical source imaging can correctly discriminate cortical activation changes in V1 corresponding to less than 3 degrees visual field changes.

Reproducibility of measures of neurophysiological activity in Wernicke's area: a magnetic source imaging study

OBJECTIVE

The purpose of this study was to evaluate the reproducibility of estimates of neurophysiological activity obtained with Magnetic Source Imaging.

METHODS

Split-half data sets were obtained from 14 healthy volunteers during performance of a continuous recognition task for spoken words. The concurrent validity of spatiotemporal activation maps obtained with this task has been previously verified through comparisons with the Wada test and electrocortical stimulation mapping. Consecutive late activity sources (> 200 ms after stimulus onset) were modeled independently as equivalent current dipoles (ECDs) and used to identify the location of language-specific cortex in the left hemisphere (Wernicke's area).

RESULTS

Linear displacement of the geometric center of the cluster of ECDs in this region ranged between 2 and 8 mm across subjects. Intraparticipant variability (range) in the onset latency of activity was +/-50 ms, while the range of change in global field power for the entire set of ECDs in Wernicke's area was less than 17% in all cases.

CONCLUSIONS

The results indicate that despite its many conceptual limitations the ECD model can provide reliable estimates of regional cortical activity associated with the engagement of linguistic processes.

SIGNIFICANCE

The results highlight the need for reproducibility studies when research questions pose particular requirements for precision of estimates of regional neurophysiological activity.

MEG study on neural activities associated with syntactic and semantic violations in spoken Korean sentences

We carried out an magnetoencephalography (MEG) study to record cortical responses elicited in the left hemisphere by ending verb phrases, which had syntactic or semantic anomalies, in Korean sentences of subject-object-verb order. Using the high temporal and spatial resolution of MEG, the study was aimed at identifying neural activities that occur during a latency course associated with the syntactic or semantic process in the spoken sentence. Major responses, distinct from the responses to normal sentences, were observed in two latency periods of about 400 and 600 ms following the onset of the verb phrase. Source localization of the grand average fields indicated separate activities in the inferior frontal region and the vicinity of the auditory cortex for the first 400-ms response to the syntactic anomaly. The region around the auditory cortex was indicated for the response to the semantic anomaly in a similar latency. The second 600-ms response indicated activity around the middle temporal gyrus inferior to the auditory cortex for both syntactic and semantic anomalies. The results are discussed with reference to the ERP components established for Indo-European languages, and the possibility of concurrent processing of syntactic and semantic aspects is suggested.

Fast robust subject-independent magnetoencephalographic source localization using an artificial neural network

We describe a system that localizes a single dipole to reasonable accuracy from noisy magnetoencephalographic (MEG) measurements in real time. At its core is a multilayer perceptron (MLP) trained to map sensor signals and head position to dipole location. Including head position overcomes the previous need to retrain the MLP for each subject and session. The training dataset was generated by mapping randomly chosen dipoles and head positions through an analytic model and adding noise from real MEG recordings. After training, a localization took 0.7 ms with an average error of 0.90 cm. A few iterations of a Levenberg-Marquardt routine using the MLP output as its initial guess took 15 ms and improved accuracy to 0.53 cm, which approaches the natural limit on accuracy imposed by noise. We applied these methods to localize single dipole sources from MEG components isolated by blind source separation and compared the estimated locations to those generated by standard manually assisted commercial software.