Relative Difference Measure Research Articles

The value and cost of complexity in predictive modelling: role of tissue anisotropic conductivity and fibre tracts in neuromodulation

Objectives. Computational methods are increasingly used to optimize transcranial direct current stimulation (tDCS) dose strategies and yet complexities of existing approaches limit their clinical access. Since predictive modelling indicates the relevance of subject/pathology based data and hence the need for subject specific modelling, the incremental clinical value of increasingly complex modelling methods must be balanced against the computational and clinical time and costs. For example, the incorporation of multiple tissue layers and measured diffusion tensor (DTI) based conductivity estimates increase model precision but at the cost of clinical and computational resources. Costs related to such complexities aggregate when considering individual optimization and the myriad of potential montages. Here, rather than considering if additional details change current-flow prediction, we consider when added complexities influence clinical decisions. Approach. Towards developing quantitative and qualitative metrics of value/cost associated with computational model complexity, we considered field distributions generated by two 4 × 1 high-definition montages (m1 = 4 × 1 HD montage with anode at C3 and m2 = 4 × 1 HD montage with anode at C1) and a single conventional (m3 = C3-Fp2) tDCS electrode montage. We evaluated statistical methods, including residual error (RE) and relative difference measure (RDM), to consider the clinical impact and utility of increased complexities, namely the influence of skull, muscle and brain anisotropic conductivities in a volume conductor model. Main results. Anisotropy modulated current-flow in a montage and region dependent manner. However, significant statistical changes, produced within montage by anisotropy, did not change qualitative peak and topographic comparisons across montages. Thus for the examples analysed, clinical decision on which dose to select would not be altered by the omission of anisotropic brain conductivity. Significance. Results illustrate the need to rationally balance the role of model complexity, such as anisotropy in detailed current flow analysis versus value in clinical dose design. However, when extending our analysis to include axonal polarization, the results provide presumably clinically meaningful information. Hence the importance of model complexity may be more relevant with cellular level predictions of neuromodulation.

Magnetoencephalography signals are influenced by skull defects

ObjectiveMagnetoencephalography (MEG) signals had previously been hypothesized to have negligible sensitivity to skull defects. The objective is to experimentally investigate the influence of conducting skull defects on MEG and EEG signals. MethodsA miniaturized electric dipole was implanted in vivo into rabbit brains. Simultaneous recording using 64-channel EEG and 16-channel MEG was conducted, first above the intact skull and then above a skull defect. Skull defects were filled with agar gels, which had been formulated to have tissue-like homogeneous conductivities. The dipole was moved beneath the skull defects, and measurements were taken at regularly spaced points. ResultsThe EEG signal amplitude increased 2–10 times, whereas the MEG signal amplitude reduced by as much as 20%. The EEG signal amplitude deviated more when the source was under the edge of the defect, whereas the MEG signal amplitude deviated more when the source was central under the defect. The change in MEG field-map topography (relative difference measure, RDM∗=0.15) was geometrically related to the skull defect edge. ConclusionsMEG and EEG signals can be substantially affected by skull defects. SignificanceMEG source modeling requires realistic volume conductor head models that incorporate skull defects.

Ear and electrode effects reduce within-group variability in middle latency response amplitude measures

Objectives: To establish normative amplitude values for relative difference measurements of the middle latency response (MLR) in normal-hearing pediatrics and to determine if these measurements provided a significant reduction of within-group variability when compared to raw, absolute amplitude measures. A relative amplitude difference is defined in the present paper as the difference in Na-Pa amplitude between two electrodes (e.g. |Na-Pa at C3 minus Na-Pa at C4|, or electrode effects) or between two ears (e.g. |Na-Pa on left ear stimulation minus Na-Pa on right ear stimulation|, or ear effects). In contrast, an absolute amplitude is defined as a single Na-Pa measurement made at one electrode for stimulation of one ear (e.g. Na-Pa measured at C3 on left ear stimulation). Design: Cross-sectional study. Study sample: 155 pediatrics with normal peripheral and central hearing, and no history of psychological, neurological, or learning disability issues. Results: Within-group variability was significantly smaller for relative differences when compared to absolute amplitude measures. Electrode effects showed significantly less variability than ear effects. Normative values for ear and electrode effects were reported. Conclusions: Relative differences may provide better utility in the clinical diagnosis of central auditory pathology in pediatrics when compared to absolute amplitude measures because these difference measures show significantly lower variability when examined across subjects.

