用有限元法计算媒质各向异性真实头模型脑电正问题

神经元所产生的脑内电活动通常可用电流源偶极子来模拟。讨论用有限元法计算电流源偶极子产生的电流与电位分布，推导出计算偶极子源在各向异性导电媒质中的有限元方程，并对其进行变形，使得在各向异性导电媒质中生成的有限元刚度阵对称。这一算法已应用于媒质各向异性的真实头模型脑电计算，得到了较为满意的结果。     

低频节律源传播方向对近真实头模型表面场电位动态参量的影响

θ振荡(4～8 Hz)是与学习、记忆等高级功能密切相关的低频脑节律,源于脑深处皮质区,头表面以额叶、颞叶最丰富。神经科学实验发现,该节律在脑内以一定方式传播,因成像技术局限和脑组织导电复杂性,其对头表面脑电节律的影响鲜有报告。依据等效偶极子电流源的脑电产生原理,以脑内低频单偶极子电流源(6 Hz正弦)偶极矩来仿真节律源驱动方向(以额叶为例定义指向),改变指向角度(以30°为移动单位),用有限元法计算电场,并对节律动态参量(大于平均值的显著能量、窄带相位)进行全局统计并对比。实验发现:脑内节律源在指向额叶表面传播时,几乎在所有指向角度下各向异性媒质会缩小显著能量空间;相反地,当源平行于额叶表面传播时,在所有指向角度下各向异性媒质会扩展显著能量空间,能量对源指向具有敏感性,而相位稳定性减小, 只与脑内节律源非线性相位时程有关,对该指向不敏感。结果表明,脑内低频节律源传播方向对头表面场电位动态参量作用不同。该研究为理解头表面低频节律动态参量提供电场计算依据,有助于理解脑内电活动、脑组织导电特性和头表面场电位动态参量之间的映射关系。     

关于脑电偶极子源定位问题

从头皮脑电分布推算脑电活动源的脑电偶极子源定位问题的研究 ,对于脑认知功能的研究有重要的科学意义和临床应用价值。本文对偶极子源定位的一些问题 ,主要包括模型的选择、偶极子源定位的方法及源定位精确性等研究前沿进行概述。     

偶极子追踪法的现状与展望

偶极子追踪法的现状与展望刘晓欲，黄远桂，蒋大宗根据头皮的脑电分布推算产生这一脑电活动的起源，这类问题被称做脑电逆问题。Ｂｒａ－ｚｉｅｒ［１］在１９４９年首次提出用电流源作为脑电发生器的观点，在此后近２０年内，由于计算方法和技术的局限，人们的研究主要集...     

关于脑电偶极子源定位问题

从头皮脑电分布推算脑电活动源的脑电偶极子源定位问题的研究，对于脑认知功能的研究有重要的科学意义和临床应用价值。本对偶极子源定位的一些问题，主要包括模型的选择、偶极子源定位的方法及源定位精确性等研究前沿进行概述。     

磁刺激内关穴的脑电源定位分析

与传统的针灸和电针刺激相比,磁刺激具有无创、无痛、不接触、易于操作等良好的安全性能.对磁刺激内关穴的脑电信号提取诱发电位并进行等效偶极子源定位分析研究.利用磁刺激仪对内关穴(PC6)及内关穴附近非穴位点(假穴)进行了相同强度、相同频率的磁刺激,分析了同步采集的脑电信号的诱发电位并对其进行了等效偶极子源定位研究.结果表明...     

利用有限差分算法研究开颅术对头皮电位分布的影响

本研究在三层同心球模型上利用有限差分法求解开颅手术后的脑电正问题,通过与开颅术前脑电正问题的解进行比较,研究颅骨上的洞对头皮电位分布的影响。仿真试验表明,颅骨开洞对头皮电位分布的影响程度取决于偶极子源的位置和方向:径向偶极子源位于洞正下方时,洞对头皮电位分布的影响最大;而切向偶极子源靠近洞边缘时的影响最大。     

