We investigated the conditions under which inhomogeneity in electrical conductivity may significantly modify the magnetic evoked field (MEF) due to primary currents (i.e., neuronal currents) in the brain. In the case of an isolated turtle cerebellum immersed in a large bath of physiological saline, our theoretical analysis showed the cerebellar surface to significantly enhance the MEF due to a primary current, by a factor of as much as two, for experimentally determined values of the conductivities of the cerebellar tissue and saline. A further parametric investigation of the conductivity effect revealed that conductivity boundaries may significantly modify the MEF due to neuronal currents located within 1 mm of a conductivity boundary, as would be the case for active neurons near an edema, an anoxic fringe such as might occur during stroke, or a ventricle in the human head. For a stationary neural source, conductivity boundaries may modify the magnitude of its MEF without affecting its temporal waveform. However, this boundary effect was found to be small for a model geometry locally approximating cortical sources in a sulcus or a fissure, where the boundary effects from adjacent sulcal walls tend to cancel each other.

译文

我们研究了电导率不均匀性可能会由于大脑中的主要电流 (即神经元电流) 而显着改变磁诱发场 (MEF) 的条件。如果将孤立的乌龟小脑浸入一个大的生理盐水浴中,我们的理论分析表明,由于一次电流,小脑表面显着提高了MEF,对于实验确定的小脑组织和盐水的电导率值,其系数高达两倍。对电导率效应的进一步的参数研究表明,由于位于电导率边界的1毫米内的神经元电流,电导率边界可能会显著地改变MEF,如在水肿、缺氧边缘 (例如在中风期间可能发生) 或人头部的心室附近的活动神经元的情况。对于固定神经源,电导率边界可能会改变其MEF的大小,而不会影响其时间波形。然而,对于局部近似于沟或裂缝中的皮质源的模型几何,发现这种边界效应很小,其中相邻沟壁的边界效应趋于相互抵消。

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