A spatial filtering technique to detect and localize multiple sources in the brain

Summary An algorithm for localization of electromagnetic activity in the central nervous system is explored. This algorithm generates a neural activity index map within the brain by passing surface recordings through a set of spatial filters. The covariance matrix of the surface recordings is used to optimize the spatial filters' responses. This approach is studied in simulated situations and in real data. The simulations show the method's capability to detect areas of activity without prior knowledge of the number of sources. The resolving power of the method increases with number of electrodes and signal-to-noise ratio, and it decreases with depth. The analysis of the electrophysiological data indicates that the method can distinguish simultaneously active areas in a realistic fashion. The analyzed recordings are bilateral median SEP responses, an epoch of spike activity showing several active regions and a recording with eye movement superimposed on spike activity. The method and the results are discussed in relation to current localization techniques.The authors wish to thank Dr. J. M. Guerit, Dr. R.E. Lasky and G. Rook for their valuable suggestions. We thank Dr. K. Hecox for his support and R. Birrenkot for preparing the figures.     

The influence of skull-conductivity misspecification on inverse source localization in realistically shaped finite element head models

Summary The electric conductivities of different tissues are important parameters of the head model and their precise knowledge appears to be a prerequisite for the localization of electric sources within the brain. To estimate the error in source localization due to errors in assumed conductivity values, parameter variations on skull conductivities are examined. The skull conductivity was varied in a wide range and, in a second part of this paper, the effect of a nonhomogeneous skull conductivity was examined. An error in conductivity of lower than 20% appears to be acceptable for fine finite element head models with average discretization errors down to 3mm. Nonhomogeneous skull conductivities, e.g., sutures, yield important mislocalizations especially in the vincinty of electrodes and should be modeled.The authors wish to thank the VW — Foundation for financial support.     

CO2 and the Cephalic Direct Current System: Studies on Humans and a Biophysical Analysis

The TCDC/CO, hypothesis is that slowly changing potentials, source resistances and source currents, measured between electrodes placed over specific diploic/emissary vein distributions on the scalp, largely reflect regional differences in subelectrode CO2 concentrations which are in equilibrium with subjacent cortical differences in metabolism and CO2 production. Studies on humans show that TCDC parameters do show the expected relationships with the effects of regional differences in CO2 concentration. An explicit quantitative formulation of the hypothesis is developed based on the best available measures of all essential parameters, and it in shown that this model permits the correct computation of all TCDC empirical norms under essentially all conditions of measurement. Although it now appears that MIX studies have a substantial theoretical foundation it must be emphasized that the analysis in this paper is based on normative—i.e. averaged—values and cannot yet be applied to isolated TCDC measurements on single subjects.     

Effectiveness of a multiple intervention strategy for the control of the tiger mosquito (Aedes albopictus) in Spain

This study was undertaken to evaluate the effectiveness of four complementary and combined strategies to minimize the presence of the invasive mosquito Aedes albopictus, firmly established in Sant Cugat del Vallès, Catalonia, Spain. A quasi-experimental design including six neighbourhoods was performed in 2008-2009. The abundance of mosquitoes was monitored through ovitraps. The multiple intervention strategy consisted of four actions: source reduction; larvicide treatments (Bacillus thuringiensis israelensis and diflubenzuron); adulticide treatments (alfacipermetrin); and cleaning up uncontrolled landfills. The results showed the number of eggs significantly reduced in the areas with intervention. In 2008, the accumulate median of eggs was 175 and 272 in the intervention and control areas, respectively. In 2009, these medians were 884 and 1668 eggs. In total, 3104 households were visited and 683 people were interviewed. During inspections inside the houses, the cooperation of citizens in 2009 was 16% higher than that in 2008 (95% CI 13-19%). These findings suggest that the strategy was effective in reducing the number of eggs. Citizen cooperation, an essential factor for success, was observed through a high level of collaboration by the home owners, who allowed entry into their private dwellings. This study could be a model for controlling the populations of Ae. albopictus in the Mediterranean region.     

Gamma oscillatory activity in a visual discrimination task

We tested the hypothesis whether images of real objects elicit stronger gamma (>25 Hz) synchronization, when compared with scrambled objects. The background of this study is a recent debate about the functional meaning of evoked and induced gamma oscillations. Brain electrical source analysis (BESA) and low resolution electromagnetic tomography analysis (LORETA) was performed on the basis of the event-related potential (ERP) data. A component at around 230 ms (termed C230) showed strongest differences between objects and scrambled objects. Time-frequency analyses were run across electrodes and within the dipole sources. We found increased gamma event-related synchronization (ERS) between 200 and 300 ms for real objects. This effect was strongest in a fronto-medial source. Induced gamma, as also shown in previous studies, reflects the more task-relevant mechanism where object representations become activated.     