Methods for high-resolution anisotropic finite element modeling of the human head: Automatic MR white matter anisotropy-adaptive mesh generation

This study proposes an advanced finite element (FE) head modeling technique through which high-resolution FE meshes adaptive to the degree of tissue anisotropy can be generated. Our adaptive meshing scheme (called wMesh) uses MRI structural information and fractional anisotropy maps derived from diffusion tensors in the FE mesh generation process, optimally reflecting electrical properties of the human brain. We examined the characteristics of the wMeshes through various qualitative and quantitative comparisons to the conventional FE regular-sized meshes that are non-adaptive to the degree of white matter anisotropy. We investigated numerical differences in the FE forward solutions that include the electrical potential and current density generated by current sources in the brain. The quantitative difference was calculated by two statistical measures of relative difference measure (RDM) and magnification factor (MAG). The results show that the wMeshes are adaptive to the anisotropic density of the WM anisotropy, and they better reflect the density and directionality of tissue conductivity anisotropy. Our comparison results between various anisotropic regular mesh and wMesh models show that there are substantial differences in the EEG forward solutions in the brain (up to RDM = 0.48 and MAG = 0.63 in the electrical potential, and RDM = 0.65 and MAG = 0.52 in the current density). Our analysis results indicate that the wMeshes produce different forward solutions that are different from the conventional regular meshes. We present some results that the wMesh head modeling approach enhances the sensitivity and accuracy of the FE solutions at the interfaces or in the regions where the anisotropic conductivities change sharply or their directional changes are complex. The fully automatic wMesh generation technique should be useful for modeling an individual-specific and high-resolution anisotropic FE head model incorporating realistic anisotropic conductivity distributions towards more accurate analysis of bioelectromagnetic problems.

Effects of local tissue conductivity on spherical and realistic head models

In this study, we consider different conductivity values based on tissue location in a human head model. We implement local conductivity (LC) to compute head surface potentials in three-, four-layered spherical and realistic head models using finite element method (FEM). Implementing LC for all head models, we obtain significant scalp potential variations in the term of relative difference measurement (RDM) and magnification (MAG) values with a maximum of 2.03±1.81 and 8.27±6.36, respectively. We also investigate the effects of conductivity variations (CVs) of head tissue layer on scalp potentials and find a maximum of 2.15±1.93 RDM and 8.57±6.61 MAG values. Our study concludes that it is important to assign LC to each tissue and it is also important to assign appropriate conductivity value in the construction of a head model for achieving accurate scalp potentials.

