The influence of skull parameters in a realastically shaped head model on the accuracy of EEG dipole localization

Magnetic resonance imaging (MRI) provides the means for the generation of head models with a high geometrical detail. Errors in the generation of realistically shaped models are likely to be made, due to the identification of the different anatomical structures. The poorly conducting skull layer plays a special role, since it is known to have a large effect on the scalp potentials and is difficult to distinguish in MRI. If source localisation is applied to EEG, then the systematic errors in the parameters of the reconstructed sources are partly due to the misspecifications of the head model. - In this paper, the influence of certain head model parameters on the systematic errors of reconstructed dipole sources is investigated. Variations in the skull conductivity and thickness, as well as local variations in the skull thickness, are considered. In order to do so, the sensitivity measure is introduced, which specifies the amount of change of a dipole parameter due to a specific model variation. Sensitivity maps are constructed for layers of dipoles underneath the brain surface. The maps of dipole sensitivities show the local distribution of the systematic errors to be expected. The computations are performed by means of a derivative method, which utilizes a linearization of the forward problem

Time/space regularization of the inward continuation problem in EEG using the Boundary Element Method

The inward continuation problem in EEG consists in the computation of the cortical potential distribution from the measured potential distribution on the scalp. Although unique this inverse problem is ill-posed. That is, low-level noise in the scalp potential data or a small error in the geometrical data can lead to unbounded errors in the solution. Regularization techniques have to be used to minimize these effects. The inverse problem is solved in two steps. First Tikhonov regularization is applied yielding a solution of the potential on the inside of the skull surface for every timestep. Than the solution of the first step is used for Twomey regularization. At each moment in time a new solution is found by using as a priori estimate the average of the first solution one timestep prior and one timestep after. This combination of spatial (Tikhonov) and temporal (Twomey) regularization improves the solution and smoothes the solution in space and time. Both simulations and the application to EEG data of a Median Nerve stimulation experiment yield encouraging results. Further comparative studies have to be carried out to evaluate the application of time/space regularization of the inward continuation problem in EEG

Face Coding Is Bilateral in the Female Brain

Background: It is currently believed that face processing predominantly activates the right hemisphere in humans, but available literature is very inconsistent. Methodology/Principal Findings: In this study, ERPs were recorded in 50 right-handed women and men in response to 390 faces (of different age and sex), and 130 technological objects. Results showed no sex difference in the amplitude of N170 to objects; a much larger face-specific response over the right hemisphere in men, and a bilateral response in women; a lack of face-age coding effect over the left hemisphere in men, with no differences in N170 to faces as a function of age; a significant bilateral face-age coding effect in women. Conclusions/Significance: LORETA reconstruction showed a significant left and right asymmetry in the activation of the fusiform gyrus (BA19), in women and men, respectively. The present data reveal a lesser degree of lateralization of brain functions related to face coding in women than men. In this light, they may provide an explanation of the inconsistencies in the available literature concerning the asymmetric activity of left and right occipito-temporal cortices devoted to fac

Second asymptomatic carotid surgery trial (ACST-2): a randomised comparison of carotid artery stenting versus carotid endarterectomy

Background: Among asymptomatic patients with severe carotid artery stenosis but no recent stroke or transient cerebral ischaemia, either carotid artery stenting (CAS) or carotid endarterectomy (CEA) can restore patency and reduce long-term stroke risks. However, from recent national registry data, each option causes about 1% procedural risk of disabling stroke or death. Comparison of their long-term protective effects requires large-scale randomised evidence. Methods: ACST-2 is an international multicentre randomised trial of CAS versus CEA among asymptomatic patients with severe stenosis thought to require intervention, interpreted with all other relevant trials. Patients were eligible if they had severe unilateral or bilateral carotid artery stenosis and both doctor and patient agreed that a carotid procedure should be undertaken, but they were substantially uncertain which one to choose. Patients were randomly allocated to CAS or CEA and followed up at 1 month and then annually, for a mean 5 years. Procedural events were those within 30 days of the intervention. Intention-to-treat analyses are provided. Analyses including procedural hazards use tabular methods. Analyses and meta-analyses of non-procedural strokes use Kaplan-Meier and log-rank methods. The trial is registered with the ISRCTN registry, ISRCTN21144362. Findings: Between Jan 15, 2008, and Dec 31, 2020, 3625 patients in 130 centres were randomly allocated, 1811 to CAS and 1814 to CEA, with good compliance, good medical therapy and a mean 5 years of follow-up. Overall, 1% had disabling stroke or death procedurally (15 allocated to CAS and 18 to CEA) and 2% had non-disabling procedural stroke (48 allocated to CAS and 29 to CEA). Kaplan-Meier estimates of 5-year non-procedural stroke were 2·5% in each group for fatal or disabling stroke, and 5·3% with CAS versus 4·5% with CEA for any stroke (rate ratio [RR] 1·16, 95% CI 0·86–1·57; p=0·33). Combining RRs for any non-procedural stroke in all CAS versus CEA trials, the RR was similar in symptomatic and asymptomatic patients (overall RR 1·11, 95% CI 0·91–1·32; p=0·21). Interpretation: Serious complications are similarly uncommon after competent CAS and CEA, and the long-term effects of these two carotid artery procedures on fatal or disabling stroke are comparable. Funding: UK Medical Research Council and Health Technology Assessment Programme

ASA-Advanced Source Analysis of Continuous and Event-Related EEG/MEG Signals

Sophisticated analysis methods for EEG and MEG play a key role in the better understanding of brain functions as measured by high-density EEG and MEG. Being commercially available since 1996, the ASA software (ANT Software BV, Enschede, Netherlands) has been gaining growing popularity among clinical and cognitive researchers. With the following article, we present an overview on the currently available functionality of the software and provide examples of its application

Realistically shaped models of the head in EEG source localization

Inverse solution techniques based on electroencephalographic (EEG) measurements have become a powerful means to gain knowledge on the functioning of the brain. A model of the head and a potential computation method are necessary to describe the EEG problem mathematically. The generation of realistically shaped three-compartment models of the head is discnsed. The isolated-problem approach for the boundary element method (BEM) is applied to develop a fast and reliable numerical solution of the EEG forward problem. Accuracy studies with this approach show that dipole positions can be reconstructed within a distance of 3 mm from the original positions. Inverse simulations indicate that the incorporation of the individual head shape may sigaxificantly influence the reconstructed dipole position but not its magnitude and orientation, in comparison with the commonly used three-sphere model. However, the presence of noise in the simulated potential data affects the solutions based on realistically shaped models more than those of the simple three-sphere model. This effect is investigated more in detail by means of visualising the objective function of the dipole optimization. The locally optimal dipole is estimated in a dense grid o f scan points in the region of interest. This enables us to gain specific information about the steepness of the objective ftmcfion as well as about possible local minima caused by the realistically shaped head model or by rtoise in the EEG potentials