应用于脑电场的新型等效源法的正演理论

将新型等效源法引入脑电场电位的计算,视激励源为偶极子,实现真实电源既可以在真实的头模型中产生电位,也可以在均质无界空间中产生电位.从无界空间中三维拉普拉斯方程解的球形等效源的有限级数项表达式可见,该半解析方法克服了头模型对电位分布的干扰,在无界空间中计算出的电位是电源的裸表现.本文利用等效分布源理论推导出无界空间中任意偶极子源的球形等效源的正演理论公式,为高分辨脑电地形图技术中的重构皮层电位提供了一种方法.     

一种处理人体骨骼肌肉层各向异性导电性的数值方法

重点讨论了应用电磁场数值计算方法求解心电图正问题和逆问题中人体骨骼肌肉层的各向异性导电性的处理方法．文中应用有限元和边界元结合的方法,构造了一个包含不同的纤维方向组合的骨骼肌肉层的三维胸腔模型,并在此模型下引入了局部坐标内的各向异性导电率向整体坐标转换的方法．据此进行的模拟计算结果以图像的方式清晰地展示了各向异性导电性对体表电位图的影响．     

Mueller矩阵在生物医学检测方面应用的实验研究

Mueller矩阵是公认的能很好地表述介质偏振特性的一种方法,由于散射光偏振在生物组织无创伤诊断技术等诸多领域中的重要应用价值,对组织散射特性的Mueller矩阵的研究成为国际上组织光学的热点之一。研究设计了一种新的测量Mueller矩阵的实验装置:斜入射正接收装置,并推导出一组后续数据处理的算法。由此所获得的Mueller矩阵空间分布图的清晰度不亚于其它所报道的,并且测量方法具有结构简单、方便、准确等优点。实验结果表明:入射角影响Mueller矩阵空间分布图;随着介质浓度的增大,随机介质后向散射Mueller矩阵各元素的空间分布图样减小;同时列举了真实生物组织样品(肌肉组织)的后向散射Mueller矩阵的实测结果,由此证明各向异性生物组织的后向散射Mueller矩阵各元素的空间分布图样与纤维的走向有关。     

Stokes参量共焦显微术用于弱各向异性物质的成像研究

提出将基于Stokes参量的偏振共焦显微成像技术应用于弱各向异性物质的成像研究。通过将分振幅Stokes参量测量法与共焦扫描成像技术相结合的方法,得到了基于Stokes参量测量的偏振共焦显微成像系统。利用该系统对具有弱各向异性的生物组织样品进行逐点测量,通过四个通道同时测量获得全部的Stokes参量。再以计算得到的偏振参量作为成像物理量进行图像重建,获得对应Stokes参量、偏振度、相位差、方位角和椭率角的空间分布图像,从而对生物组织实现细胞水平的偏振显微成像研究。实验结果表明:基于Stokes参量的偏振共焦显微成像技术能够获得弱各向异性的生物组织样品的显微图像,并通过比较样品的Stokes参量及相关偏振参量的分布图像,提取样品全部的偏振信息,从而为生物组织的特性研究提供更丰富的信息。     

混浊介质后向散射特性的Mueller矩阵实验测量

Mueller矩阵是一种公认的能很好地表述介质偏振特性的方法.由于散射光偏振在生物组织无创伤诊断技术等诸多领域中的重要应用价值,对组织散射特性的Mueller矩阵的研究成为国际上组织光学的热点之一.与现有测量Mueller矩阵的实验方法相比,斜入射正接收装置用来测量Mueuer矩阵是一个更加行之有效的方法,再结合一种新的算法来处理后续数据,由此所获得的Mueller矩阵空间分布图的清晰度不亚于其它文献的报道.这种测量方法结构更简单,具有测量更方便、准确等优点.结果表明:入射角影响Mueller矩阵的空间分布图.随着介质浓度的增大,随机介质后向散射Mueller矩阵各元素的空间分布图样减小.     

基于模拟退火法由脑磁图推测电流偶极子参数

利用模拟退火（Ｓｉｍｕｌａｔｅｄ Ａｎｎｅａｌｉｎｇ） 算法,由脑磁图（ ＭＥＧ） 数据反演脑内作为磁源的单电流偶极子参数,可以得到理想的结果。在上述工作的基础上,对脑内多电流偶极子参数的反演,则呈现如下状况：即以少于实际源数目的偶极子为源假设反演,目标函数得不到极小优化。反之,目标函数可以得到极小优化, 但出现多余的伪偶极子, 且这些伪偶极子在多次不同条件的反演结果中,处于不稳定状态。若将多次反演结果中处于不稳定状态的偶极子作为伪偶极子的判据而将其排除,则可以得到一种判断磁源偶极子数目的方法     