OpenMEEG: opensource software for quasistatic bioelectromagnetics

BackgroundInterpreting and controlling bioelectromagnetic phenomena require realistic physiological models and accurate numerical solvers. A semi-realistic model often used in practise is the piecewise constant conductivity model, for which only the interfaces have to be meshed. This simplified model makes it possible to use Boundary Element Methods. Unfortunately, most Boundary Element solutions are confronted with accuracy issues when the conductivity ratio between neighboring tissues is high, as for instance the scalp/skull conductivity ratio in electro-encephalography. To overcome this difficulty, we proposed a new method called the symmetric BEM, which is implemented in the OpenMEEG software. The aim of this paper is to present OpenMEEG, both from the theoretical and the practical point of view, and to compare its performances with other competing software packages.MethodsWe have run a benchmark study in the field of electro- and magneto-encephalography, in order to compare the accuracy of OpenMEEG with other freely distributed forward solvers. We considered spherical models, for which analytical solutions exist, and we designed randomized meshes to assess the variability of the accuracy. Two measures were used to characterize the accuracy. the Relative Difference Measure and the Magnitude ratio. The comparisons were run, either with a constant number of mesh nodes, or a constant number of unknowns across methods. Computing times were also compared.ResultsWe observed more pronounced differences in accuracy in electroencephalography than in magnetoencephalography. The methods could be classified in three categories: the linear collocation methods, that run very fast but with low accuracy, the linear collocation methods with isolated skull approach for which the accuracy is improved, and OpenMEEG that clearly outperforms the others. As far as speed is concerned, OpenMEEG is on par with the other methods for a constant number of unknowns, and is hence faster for a prescribed accuracy level.ConclusionsThis study clearly shows that OpenMEEG represents the state of the art for forward computations. Moreover, our software development strategies have made it handy to use and to integrate with other packages. The bioelectromagnetic research community should therefore be able to benefit from OpenMEEG with a limited development effort.

The effect of mineral surface nature on the mechanical properties of mineral-filled polypropylene composites

The adhesion between the polymer matrix and the inorganic filler particles are the goal of various and tremendous studies. This issue is still occupying a big part of the researchers thinking to find a proper solution, however, its difficulty returns to different factors playing roles in it. Among these factors are the filler surface, i.e., hydrophobicity and hydrophilicity, functional groups on the surface, as well as mineral filler particle size distribution, and particle shape. Therefore, in the current study, the difference in mechanical properties for two mineral fillers; namely, silica and talc, differs in their surface and rheology properties were investigated. Results showed that the difference in the mechanical properties of the same matrix when the inorganic filler is different either in type or loading. Talc showed the better results in terms of Young’s modulus and impact strength, where silica showed higher values in terms of yield stress. Moreover, both minerals addition resulted drop in all strain measures, yet silica showed relatively higher measures than talc, but the relative difference measures between the two minerals decrease with increasing their percentage of additions. In brief, the introduction of inorganic fillers to polypropylene (PP) increases toughness, stiffness and strength of the mineral-filled PP end-products.

Revised δ34S reference values for IAEA sulfur isotope reference materials S‐2 and S‐3

Revised delta(34)S reference values with associated expanded uncertainties (95% confidence interval (C.I.)) are presented for the sulfur isotope reference materials IAEA-S-2 (22.62 +/- 0.16 per thousand) and IAEA-S-3 (-32.49 +/- 0.16 per thousand). These revised values are determined using two relative-difference measurement techniques, gas source isotope ratio mass spectrometry (GIRMS) and double-spike multi-collector thermal ionization mass spectrometry (MC-TIMS). Gas analyses have traditionally been considered the most robust for relative isotopic difference measurements of sulfur. The double-spike MC-TIMS technique provides an independent method for value-assignment validation and produces revised values that are both unbiased and more precise than previous value assignments. Unbiased delta(34)S values are required to anchor the positive and negative end members of the sulfur delta (delta) scale because they are the basis for reporting both delta(34)S values and the derived mass-independent Delta(33)S and Delta(36)S values.

Methods and Software for Estimating Health Disparities: The Case of Children's Oral Health

The National Center for Health Statistics recently issued a monograph with 11 guidelines for reporting health disparities. However, guidelines on confidence intervals (CIs) cannot be readily implemented with the complex sample surveys often used for disease surveillance. In the United States, dental caries (decay) is the most common chronic childhood disease-5 times more common than asthma. Racial/ethnic minorities, immigrants, and persons of lower socioeconomic position (SEP) have a greater prevalence of caries. The authors provide methods for applying National Center for Health Statistics guidelines to complex sample surveys (health disparity indices and absolute and relative difference measures assessing associations of race/ethnicity and SEP to health outcomes with CIs); illustrate the application of those methods to children's untreated caries; provide relevant software; and report results from a simulation varying prevalence. They use data on untreated caries from the California Oral Health Needs Assessment of Children 2004-2005 and school percentage of participation in free/reduced-price lunch programs to illustrate the methods. Absolute and relative measures, the Slope Index of Inequality, the Relative Index of Inequality (mean and ratio), and the Health Concentration Index were estimated. Taylor series linearization and rescaling bootstrap methods were used to estimate CIs. Oral health differed significantly between White children and all non-White children and was significantly related to SEP.

