Zhang Z, Jewett D L, Goodwill G
Research Division, Abratech Corporation, Sausalito, California 94965.
Brain Topogr. 1994 Summer;6(4):283-98. doi: 10.1007/BF01211174.
Insidious errors in dipole modeling due to shell model misspecification in a spherical model were examined analyzing multiple time-points using the constraints of a commonly-used DSL (Dipole Source Localization) method. The computer simulation examined the differences in the fit dipole parameters for the same generator under two circumstances: 1) when computed as a single dipole active alone, and 2) when computed as a member of a simultaneously-active dipole pair. The computations were done using a simplification by which the dipole parameters computed from multiple time-points can be correctly assessed by computing dipole parameters at only two virtual time-points. Using multiple time-points in the DSL generally resulted in less error than if only a single time-point was used. However, how much improvement cna be achieved by using multiple time-points, as compared with a single time-point, is a function of many factors, such as the location and orientation of the dipoles, and the relative magnitudes and overlap of the waveforms (i.e., time-varying magnitudes) of the dipoles, as well as the model used in the fitting. Further, it was shown that it is incorrect to assume that a multiple-time-point DSL will compute a zero magnitude for generators during quiescent intervals. Additionally, it was shown that a "correction" to reduce error for one pair of waveforms will not be applicable to other waveforms. Also, even if location errors are eliminated, magnitude and orientation errors can still be shown to be present. Finally, iterative reduction of the least-square error between the observed and predicted surface maps leads to increasing errors in dipole parameters. We conclude that a DSL with model misspecification can contain insidious (undetectable) errors.
通过使用常用的偶极子源定位（DSL）方法的约束条件，对多个时间点进行分析，研究了球形模型中由于壳模型指定错误而导致的偶极子建模中的潜在误差。计算机模拟研究了同一发生器在两种情况下拟合偶极子参数的差异：1）单独作为单个活动偶极子计算时；2）作为同时活动的偶极子对的成员计算时。计算是通过一种简化方法进行的，即通过仅在两个虚拟时间点计算偶极子参数，就可以正确评估从多个时间点计算得到的偶极子参数。在DSL中使用多个时间点通常比仅使用单个时间点产生的误差更小。然而，与单个时间点相比，使用多个时间点能实现多大程度的改进，取决于许多因素，例如偶极子的位置和方向、偶极子波形的相对大小和重叠（即时变大小），以及拟合中使用的模型。此外，结果表明，假设在静止间隔期间多时间点DSL会为发生器计算出零大小是不正确的。另外，结果表明，为减少一对波形的误差而进行的“校正”不适用于其他波形。而且，即使消除了位置误差，仍然可以显示存在大小和方向误差。最后，观测表面图与预测表面图之间最小二乘误差的迭代减小会导致偶极子参数误差增加。我们得出结论，存在模型指定错误问题的DSL可能包含潜在（不可检测）的误差。