生物磁的奥秘

生物电流是生命过程中的普遍现象。大脑和神经系统活动、心脏跳动和肌肉运动,都伴随着生物电流。电的流动产生磁,因而有脑电、神经电、心电、肌电,就有脑磁场、神经磁场、心磁场和肌磁场。这些是生命过程中普遍的基本的生物磁现象。 此外,人体组织内含有一些磁性物质。例如血红蛋白、肌红蛋白、铁蛋白中均含有铁;血浆铜蓝蛋     

高斯光束通过生物体混浊介质模型的测量

将各向同性的沸浊介质材料作为生物组织的光学模型,用2mw单模He-Ne高斯激光透过该混浊介质材料,测量透射率、偏振度及透射光强的分布,得到了偏振度（ν）与透射率（T）的关系曲线。当透射率T＜30％时,偏振度ν→0,高斯激光束透过混浊介质材料,受散射影响,光强偏离高斯分布,并提出了透射光强分布公式。     

经颅磁刺激:生理、心理、脑成像及其临床应用

非侵入性的经颅磁刺激 (TranscranialMagneticStimulation ,TMS)可以无痛地产生感应性电流来激活皮层 ,从而改变大脑内的生理过程。通过改变TMS的参数可以观测到不同的生理和心理效应。TMS对大脑的不同的调节方式体现在对感觉的调节、对认知功能和行为表现的促进或抑制上。TMS还可以与神经病学、精神病学和药理学研究相结合。无框架的TMS立体定位技术可以提高用于解剖定位的TMS以及用于指引脑外科手术的准确性。最后 ,中医针灸的理论也可以用于理解TMS在脑内调节作用的频率效应     

中脑网状结构在针刺镇痛中的作用及其传导通路的初步分析

我们在过去的实验中曾经观察到:(1)针刺对血压反应的调整作用在去大脑动物上仍可出现(刘觐龙等,1964);(2)通过埋藏电极用脉冲电刺激中脑网状结构可以出现镇痛作用,并可对针刺的效应产生影响(广西医学院针麻研究小组,1973)。为了进一步研究中脑网状结构在针刺镇痛中的作用,进行了以下三项实验工作: 1.中脑网状结构神经元对于伤害性刺激有哪些反应形式?这些神经元在中脑内分布如何?针刺能否对这些反应产生影响?产生什么样的影响? 2.伤害性刺激和针刺对中脑网状结构神经元的作用,是否有赖于高一级的中枢结构     

EEG and implanted sources in the brain

Localisation procedures are based on models of the EEG that are relatively simple. The models are based on assumptions and choices of parameters that can be mistaken. Thus, it is crucial to validate the localisation procedures used in EEG. One of the options is to use the data obtained with electrodes that are implanted within the brain of an epileptic patient as part of the pre-surgical evaluation. When one of two neighbouring electrodes is used as a current source and the other as a current sink this can be regarded as a current dipole. The current injected has to be below the threshold for activation of cells. The position of this dipole can be deduced from magnetic resonance or X-ray images. The current dipole gives rise to a potential distribution at the scalp that can be measured by EEG. The measurements can be compared with the potential distribution that is calculated in a forward computation. Another method is to use the measured potential at the scalp to localize the source and to compare the result with the actual position of the dipole. In this paper the measured potential distributions at the scalp due to implanted dipoles were used to evaluate different volume conductor models. Since intracerebral and subdural electrodes were introduced through trephine holes over the fronto-central areas, and the diameter of the holes was rather large, approximately 23 mm, special effort was put into modelling the skull. Two important assumptions could be validated in this study: the electric currents within the head are Ohmic and a dipole can be used to model the induced electric activity of pairs of contacts on subdural electrodes or intra cerebral electrodes.     

Polarized fluorescent emission from probes near dielectric interfaces

The electromagnetic field surrounding and emitted by a dipolar molecular probe very near to a dielectric interface is the sum of the real dipole field and the field of the image dipole induced inside the dielectric interface. The total charge distribution, made up of the real and image dipoles in close proximity to each other, approximates a quadrupole distribution and emits a light intensity pattern similar to that of an oscillating electric quadrupole. The electromagnetic field emitted by this system contains information that can be directly related to the spatial and orientational distributions of the dipole near the interface. Experimental methods are discussed that utilize this system for determining the spatial and orientational distribution of fluorescent probes in biological material.     

The influence of conductivity changes in boundary element compartments on the forward and inverse problem in electroencephalography and magnetoencephalography.

Source localization based on magnetoencephalographic and electroencephalographic data requires knowledge of the conductivity values of the head. The aim of this paper is to examine the influence of compartment conductivity changes on the neuromagnetic field and the electric scalp potential for the widely used three compartment boundary element models. Both the analysis of measurement data and the simulations with dipoles distributed in the brain produced two significant results. First, we found the electric potentials to be approximately one order of magnitude more sensitive to conductivity changes than the magnetic fields. This was valid for the field and potential topology (and hence dipole localization), and for the amplitude (and hence dipole strength). Second, changes in brain compartment conductivity yield the lowest change in the electric potentials topology (and hence dipole localization), but a very strong change in the amplitude (and hence in the dipole strength). We conclude that for the magnetic fields the influence of compartment conductivity changes is not important in terms of dipole localization and strength estimation. For the electric potentials however, both dipole localization and strength estimation are significantly influenced by the compartment conductivity.     