Evaluating Clinical Accuracy of Continuous Glucose Monitoring Devices:Other Methods

With more and more continuous glucose monitoring devices entering the market, the importance of adequate accuracy assessment grows. This review discusses pros and cons of Regression Analysis and Correlation Coefficient, Relative Difference measures, Bland Altman plot, ISO criteria, combined curve fitting, and epidemiological analyses, the latter including sensitivity, specificity and positive predictive value for hypoglycaemia. Finally, recommendations for much needed head-to-head studies are given. This paper is a revised and adapted version of How to assess and compare the accuracy of continuous glucose monitors?, Diabetes Technology and Therapeutics 2007, in press, published with permission of the editor.

Mapping and correction of vascular hemodynamic latency in the BOLD signal

Correlation and causality metrics can be applied to blood-oxygen level-dependent (BOLD) signal time series in order to infer neural synchrony and directions of information flow from fMRI data. However, the BOLD signal reflects both the underlying neural activity and the vascular response, the latter of which is governed by local vasomotor physiology. The presence of potential vascular latency differences thus poses a confound in the detection of neural synchrony as well as inferences about the causality of neural processes. In the present study, we investigate the use of a breath holding (BH) task for characterizing and correcting for voxel-wise neurovascular latency differences across the whole brain. We demonstrate that BH yields reliable measurements of relative timing differences between voxels, and further show that a BH-derived latency correction can impact both functional connectivity maps of the resting-state default-mode network and activation maps of an event-related working memory (WM) task.

The resting frontal alpha asymmetry across the menstrual cycle: A magnetoencephalographic study

Gonadotropic hormones play an important role in the regulation of emotion. Previous studies have demonstrated that estrogen can modulate appetitive (approach/positive) and aversive (avoidance/negative) affective behaviors during the menstrual cycle. Frontal alpha asymmetry (a measure of relative difference of the alpha power between the two anterior hemispheres) has been associated with the trait and state reactivity of different affective styles. We studied the pattern change of frontal alpha asymmetry across the menstrual cycle. 16 healthy women participated in this resting magneto-encephalographic (MEG) study during the peri-ovulatory (OV) and menstrual (MC) phases. Our results showed significant interaction of resting MEG alpha activity between hemispheric side and menstrual phases. Difference in spontaneous frontal alpha asymmetry pattern across the menstrual cycle was also noted. Relatively higher right frontal activity was found during the OV phase; relatively higher left frontal activity was noted during the MC phase. The alteration of frontal alpha asymmetry might serve a sub-clinical correlate for hormonal modulation effect on dynamic brain organization for the predisposition and conceptualization of different affective styles across the menstrual cycle.

Absolute versus relative difference measures of priming: Which is appropriate when baseline scores change with age?

It is often of theoretical interest to know if implicit memory (repetition priming) develops across childhood under a given circumstance. Methodologically, however, it is difficult to determine whether development is present when baseline performance for unstudied items improves with age. Calculation of priming in absolute (priming=studied – unstudied) or relative‐to‐baseline terms can lead to different conclusions. In first noting this problem, Parkin (1993) suggested using the Snodgrass (1989a) calculation of relative priming [priming=(studied – unstudied)/(maximum – unstudied)], and most developmental studies have since adopted this procedure. Here, we question the Snodgrass method because the Snodgrass method's results are not replicated in the picture identification task when baselines are equated experimentally across age groups. Instead, results support an absolute measure of priming. Theoretically, we argue against its core assumption; namely, that children and adults always lie on the same learning curve, with an equal maximum performance level and equal rate of learning.

