Principal components and Multidimensional scaling of auditory and visual event-related potential topography

Summary Principal components analysis and metric multidimensional scaling were used to assess auditory and visual event-related potential topography in healthy late-middle-aged and elderly adults (n=20). Binaurally elicited auditory evoked potentials and full-field checkerboard pattern reversal visual event-related potentials were recorded from 28 scalp sites. The zero-lag, cross-correlations of all waveforms for 300 msec post-stimulus epochs were obtained and separate analyses of the auditory and visual data were performed. Ultimately, three of the four dimensions identified in the principal components and multidimensional scaling solutions were similar and represented electrode site differences in (1) the anterior-posterior plane; (2) laterality; and, (3) the proximal-distal relation to midline. The remaining multidimensional scaling axis appeared to reflect effects specific to the modality of stimulation. Under auditory stimulation, the temporal and central-parietal sites were distinct from other scalp regions, whereas under visual stimulation, the occipital and frontal sites were distinctive. Although the results of the principal component analyses were conceptually similar to the multidimensional scaling outcomes, there were consistent differences between them. The findings provide empirical support for the validity of these multivariate methods in topographic analysis.     

Analysis of Pattern-Reversal Visual Evoked-Potential Topography: Replicability in Normal Subjects and Comparisons with Alzheimer''s Disease

This paper reports two studies using multivariate analysis to aid the interpretation of cross-coherence of multiple electrode sites in evoked potential responses. In the first study, the replicability of principal components and multidimensional scaling was evaluated by applying both methods to pattern-reversal visual evoked potentials recorded from 28 scalp sites in two subgroups of sex- and age-matched healthy subjects, and then comparing the results. Four dimensions replicated in the two initial multidimensional scaling solutions and appeared to reflect differences in: 1) anterior versus posterior scalp areas, 2) laterality, 3) separation of frontal and occipital sites from other scalp regions, and 4) proximal versus distal placements. The initial principal components did not match well in the two groups, but rotation to congruence improved their replicability and ultimately yielded axes similar to those of the multidimensional scaling dimensions. In the second study, Alzheimer's disease and sex- and age-matched control subjects were evaluated. The four axes identified were the same as those described above, but after the solutions were rotated to align them, the group differences appeared negligible. Examination of the components and dimensions from both studies showed some consistent departures from being merely reflectors of site location, and the apparently visual dimension appeared clearly in all four groups. Judged on the basis of initial interpretability and replicability of the solutions, the results suggest that multidimensional scaling, with appropriate transformation, may provide an effective tool for analyzing pattern-reversal visual evoked-potential topography.     

The usefulness of the Laplacian in principal component analysis and dipole source localization

Summary Evoked potentials are difficult to analyze because multiple sources are active simultaneously. Principal component analysis and dipole localization are two techniques that have been used to disentangle overlapping sources. Both of these techniques have problems. Principal component analysis suffers from a rotation ambiguity. Dipole localization suffers from biases when the model used to derive the sources from the scalp potentials is misspecified. Using computer simulations we demonstrate that by applying both of these techniques to the Laplacian of the voltages rather than to the raw voltages the problems associated with the two techniques are reduced. Computer programs for the analyses are presented in an Appendix.Supported by grant EY04776 from the National Institutes of Health. We would like to thank Amnon Silverstein for his help with icosohedra.     

Laplacian心电图的研究进展

Laplacian心电图技术是利用表面Laplacian值的概念,通过对体表电位进行微分,得以更准确、全面地反映心脏电活动的情况。本文详细介绍了Laplacian心电图研究的发展和现状,主要包括理论背景、实际测量、数据处理方法以及临床应用等方面,并指出了Laplacian心电图的研究困难及发展前景。     

Scalp topography of SEP late components in patients with supra-tentorial lesions

Summary Somatosensory evoked potentials (SEP) to simultaneous bilateral median nerve stimulation were recorded in 7 patients with unilateral brain lesions, 8 patients with degenerative dementia, and 5 normal volunteers. Right-left amplitude difference was compared from serial topographic images and the amplitude was compared at homologous electrodes. In patients with unilateral lesions, the amplitude from the frontopolar, frontal, anterior-temporal, and/or occipital electrodes was smaller on the affected side at 240 and 360 msec, regardless of whether the subjects showed sensory deficit or not. No significant laterality was seen in the primary sensorimotor areas. Distribution obtained from the patients with degenerative dementia was quite symmetrical. A significant right-left amplitude difference was seen at the anterior temporal site at 180 and 240 msec in normal controls, although an almost symmetrical distribution was obtained on the topographic images. The late components of SEP are not associated with the primary somatosensory function, but possibly with other cortical pathways.     

