Rogers Nicholas, Thunemann Martin, Devor Anna, Gilja Vikash
Department of Physics, University of California, San Diego, La Jolla, CA, United States.
Department of Radiology, University of California, San Diego, La Jolla, CA, United States.
Front Neurosci. 2020 Aug 7;14:763. doi: 10.3389/fnins.2020.00763. eCollection 2020.
Volume conduction of electrical potentials in the brain is highly influenced by the material properties and geometry of the tissue and recording devices implanted into the tissue. These effects are very large in EEG due to the volume conduction through the skull and scalp but are often neglected in intracranial electrophysiology. When considering penetrating electrodes deep in the brain, the assumption of an infinite and homogenous medium can be used when the sources are far enough from the brain surface and the electrodes to minimize the boundary effect. When the electrodes are recording from the brain's surface the effect of the boundary cannot be neglected, and the large surface area and commonly used insulating materials in surface electrode arrays may further increase the effect by altering the nature of the boundary in the immediate vicinity of the electrodes. This gives the experimenter some control over the spatial profiles of the potentials by appropriate design of the electrode arrays. We construct a simple three-layer model to describe the effect of material properties and geometry above the brain surface on the electric potentials and conduct empirical experiments to validate this model. A laminar electrode array is used to measure the effect of insulating and relatively conducting layers above the cortical surface by recording evoked potentials alternating between a dried surface and saline covering layer, respectively. Empirically, we find that an insulating boundary amplifies the potentials relative to conductive saline by about a factor of 4, and that the effect is not constrained to potentials that originate near the surface. The model is applied to predict the influence of array design and implantation procedure on the recording amplitude and spatial selectivity of the surface electrode arrays.
大脑中电位的容积传导受组织的材料特性、几何形状以及植入组织中的记录设备的极大影响。由于通过颅骨和头皮的容积传导，这些影响在脑电图（EEG）中非常大，但在颅内电生理学中常常被忽视。当考虑大脑深部的穿透电极时，当源离脑表面和电极足够远时，可以使用无限均匀介质的假设来最小化边界效应。当电极从脑表面进行记录时，边界效应不能被忽视，并且表面电极阵列中较大的表面积和常用的绝缘材料可能会通过改变电极紧邻区域的边界性质进一步增加这种效应。这使得实验者能够通过适当设计电极阵列来对电位的空间分布进行一定程度的控制。我们构建了一个简单的三层模型来描述脑表面上方的材料特性和几何形状对电位的影响，并进行实证实验来验证该模型。使用层状电极阵列通过分别记录在干燥表面和盐水覆盖层之间交替的诱发电位来测量皮质表面上方绝缘层和相对导电层的影响。从经验上看，我们发现绝缘边界相对于导电盐水使电位放大了约4倍，并且这种效应并不局限于起源于表面附近的电位。该模型被应用于预测阵列设计和植入程序对表面电极阵列记录幅度和空间选择性的影响。