脑电记录中"参考电极位置"的设置研究

目的：探讨一种可以把头皮脑电活动（EEG）记录中的“参考电极位置”近似地转化到无穷远点的新技术。方法：基于由头皮记录确定神经电活动源或其等效源不受参考电极影响的事实，用基于普通参考电极描记到的头皮EEG记录近似地重建出一组等效源，然后由等效源计算出以无穷远点为参考电极位置的头皮记录。结果：用在导体中假设神经电活动源的方式进行的仿真研究表明，当电源位于最重要的皮层表面区域时，可以明显降低参考电极对EEG记录的不良影响。结论：EEG记录的“参考电极位置”可以被近似地转换到无穷远点，从而近似地得到一个以零电位为参考的标准记录。     

想象左右手运动的脑电特征提取及分类研究

针对想象运动的脑机接口(BCI)系统存在分类准确率低、抗干扰能力差等不足,提出一种将离散小波变换(DWT)和BP神经网络相结合的脑电识别方法(DWT-BP法).通过计算想象左、右手运动的C3、C4的平均功率,合理确定时间窗设置,对时间窗内的平均功率信号进行离散小波变换,并选取尺度6上的逼近系数A6的组合信号作为脑电信号特征,以BP神经网络为分类器实现对脑电观测数据的分析.实验结果表明,DWT-BP方法能够较准确地提取脑电信号的本质特征,具有较好的抗干扰能力和分类性能,以及识别运动想象脑电信号的有效性,同时为实现运动想象在线BCI系统打下基础.     

视听联合刺激的跨感觉通道诱发电位源的分析

运用偶极子源分析方法分析视听单独刺激和联合刺激下脑区的区别和联系.采用64道脑电采集系统,记录了7名被试者在视觉、听觉和视听联合刺激下的脑诱发电位,并分别在初期、早期和晚期时间段对这3种任务做偶极子源分析.通过比较视听联合刺激和视听单独刺激下偶极子源的变化,可以得到视听联合刺激下的源不等于单独视听刺激下的源的简单叠加,从而提示存在视听相互作用的区域.     

Dealing with mismatched fMRI activations in fMRI constrained EEG cortical source imaging: a simulation study assuming various mismatch types

Although fMRI constrained EEG source imaging could be a promising approach to enhancing both spatial and temporal resolutions of independent fMRI and EEG analyses, it has been frequently reported that a hard fMRI constraint may cause severe distortion or elimination of significant EEG sources when there are distinct mismatches between fMRI activations and EEG sources. If estimating actual EEG source locations is important and fMRI prior information is used as an auxiliary tool to enhance the concentration of widespread EEG source distributions, it is reasonable to weaken the fMRI constraint when significantly mismatched sources exist. The present study demonstrates that the mismatch problem may be partially solved by extending the prior fMRI activation regions based on the conventional source imaging results. A hard fMRI constraint is then applied when there is no distinct mismatch, while a weakened fMRI constraint is applied when there are significant mismatches. A preliminary simulation study assuming different types of mismatches such as fMRI invisible, extra, and discrepancy sources demonstrated that this approach can be a promising option to treat mismatched fMRI activations in fMRI constrained EEG source imaging.     

基于自回归模型和相位斜率指数的动态脑连接分析

随着脑网络理论的发展,人们越来越关注不同脑区域之间的功能性和效应性连接。在常见的脑活动无创检测方法中,脑电图（EEG）具有较高的时间分辨率,适于进行效应性连接分析。提出一种估计不同通道的EEG信号间的效应性连接的方法,即自回归相位斜率指数（AR-PSI）。该方法结合多元自回归（MVAR）模型对短时数据进行谱估计频率分辨率高的特点和相位斜率指数(PSI)对源信号混叠不敏感的特点。与传统的格兰杰因果模型相比,它可以有效地摒除由于容积导体效应造成的信号混叠所带来的干扰;与传统的PSI相比,它在短时数据上能够更准确地估计不同通道间EEG信号的效应性连接。首先,分别生成具有强效应性连接和相互独立的混叠噪声这两组信号进行模拟实验,结果表明AR-PSI方法确实能够更有效地检测出信号中存在的效应连接,排除信号线性混叠可能引起的误检。然后,应用此方法并结合滑动窗技术,对Stroop实验记录到的EEG数据进行网络连接的动态分析,发现250～500 ms和550～800 ms时间段内脑网络连接密度在两种条件下存在显著性差异。分析结果,显示语义和颜色不一致条件的刺激能够引起脑网络连接密度更迅速地增加,且连接跨度更大。     

