Barlow J S
Electroencephalogr Clin Neurophysiol. 1984 Aug;58(2):161-74. doi: 10.1016/0013-4694(84)90030-0.
A multichannel analog filter for minimizing EMG artifact in routine EEG recordings is described. A 20-channel readily portable version has been implemented which can be easily interfaced with standard electroencephalographs at the auxiliary input/output connectors, in such a way that the filter can be inserted or removed for all channels simultaneously, or, selected channels can be filtered individually. That such an on-line filter be explored was suggested by the remarkable results for minimizing EMG artifact from EEG recordings at seizure onset by off-line computer-based digital filtering recently reported by Gotman, Ives and Gloor. A 4-pole Butterworth filter, i.e., having a maximally flat amplitude response in the passband and having a cut-off frequency, i.e., 30% attenuation point, of 12.5 Hz was selected, the same cut-off frequency as that of the digital filter used by Gotman et al. With this cut-off frequency, which represents a compromise, residual EMG artifact is relatively small. With higher cut-off frequencies, EMG artifact can appear as pseudo-beta activity. Minimization of pseudo-beta activity is obtained, however, at the expense of the elimination of true beta activity. On the other hand, activity of alpha frequencies or lower can be readily identified in the filter output, even with severe EMG artifact in the unfiltered EEG. Other problems in EMG filtering are discussed. Since the analog filter has a non-linear phase characteristic, in contrast to the linear phase characteristic of the finite-impulse-response digital filter used by Gotman et al., the possibility existed that distortion of the wave form of EEG spikes could result, as well as attenuation of their amplitudes. However, the wave forms of filtered spikes in the write-out of the analog filter were found to be essentially identical with those from a one-channel on-line finite-impulse-response computer-based filter having a linear phase characteristic. This finding indicates that phase shift in the analog filter is not a problem, at least for clinical EEG.
本文描述了一种用于在常规脑电图记录中最小化肌电图伪迹的多通道模拟滤波器。已实现了一个20通道的便携式版本，它可以很容易地在辅助输入/输出连接器处与标准脑电图仪接口，使得滤波器可以同时对所有通道插入或移除，或者可以单独对选定的通道进行滤波。最近Gotman、Ives和Gloor报道了基于计算机离线数字滤波在癫痫发作开始时最小化脑电图记录中肌电图伪迹的显著结果，这提示了探索这种在线滤波器的可能性。选择了一个4阶巴特沃斯滤波器，即其在通带具有最大平坦幅度响应且截止频率（即30%衰减点）为12.5Hz，与Gotman等人使用的数字滤波器的截止频率相同。这个截止频率是一种折衷选择，此时残余肌电图伪迹相对较小。截止频率更高时，肌电图伪迹可能表现为伪β活动。然而，最小化伪β活动是以消除真正的β活动为代价的。另一方面，即使在未滤波的脑电图中存在严重的肌电图伪迹，在滤波器输出中也能很容易地识别出α频率或更低频率的活动。文中还讨论了肌电图滤波中的其他问题。由于模拟滤波器具有非线性相位特性，与Gotman等人使用的有限脉冲响应数字滤波器的线性相位特性相反，存在脑电图尖峰波形失真以及其幅度衰减的可能性。然而，发现模拟滤波器记录输出中滤波后的尖峰波形与具有线性相位特性的单通道在线有限脉冲响应计算机滤波器的尖峰波形基本相同。这一发现表明，至少对于临床脑电图来说，模拟滤波器中的相位偏移不是问题。