Response of juvenile scalloped hammerhead sharks to electric stimuli

Sharks can use their electrosensory system to detect electric fields in their environment. Measurements of their electrosensitivity are often derived by calculating the voltage gradient from a model of the charge distribution for an ideal dipole. This study measures the charge distribution around a dipole in seawater and confirms the close correspondence with the model. From this, it is possible to predict how the sharks will respond to dipolar electric fields comprised of differing parameters. We tested these predictions by exposing sharks to different sized dipoles and levels of applied current that simulated the bioelectric fields of their natural prey items. The sharks initiated responses from a significantly greater distance with larger dipole sizes and also from a significantly greater distance with increasing levels of electric current. This study is the first to provide empirical evidence supporting a popular theoretical model and test predictions about how sharks will respond to a variety of different electric stimuli.     

Measurement of Dipole Potential in Bilayer Lipid Membranes by Dielectric Spectroscopy

Planar bilayer lipid membranes formed from egg phosphatidylcholine in aqueous media containing the lipophilic anion, dipicrylamine (DPA), were studied by dielectric spectroscopy over a frequency range of 10 Hz–10 MHz. The membranes showed dielectric relaxation due to the translocation of DPA between the membrane interfaces. Incorporating either cholesterol or 6-ketocholestanol into the membranes increased the characteristic frequency of the relaxation, which is proportional to the translocation rate constant of DPA. The results suggested that the sterol dipoles induced positive potential changes within the membrane interior. The changes of the dipole potential were 70 mV for cholesterol and 150 mV for 6-ketocholestanol when the sterol mole fraction was 0.67. The opposite effect was caused by phloretin added to the aqueous media, and the maximum dipole potential change was ?90 mV at 100 μM.     

Permeation of ions across the potassium channel: brownian dynamics studies

The physical mechanisms underlying the transport of ions across a model potassium channel are described. The shape of the model channel corresponds closely to that deduced from crystallography. From electrostatic calculations, we show that an ion permeating the channel, in the absence of any residual charges, encounters an insurmountable energy barrier arising from induced surface charges. Carbonyl groups along the selectivity filter, helix dipoles near the oval chamber, and mouth dipoles near the channel entrances together transform the energy barrier into a deep energy well. Two ions are attracted to this well, and their presence in the channel permits ions to diffuse across it under the influence of an electric field. Using Brownian dynamics simulations, we determine the magnitude of currents flowing across the channel under various conditions. The conductance increases with increasing dipole strength and reaches its maximum rapidly; a further increase in dipole strength causes a steady decrease in the channel conductance. The current also decreases systematically when the effective dielectric constant of the channel is lowered. The conductance with the optimal choice of dipoles reproduces the experimental value when the dielectric constant of the channel is assumed to be 60. The current-voltage relationship obtained with symmetrical solutions is linear when the applied potential is less than approximately 100 mV but deviates from Ohm's law at a higher applied potential. The reversal potentials obtained with asymmetrical solutions are in agreement with those predicted by the Nernst equation. The conductance exhibits the saturation property observed experimentally. We discuss the implications of these findings for the transport of ions across the potassium channels and membrane channels in general.     

Monte-Carlo Simulation of Molten CsCl Using a ‘Deformation Dipole’ Polarisable Ion Potential

Abstract

We have simulated the structure of molten CsCl at 943 K using a form of interionic potential, based on the deformation dipole model of Hardy and Karo, where ions have both point charges and dipoles. The magnitude of the dipole on an ion is determined by two factors, one being the local electric field and the other the repulsive potential due to overlap with neighbouring ions. Parameters are determined directly from the crystal lattice constant, compressibility and dielectric constants. Good agreement with experimental results is obtained. The potential is compared to the shell model of ionic polarisability which does not produce good agreement with experiment.     

The different screening of electric charges and dipoles near a dielectric interface

The electric fields within a planar slab of material due to both charges and dipoles within the slab and near to its surface are simply calculated using the method of images. If the slab is immersed in a fluid of high dielectric constant the electric field within the slab due to the charge is always reduced but that of a suitably oriented electric dipole is enhanced by as much as a factor of two.     

Dielectric relaxation and orientation of DNA molecules

A conductivity dispersion has been measured at very low frequencies (VLF) on several concentrated DNA solutions. By measuring simultaneously their electric birefringence decay, it is shown that the dielectric relaxation (which is related to the conductivity dispersion) is due to the molecular orientation. Different polarization mechanisms are discussed. It is concluded that the DNA polarizability measured in the VLF range can only be explained by the orientation of a permanent ionic dipole. It is suggested that such permanent dipoles could be caused by small differences in the ionic composition between the two molecular “ends;” the difference could either be stable (asymmetrical localization of protein impurities for instance) or transient (fluctuating dipoles explained by the Kirkwood-Schumaker theory).