A technique to consider mismatches between fMRI and EEG/MEG sources for fMRI-constrained EEG/MEG source imaging: a preliminary simulation study

fMRI-constrained EEG/MEG source imaging can be a powerful tool in studying human brain functions with enhanced spatial and temporal resolutions. Recent studies on the combination of fMRI and EEG/MEG have suggested that fMRI prior information could be readily implemented by simply imposing different weighting factors to cortical sources overlapping with the fMRI activations. It has been also reported, however, that such a hard constraint may cause severe distortions or elimination of meaningful EEG/MEG sources when there are distinct mismatches between the fMRI activations and the EEG/MEG sources. If one wants to obtain the actual EEG/MEG source locations and uses the fMRI prior information as just an auxiliary tool to enhance focality of the distributed EEG/MEG sources, it is reasonable to weaken the strength of fMRI constraint when severe mismatches between fMRI and EEG/MEG sources are observed. The present study suggests an efficient technique to automatically adjust the strength of fMRI constraint according to the mismatch level. The use of the proposed technique rarely affects the results of conventional fMRI-constrained EEG/MEG source imaging if no major mismatch between the two modalities is detected; while the new results become similar to those of typical EEG/MEG source imaging without fMRI constraint if the mismatch level is significant. A preliminary simulation study using realistic EEG signals demonstrated that the proposed technique can be a promising tool to selectively apply fMRI prior information to EEG/MEG source imaging.     

基于小波神经网络的心电信号滤波研究

针对心电(ECG)信号检测中存在的主要噪声,本文研究了基于小波神经网络(WNN)的ECG信号滤波理论。提出一种通过WNN非线性逼近能力构建的针对ECG信号的非线性滤波器算法和滤波策略,实现对ECG信号中基线漂移、肌电干扰、工频干扰噪声的滤除;给出了网络训练算法和滤波实验,滤波后信号与期望信号误差范围在微伏级,验证了本文提出的基于WNN的心电非线性滤波器对心电主要噪声快速滤波的良好效果,最后讨论了影响WNN用于心电滤波的几个关键问题。     

On the EEG/MEG forward problem solution for distributed cortical sources

In studies of EEG/MEG problems involving cortical sources, the cortex may be modeled by a 2-D manifold inside the brain. In such cases the primary or impressed current density over this manifold is usually approximated by a set of dipolar sources located at the vertices of the cortical surface tessellation. In this study, we analyze the different errors induced by this approximation on the EEG/MEG forward problem. Our results show that in order to obtain more accurate solutions of the forward problems with the multiple dipoles approximation, the moments of the dipoles should be weighted by the area of the surrounding triangles, or using an alternative approximation of the primary current as a constant or linearly varying current density over plane triangular elements of the cortical surface tessellation. This should be taken into account when computing the lead field matrix for solving the EEG/MEG inverse problem in brain imaging methods.     

Modeling and source localization of MEG activities

Summary During the past decade, substantial advances in the understanding of the functional organization of the human brain have been made through the technique of MEG topographic mapping. Most of these investigations were concerned with the estimation and localization of sources which were modeled as single current dipoles positioned in a semi-infinite volume conductor with homogeneous conductivity. However, the sources in the brain are complex, and the head as a volume conductor consists of different materials with different electrical conductivities. The influence of these inhomogeneities on the MEG topography is studied by a computer simulation, modeling the sources as single or multiple dipoles located in inhomogeneous volume conductors. The computer simulation suggests some important aspects in estimation of source localization. The sources of MEG activities in human subject during sleep are also studied. A comparison of simulated MEG topographic patterns with measured data suggests that the sources of K-complexes can be modeled by two current dipoles. Sources for delta waves are analyzed by the FFT technique. The results show that the frequency distributions are different for delta waves measured by MEG and EEG techniques, leading us to conclude that at least two different sources are present. The MEG measurements have an advantage to provide important information concerning brain function which cannot be obtained using the EEG measurements.This work was supported in part by grants 63850090 and 01790390 from the Ministry of Education Science and Culture, Japan, and by grants from Nakatani Electric Measuring Technology Association, and the Mitsubishi Foundation.     