Source localization of scalp-EEG interictal spikes in posterior cortex epilepsies investigated by HR-EEG and SEEG

Purpose:   To determine the validity of scalp-electroencephalography (EEG)-interictal spike (IIS) source localization in posterior cortex epilepsies (PCE).
Methods:   Eleven patients with drug-resistant PCE were studied with high-resolution EEG (HR-EEG) and stereoelectroencephalography (SEEG). Sixty-four scalp channels, a realistic head model, and different algorithms [multiple signal classification (MUSIC) and equivalent current dipoles] were used. Results were compared to intracerebral SEEG recordings. For SEEG, a semiautomatic detection of intracerebral IIS was used, allowing a classification of intracerebral IIS into one of three groups: Medial, lateral, and mediolateral.
Results:   In the medial group (two patients), scalp-EEG IIS were usually absent for one patient whereas for the other, scalp-EEG was misleading. Indeed, scalp-EEG IIS had a posterior projection, predominantly contralateral to the source. In the lateral group (two patients), scalp-EEG IIS were subcontinuous and accurately localized. In the mediolateral group (seven patients), intracerebral interictal distribution was complex and bilateral for four of seven patients. Source localizations were able to determine only a part, whether lateral or medial, of the intracerebral interictal distribution.
Discussion:   The accuracy of scalp-EEG IIS source localization is dependant on the type of intracerebral interictal distribution. In the most frequent type of PCE, patients proved to have a complex interictal distribution between both medial and lateral cortices, and source localizations always  underestimated intracerebral IIS. In cases where intracranial sources were quite focal, surface EEG sources were localized with accuracy, even in medial occipital lobe structures.     

High-resolution EEG (HR-EEG) and magnetoencephalography (MEG)

HR-EEG (high resolution EEG) and MEG (magnetoencephalography) allow the recording of cerebral electromagnetic activities with excellent temporal resolution. These tools have also considerably progressed in spatial resolution and now constitute real methods of Electric and Magnetic Source Imaging. Their limits and the precision of the results obtained are discussed in distinct types of partial epilepsy. HR-EEG and MEG allow localization of scalp-EEG interictal spikes and more rarely ictal activities. They now contribute to the presurgical evaluation of pharmacoresistant partial epilepsies. These investigations appear to be of particular importance in presurgical assessment of MRI-negative epilepsy.     

“二重解析”技术在颗粒物来源解析中的应用

运用化学质量平衡(CMB)模型是目前进行颗粒物的来源解析工作中最重要最实用的受体模型。"二重解析"技术,解决了CMB模型中共线性源的共线性问题,是对CMB模型的重要补充和创新。     

Can individually targeted and optimized multi-channel tDCS outperform standard bipolar tDCS in stimulating the primary somatosensory cortex?

BackgroundTranscranial direct current stimulation (tDCS) has emerged as a non-invasive neuro-modulation technique. Most studies show that anodal tDCS increases cortical excitability, however, with variable outcomes. Previously, we have shown in computer simulations that our multi-channel tDCS (mc-tDCS) approach, the distributed constrained maximum intensity (D-CMI) method can potentially lead to better controlled tDCS results due to the improved directionality of the injected current at the target side for individually optimized D-CMI montages.ObjectiveIn this study, we test the application of the D-CMI approach in an experimental study to stimulate the somatosensory P20/N20 target source in Brodmann area 3b and compare it with standard bipolar tDCS and sham conditions.MethodsWe applied anodal D-CMI, the standard bipolar and D-CMI based Sham tDCS for 10 min to target the 20 ms post-stimulus somatosensory P20/N20 target brain source in Brodmann area 3b reconstructed using combined magnetoencephalography (MEG) and electroencephalography (EEG) source analysis in realistic head models with calibrated skull conductivity in a group-study with 13 subjects. Finger-stimulated somatosensory evoked fields (SEF) were recorded and the component at 20 ms post-stimulus (M20) was analyzed before and after the application of the three tDCS conditions in order to read out the stimulation effect on Brodmann area 3b.ResultsAnalysis of the finger stimulated SEF M20 peak before (baseline) and after tDCS shows a significant increase in source amplitude in Brodmann area 3b for D-CMI (6–16 min after tDCS), while no significant effects are found for standard bipolar (6–16 min after tDCS) and sham (6–16 min after tDCS) stimulation conditions. For the later time courses (16–26 and 27–37 min post-stimulation), we found a significant decrease in M20 peak source amplitude for standard bipolar and sham tDCS, while there was no effect for D-CMI.ConclusionOur results indicate that targeted and optimized, and thereby highly individualized, mc-tDCS can outperform standard bipolar stimulation and lead to better control over stimulation outcomes with, however, a considerable amount of additional work compared to standard bipolar tDCS.     

Consistency of the blind source separation computed with five common algorithms for magnetoencephalogram background activity

Blind source separation (BSS) is widely used to analyse brain recordings like the magnetoencephalogram (MEG). However, few studies have compared different BSS decompositions of real brain data. Those comparisons were usually limited to specific applications. Therefore, we aimed at studying the consistency (i.e., similarity) of the decompositions estimated for real MEGs from 26 subjects using five widely used BSS algorithms (AMUSE, SOBI, JADE, extended-Infomax and FastICA) for five epoch lengths (10 s, 20 s, 40 s, 60 s and 90 s). A statistical criterion based on Factor Analysis was applied to calculate the number of components into which each epoch would be decomposed. Then, the BSS techniques were applied. The results indicate that the pair of algorithms ‘AMUSE–SOBI’, followed by ‘JADE–FastICA’, provided the most similar separations. On the other hand, the most dissimilar outcomes were computed with ‘AMUSE–JADE’ and ‘SOBI–JADE’. The BSS decompositions were more similar for longer epochs. Furthermore, additional analyses of synthetic signals supported the results of the real MEGs. Thus, when selecting BSS algorithms to explore brain signals, the techniques offering the most different decompositions, such as AMUSE and JADE, may be preferred to obtain complementary, or at least different, perspectives of the underlying components.