Discussing the Capabilities of Laplacian Minimization

This paper discusses the properties and capabilities of linear inverse solutions to the neuroelectromagnetic inverse problem obtained under the assumption of smoothness (Laplacian Minimization). Simple simulated counterexamples using smooth current distributions as well as single or multiple active dipoles are presented to refute some properties attributed to a particular implementation of the Laplacian Minimization coined LORETA. The problem of the selection of the test sources to be used in the evaluation is addressed and it is demonstrated that single dipoles are far from being the worst test case for a smooth solution as generally believed. The simulations confirm that the dipole localization error cannot constitute the tool to evaluate distributed inverse solutions designed to deal with multiple sources and that the necessary condition for the correct performance of an inverse is the adequate characterization of the source space, i.e., the characterization of the properties of the actual generators.     

On the Estimation of the Laplacian Electrocardiogram during Ventricular Activation

Body surface Laplacian electrocardiogram (ECG) mapping using a set of disk electrodes is explored by both computer simulation and human experiments in 12 healthy subjects. The Laplacian ECG was estimated from body surface potentials using finite difference estimation algorithms. The performance of the finite difference Laplacian estimators was evaluated by both computer simulation and human experiments. The present experimental results show that the two types of finite difference Laplacian estimates are highly correlated and have a consistent spatial distribution over the anterolateral chest during normal ventricular activation. The present computer simulation and human experiment results suggest the feasibility of estimating the body surface Laplacian maps (BSLMs) from potentials using the finite difference algorithm over the anterior chest in male subjects. The noise levels of the BSLMs over the anterolateral chest were quantitatively compared to the noise levels in corresponding body surface potential maps (BSPMs) in 12 healthy subjects. The simulation and experiment results indicate that the noise to signal ratios in the BSLMs over the anterolateral chest during ventricular activation is about 5 times that of the BSPMs, when no signal processing is performed. © 1999 Biomedical Engineering Society. PAC99: 8719Hh, 8719Nn     

Receptive field scatter,topography and map variability in different layers of the hindpaw representation of rat somatosensory cortex

We recorded neurons extracellularly in layers II/III, IV, and V of the hindpaw representation of primary somatosensory cortex in anesthetized rats and studied laminar features of receptive fields (RFs) and representational maps. On average, RFs were smallest in layer IV and largest in layer V; however, for individual penetrations we found substantial deviations from this rule. Within the hindpaw representation, a distinct rostrocaudal gradient of RF size was present in all layers. While layer V RFs were generally largest independent of this gradient, layer IV RFs recorded caudally representing the proximal portions of the paw were larger than layer II/III RFs recorded rostrally representing the digits. The individual scatter of the locations of RFs across laminar groups was in the range of several millimeters, corresponding to about 25% of the average RF diameter. The cutaneous representations of the hindpaw in extragranular layers were confined to the areal extent defined by responsive sites in layer IV. Comparison between RFs determined quantitatively and by handplotting showed a reliable correspondence. Repeated measurements of RFs revealed spontaneous fluctuations of RF size of no more than 5% of the initial condition over an observation period of several hours. The topography and variability of cortical maps of the hindpaw representation were studied with a quantitative interpolation method taking into account the geometric centers of RFs and the corresponding cortical recording sites. On average, the overall topography in terms of preservation of neighborhood relations was present in all layers, although some individual maps showed severe distortions of topography. Factors contributing to map variability were overall position of the representation on the cortical surface, internal topography and spatial extent. Interindividual variability of map layout was always highest in the digit representations. Local topographic orderliness was lowest in layer V, but comparable in layers II/III and IV. Within layer IV, the lowest orderliness was observed in the digit representations. Our data emphasize a substantial variability of RF size, overlap and position across layers and within layers. At the level of representational maps, we found a similar degree of variability that often co-varied across layers, with little evidence for significant layer specificity. Laminar differences are likely to arise from the specific input-output pattern, layer-specific cell types and the connectivity between different layers. Our findings emphasizing similarities in the variability across layers support the notion of tightly coupled columnar interactions between different layers.     