A computer-aided setup for measurements of the relative resistance differences of super-precision resistors

The results of investigations on the creation of a setup for measuring relative resistance differences with an rms deviation of 3×10−8 are described. The measuring circuit, conversion function, analysis of error structure, requirements for assemblies and units of setup, and results of experimental studies confirming the possibility of obtaining readouts with an rms deviation of 5×10−8 in the range of 100 Ω-1 kΩ and 3×10−8 in the range of 1–10 kΩ are presented.

Representation of bioelectric current sources using Whitney elements in the finite element method

Bioelectric current sources of magneto- and electroencephalograms (MEG, EEG) are usually modelled with discrete delta-function type current dipoles, despite the fact that the currents in the brain are naturally continuous throughout the neuronal tissue. In this study, we represent bioelectric current sources in terms of Whitney-type elements in the finite element method (FEM) using a tetrahedral mesh. The aim is to study how well the Whitney elements can reproduce the potential and magnetic field patterns generated by a point current dipole in a homogeneous conducting sphere. The electric potential is solved for a unit sphere model with isotropic conductivity and magnetic fields are calculated for points located on a cap outside the sphere. The computed potential and magnetic field are compared with analytical solutions for a current dipole. Relative difference measures between the FEM and analytical solutions are less than 1%, suggesting that Whitney elements as bioelectric current sources are able to produce the same potential and magnetic field patterns as the point dipole sources.

Use of the isolated problem approach for multi-compartment BEM models of electro-magnetic source imaging

The isolated problem approach (IPA) is a method used in the boundary element method (BEM) to overcome numerical inaccuracies caused by the high-conductivity difference in the skull and the brain tissues in the head. Hämäläinen and Sarvas (1989 IEEE Trans. Biomed. Eng. 36 165–71) described how the source terms can be updated to overcome these inaccuracies for a three-layer head model. Meijs et al (1989 IEEE Trans. Biomed. Eng. 36 1038–49) derived the integral equations for the general case where there are an arbitrary number of layers inside the skull. However, the IPA is used in the literature only for three-layer head models. Studies that use complex boundary element head models that investigate the inhomogeneities in the brain or model the cerebrospinal fluid (CSF) do not make use of the IPA. In this study, the generalized formulation of the IPA for multi-layer models is presented in terms of integral equations. The discretized version of these equations are presented in two different forms. In a previous study (Akalın-Acar and Gençer 2004 Phys. Med. Biol. 49 5011–28), we derived formulations to calculate the electroencephalography and magnetoencephalography transfer matrices assuming a single layer in the skull. In this study, the transfer matrix formulations are updated to incorporate the generalized IPA. The effects of the IPA are investigated on the accuracy of spherical and realistic models when the CSF layer and a tumour tissue are included in the model. It is observed that, in the spherical model, for a radial dipole 1 mm close to the brain surface, the relative difference measure (RDM*) drops from 1.88 to 0.03 when IPA is used. For the realistic model, the inclusion of the CSF layer does not change the field pattern significantly. However, the inclusion of an inhomogeneity changes the field pattern by 25% for a dipole oriented towards the inhomogeneity. The effect of the IPA is also investigated when there is an inhomogeneity in the brain. In addition to a considerable change in the scale of the potentials, the field pattern also changes by 15%. The computation times are presented for the multi-layer realistic head model.

Dentoskeletal treatment changes in Class II subdivision malocclusions in submentovertex and posteroanterior radiographs