Localization of coherent sources by simultaneous MEG and EEG beamformer

Simultaneous magnetoencephalography (MEG) and electroencephalography (EEG) analysis is known generally to yield better localization performance than a single modality only. For simultaneous analysis, MEG and EEG data should be combined to maximize synergistic effects. Recently, beamformer for simultaneous MEG/EEG analysis was proposed to localize both radial and tangential components well, while single modality analyses could not detect them, or had relatively higher location bias. In practice, most interesting brain sources are likely to be activated coherently; however, conventional beamformer may not work properly for such coherent sources. To overcome this difficulty, a linearly constrained minimum variance (LCMV) beamformer may be used with a source suppression strategy. In this work, simultaneous MEG/EEG LCMV beamformer using source suppression was formulated firstly to investigate its capability over various suppression strategies. The localization performance of our proposed approach was examined mainly for coherent sources and compared thoroughly with the conventional simultaneous and single modality approaches, over various suppression strategies. For this purpose, we used numerous simulated data, as well as empirical auditory stimulation data. In addition, some strategic issues of simultaneous MEG/EEG analysis were discussed. Overall, we found that our simultaneous MEG/EEG LCMV beamformer using a source suppression strategy is greatly beneficial in localizing coherent sources.     

Neuronal Networks in West Syndrome as Revealed by Source Analysis and Renormalized Partial Directed Coherence

West syndrome is a severe epileptic encephalopathy of infancy with a poor developmental outcome. This syndrome is associated with the pathognomonic EEG feature of hypsarrhythmia. The aim of the study was to describe neuronal networks underlying hypsarrhythmia using the source analysis method (dynamic imaging of coherent sources or DICS) which represents an inverse solution algorithm in the frequency domain. In order to investigate the interaction within the detected network, a renormalized partial directed coherence (RPDC) method was also applied as a measure of the directionality of information flow between the source signals. Both DICS and RPDC were performed for EEG delta activity (1–4 Hz) in eight patients with West syndrome and in eight patients with partial epilepsies (control group). The brain area with the strongest power in the given frequency range was defined as the reference region. The coherence between this reference region and the entire brain was computed using DICS. After that, the RPDC was applied to the source signals estimated by DICS. The results of electrical source imaging were compared to results of a previous EEG-fMRI study which had been carried out using the same cohort of patients. As revealed by DICS, delta activity in hypsarrhythmia was associated with coherent sources in the occipital cortex (main source) as well as the parietal cortex, putamen, caudate nucleus and brainstem. In patients with partial epilepsies, delta activity could be attributed to sources in the occipital, parietal and sensory-motor cortex. In West syndrome, RPDC showed the strongest and most significant direction of ascending information flow from the brainstem towards the putamen and cerebral cortex. The neuronal network underlying hypsarrhythmia in this study resembles the network which was described in previous EEG-fMRI and PET studies with involvement of the brainstem, putamen and cortical regions in the generation of hypsarrhythmia. The RPDC suggests that brainstem could have a key role in the pathogenesis of West syndrome. This study supports the theory that hypsarrhythmia results from ascending brainstem pathways that project widely to basal ganglia and cerebral cortex.     

Finite-element model of the human head: scalp potentials due to dipole sources

Three-dimensional finite-element models provide a method to study the relationship between human scalp potentials and neural current sources inside the brain. A new formulation of dipole-like current sources is developed here. Finiteelement analyses based on this formulation are carried out for both a threeconcentric-spheres model and a human-head model. Differences in calculated scalp potentials between these two models are studied in the context of the forward and inverse problems in EEG. The effects of the eye orbit structure on surface potential distribution are also studied.