体表势表面拉普拉斯的三电极传感方法实现及其实验研究

体表势表面拉普拉斯是一种高空间分辨率的分析方法，在体表心电分析时可定位兴奋到达时刻。但由于信号小，对其测量要求放大器有高输入阻抗及高共模抑制比。本研究足研究一种集高性能信号调理与新的三极同心圆环传感电极于一体的表面拉普拉斯传感器，进行有关的实验研究，并与理论计算做比对，验证了表面拉普拉斯直接测量的可行性，为表面拉普拉斯用于心脏疾病的诊断奠定了基础。     

Local Polynomial Estimate of Surface Laplacian

This paper describes a method for estimating the surface Laplacian of brain potentials. The method consists of two steps: local surface approximation by its tangent plane and local polynomial fitting. Compared to previous methods for estimating surface Laplacian, this method has some new features. First, it can estimate the surface Laplacian at any point of the scalp, including the locations of the peripheral electrodes. Secondly, it estimates the brain potential and the surface Laplacian at any point simultaneously. This reduces the risk of error propagation, which occurs when the brain potential is interpolated first and the surface Laplacian is then computed based on the interpolated brain potential. Finally, the method automatically adapts to noisy data by using more or less measurements at neighboring electrodes based on estimated noise level. Simulations suggest that this method is effective. Application to event-related potentials are also presented.     

Steady-State Visually Evoked Potential topography associated with a visual vigilance task

Summary This paper describes data which demonstrate a correlation between the magnitude of the Steady-State Visually Evoked Potential (SSVEP) and visual vigilance. The SSVEP was recorded from 64 scalp sites and elicited by a 13Hz uniform visual flicker presented continuously while subjects undertook a visual vigilance task. Fifteen right-handed males were required to view three times a series of 180 geometrical shapes comprising a sequence of 60 squares, 60 circles and a further 60 squares. Each viewing of the 180 shapes constituted a trial. Trials 1 and 2 were identical while trial 3 differed from the first two in that one of the circles was modified. Subjects were ignorant as to the location of the modified circle and prior to the third trial, were challenged to identify the modified circle. A comparison of trials 2 and 3 indicated that the appearance of the modified circle was associated with an attenuation of the SSVEP in the occipito/parietal region. The same comparison indicated a pronounced SSVEP attenuation in the centro/parietal region during the interval that subjects were anticipating the appearance of the modified circle. These results suggest a distinction between the cortical activation patterns occurring during different phases of a visual vigilance task.This work was supported in part, by a grant from the Rebecca L. Cooper Foundation. The authors are happy to acknowledge the assistance of Mr. M. Cowey in the recording and analysis of the data, Dr. J. Currie for his helpful advice on conceptual and methodological issues, Prof. P. Finch and Mr. N. Garnham for their statistical advice and assistance, Dr. E. Gordon for his helpful comments on an earlier draft of this paper and Mr. F. Musci for his development of the task presentation software.     

A Radial-Basis Function Based Surface Laplacian Estimate for a Realistic Head Model

Scalp surface Laplacian (SL) is widely used to enhance spatial resolution and sensitivity for electroencephalograph (EEG) recordings. In this paper, a radial-basis function (RBF) based surface Laplacian (RBFL) estimate is proposed for realistic head model, in which RBF is used as basis function to interpolate the surface of a head model and the surface potentials. The efficiency of RBFL was confirmed through a comparative study to the global realistic geometry spline Laplacian (GSL) with both simulation studies and human visual evoked potential. The simulations include both feasibility in a 3-concentric sphere head model and influence of head model noise and potential noise, effects of border source and sources separation distance in a realistic head model. All the comparisons show RBFL can provide comparatively better results than GSL and hence RBFL provide another efficient method for high-resolution EEG mapping.     