The objective of this study was to evaluate the dentoskeletal changes consequent to orthodontic treatment in subjects with Class II subdivision malocclusions, treated with asymmetric extractions, compared with a normal-occlusion control group. The sample consisted of 3 groups, with 30 subjects in each: normal-occlusion subjects (group 1), untreated Class II subdivision subjects (group 2), and Class II subdivision patients treated with asymmetric extractions (group 3). All subjects had a full complement of permanent teeth at the beginning of treatment. The average ages of the subjects were 22.42, 15.76, and 18.57 years, respectively, in groups 1, 2, and 3. Measurements of relative differences in the spatial position of dental and skeletal bilateral landmarks were obtained from the submentovertex and posteroanterior cephalometric (PA) radiographs. The t test for independent samples was used to compare group 1 with groups 2 and 3 at different times. Results from the submentovertex radiograph showed that asymmetric extractions in Class II subdivision malocclusions will maintain the differences in the anteroposterior positions of right and left, maxillary and mandibular first molars, as would be expected with the treatment protocols used. There were no significant skeletal changes that could be attributed to the treatment approaches investigated or transverse collateral effects with the asymmetric mechanics used. It was also demonstrated that treatment of Class II subdivision malocclusions with asymmetric extractions produced corrections of maxillary and mandibular dental midline deviations with the midsagittal plane, without canting the occlusal plane or any other investigated horizontal plane, as seen in the PA radiograph. Treatment of Class II subdivision malocclusions with asymmetric extractions constitutes a beneficial approach to this problem.

Reduction of CCD observations of visual binaries using the “Tepui” function as PSF

429 CCD measurements of relative positions and magnitude differences in V and R photometric bands for 165 visual double and multiple stars are given. CCD frames were taken at the 1.52 m Spanish telescope of the Spanish-German Center of Astronomy at Calar Alto (Almería, Spain). During the reduction process a “Tepui" function was used as the PSF function.

Solution of Poisson’s equation in a volume conductor using resistor mesh models: Application to event related potential imaging

In electroencephalography (EEG) and event related potentials (ERP), localizing the electrical sources at the origin of scalp potentials (inverse problem) imposes, in a first step, the computation of scalp potential distribution from the simulation of sources (forward problem). This article proposes an alternative method for mimicing both the electrical and geometrical properties of the head, including brain, skull, and scalp tissue with resistors. Two resistor mesh models have been designed to reproduce the three-sphere reference model (analytical model). The first one (spherical resistor mesh) closely mimics the geometrical and electrical properties of the analytical model. The second one (cubic resistor mesh) is designed to conveniently handle anatomical data from magnetic resonance imaging. Both models have been validated, in reference to the analytical solution calculated on the three-sphere model, by computing the magnification factor and the relative difference measure. Results suggest that the mesh models can be used as robust and user-friendly simulation or exploration tools in EEG/ERP.

Reporting of numerical and statistical differences in abstracts: improving but not optimal.

The reporting of relative risk reductions (RRRs) or absolute risk reductions (ARRs) to quantify binary outcomes in trials engenders differing perceptions of therapeutic efficacy, and the merits of P values versus confidence intervals (CIs) are also controversial. We describe the manner in which numerical and statistical difference in treatment outcomes is presented in published abstracts. A descriptive study of abstracts published in 1986 and 1996 in 8 general medical and specialty journals. controlled, intervention trials with a binary primary or secondary outcome. Seven items were recorded: raw data (outcomes for each treatment arm), measure of relative difference (e.g., RRR), ARR, number needed to treat, P value, CI, and verbal statement of statistical significance. The prevalence of these items was compared between journals and across time. Of 5,293 abstracts, 300 met the inclusion criteria. In 1986, 60% of abstracts did not provide both the raw data and a corresponding P value or CI, while 28% failed to do so in 1Dr. Hux is a Career Scientist of the Ontario Ministry of Health and receives salary support from the Institute for Clinical Evaluative Sciences in Ontario.996 ( P <.001; RRR of 53%; ARR of 32%; CI for ARR 21% to 43%). The variability between journals was highly significant ( P <.001). In 1986, 100% of abstracts lacked a measure of absolute difference while 88% of 1996 abstracts did so ( P <.001). In 1986, 98% of abstracts lacked a CI while 65% of 1996 abstracts did so ( P <.001). The provision of quantitative outcome and statistical quantitative information has significantly increased between 1986 and 1996. However, further progress can be made to make abstracts more informative.