P300 topography and modality effects from a single-stimulus paradigm

The P300 event-related brain potential (ERP) was elicited with auditory and visual stimuli in two different tasks. The oddball paradigm presented both target and standard stimuli; the single-stimulus paradigm presented a target but no standard stimulus, with the intertarget interval the same as that for the oddball condition. Target probability was .20 for the oddball task, with target stimuli occurring at the same temporal frequency in the single-stimulus paradigm. Scalp topography was assessed with 15 electrode locations. P300 amplitude was larger and latency was longer for the oddball than for single-stimulus procedure. P300 from auditory stimuli was smaller and shorter in latency than that from the visual stimuli, and both modalities showed similar but not identical scalp topographies. The findings suggest that the single-stimulus paradigm may be useful in experimental and applied contexts that require very simple ERP task conditions.     

P300 topography in Alzheimer's disease

This study assessed whether P300 scalp topography distinguishes subjects with Alzheimer's disease (AD) from controls. Specifically, it was predicted that the AD group would show maximum P300 amplitude over frontal areas and t he largest P300 reduction over parietal and left hemisphere areas. These hypotheses were tested using a standard auditory oddball paradigm to compare 26 AD subjects and 26 controls matched on age, sex, handedness, and education. P300 was measured at frontal, central, parietal, and occipital sites over left and right hemispheres and along the midline. Results revealed that the distribution of P300 was different for the two groups such that the controls manifested a maximum over parietal areas, whereas the AD subjects showed a maximum at frontal sites with the largest reductions in P300 over parietal areas. No hemispheric differences in P300 were found. These results are consistent with the hypothesis that P300 represents the activity of multiple neural generators that are differentially disrupted by the disease process.     

Body surface Laplacian mapping of atrial depolarization in healthy human subjects

In the present study, we report body surface Laplacian mapping of atrial depolarization under sinus rhythm in 8 healthy male subjects. For each subject, 95 unipolar disk electrodes with inter-electrode distance of 2 cm were used to record simultaneously potential ECGs over the anterior chest. The Laplacian ECG was then estimated during the P wave using a novel spline Laplacian technique. The body surface potential map (BSPM) and body surface Laplacian map (BSLM) at different time instants or time intervals of the P wave were constructed and compared. The present results showed that the BSPMs during the P wave were characterized by the rotation of a pair of positive/negative potential distribution from right to left around the anterior torso. On the other hand, the corresponding BSLMs revealed more spatial details, including two positive activities (denoted as P1 and P2, appeared in all 8 subjects), and three negative activities (denoted as N1, N2, and N3, appeared in 7, 7, and 4 subjects, respectively). The separation of these activities and their evolving patterns were also compared and confirmed by computer simulation using a realistic geometry heart-torso model. The above findings may be directly related to the underlying activation sequence during atrial depolarization in healthy subjects, suggesting the potential clinical applications of the Laplacian ECG technique.     

Dynamic functional topography of cognitive tasks

Summary Improved neuroelectric recording and analysis tools are yielding increasingly specific information about the spatial and temporal features of neurocognitive processes. Such tools include recordings with up to 125 channels, digital signal processing techniques, and correlation of neuroelectric measures with anatomical information from magnetic resonance images. These tools, and their application to the study of cognitive functions, are presented in this paper.Acknowledgements: This work was supported by grants from The National Institute of Neurological Diseases, The National Institute of Mental Health, The Air Force Office of Scientific Research and The National Science Foundation. The author gratefully acknowledges the efforts of his scientific collaborators, Drs. S. Bressler, P. Brickett, B. Cutillo, R. Fowler-White, D. Greer, J. Illes, J. Le, N. Morgan and B. Reutter for their contributions to the original research presented here.     

Comparison of Event-Related Potentials of Young Children and Adults in a Visual Recognition and Word Reading Task

An experiment is described in which event-related potentials (ERPs) of a group of young children were compared with ERPs of a group of young adults. Both groups were required to perform a simple word-reading task and a picture-recognition task. Principal components analyses (PCAs) were performed on the averaged ERPs in two different ways: a) separately for each of four combinations of tasks and age groups, and b) separately for each age group (pooled across tasks). The results demonstrated that ERP components of children and adults differed both qualitatively and quantitatively. First, children's ERPs were characterized by a long-latency negative component (N500) and a slow positive wave (SW) component, and adults’ ERPs were characterized by two late positive components (P340 and SW respectively). Second, both children and adults showed an earlier positive component that varied in peak latency between 280 ms for children and 240 ms for adults. In addition, adults showed a marked increase in SW positivity in the word-reading task as compared with the picture-recognition task, while task effects were less manifest in the components of children. These results support the notion that children and adults differed both in speed as well as in their mode of processing under the different task requirements.     

Development of a Tri-polar Concentric Ring Electrode for Acquiring Accurate Laplacian Body Surface Potentials

Potentials recorded on the body surface from the heart are of a spatial and temporal function. The 12-lead electrocardiogram (ECG) provides a useful means of global temporal assessment; however, it yields limited spatial information due to the smoothing effect caused by the volume conductor. In an attempt to circumvent the smoothing problem, researchers have used the five-point method (FPM) to numerically estimate the analytical solution of the Laplacian with an array of monopolar electrodes. Researchers have also developed a bipolar concentric ring electrode system to estimate the analytical Laplacian, and others have used a quasi-bipolar electrode configuration. In a search to find an electrode configuration with a close approximation to the analytical Laplacian, development of a tri-polar concentric ring electrode based on the nine-point method (NPM) was conducted. A comparison of the NPM, FPM, and discrete form of the quasi-bipolar configuration was performed over a 400 × 400 mesh with 1/400 spacing by computer modeling. Different properties of bipolar, quasi-bipolar and tri-polar concentric ring electrodes were evaluated and compared, and verified with tank experiments. One-way analysis of variance (ANOVA) with post hoc t-test and Bonferroni corrections were performed to compare the performance of the various methods and electrode configurations. It was found that the tri-polar electrode has significantly improved accuracy and local sensitivity. This paper also discusses the development of an active sensor using the tri-polar electrode configuration. A 1-cm active Laplacian tri-polar sensor based on the NPM was tested and deemed feasible for acquiring Laplacian cardiac surface potentials.     

肝静脉在背驮式肝移植术中的应用

按背驮式原位肝移植术要求,对17例成人肝和肝短静脉进行解剖观察与测量。结果显示:左、中、右肝静脉主干长为22.8±8.8mm,50.98±23.94mm,22.8±9.5mm;管径分别为10.74±2.86mm,9.5±3.75mm,15.6±4.05mm。右肝静脉除1例外,都以单独1支形式注入下腔静脉。中肝静脉和左肝静脉分三种形式注入下腔静脉:Ⅰ型4例,占23.5％,Ⅱ型12例,占70.6>6％;Ⅲ型1例,占5.9％。它们不在同一个平面注入下腔静脉。右、中肝静脉之间距离7～23mm,平均15.6mm。文章结合解剖学发现着重讨论了病肝切除和肝静脉成形术中的有关问题。     

Malignant melanoma - its precursors and its topography of proliferation

Summary DNA-content and size of the nuclear areas in different zones of malignant melanomas of different histological types and in dysplastic naevi were measured in order to provide information on the histogenesis and proliferative behaviour of human malignant melanoma. The results were compared with those from normal epidermis, common naevi, and reactive melanocytic hyperplasias. The mitotic index of melanomas -divided into different topographic zones in an analogous way - was also determined. The DNA-histographs of all naevi and reactive melanocytic hyperplasias showed a diploid maximum, but the dysplastic naevi had a larger proportion of nuclei with hyperdiploid and tetraploid DNA-content, indicating an increased proliferative activity. The mean values (X) of nuclear areas in dysplastic naevi (DN) were about the same as in common naevi (CN) and slightly lower than in superficial spreading melanomas (SSM). The coefficient of variability (cv) as an indicator of anisokaryosis was markedly higher in DN (27.8) and SSM (29.3) than in CN (20.2). In DNA-content we found similar results: almost no difference in mean values, but DN taking an intermediate position between CN and SSM with respect to cv (CN: 12.3; DN: 21.0; SSM: 36.6). There was no unequivocal evidence in these data for DN being a precancerous stage. Superficial melanomas with a nodular component (SSM/NM) differed from SSM and NM by increased DNA-content and greater variability of nuclear areas and showed the clearest features of malignancy in their DNA-histographs. The mitotic indices had rather low values in SSM and intraepidermal marginal zones of SSM/NM on one hand and markedly higher values in NM and nodular parts of SSM/NM on the other. The highest mitotic counts were found in the three investigated metastases.Arbeitsgemeinschaft Malignes Melanom Eppendorf AMMEWe gratefully acknowledge the generous support of the Hamburger Stiftung zur Förderung der Krebsbekämpfung