Alarcon G, Guy C N, Binnie C D, Walker S R, Elwes R D, Polkey C E
Imperial College of Science, Technology, and Medicine, Physics Department (Biophysics), Blackett Laboratory, London, UK.
J Neurol Neurosurg Psychiatry. 1994 Apr;57(4):435-49. doi: 10.1136/jnnp.57.4.435.
The hypothesis that focal scalp EEG and MEG interictal epileptiform activity can be modelled by single dipoles or by a limited number of dipoles was examined. The time course and spatial distribution of interictal activity recorded simultaneously by surface electrodes and by electrodes next to mesial temporal structures in 12 patients being assessed for epilepsy surgery have been studied to estimate the degree of confinement of neural activity present during interictal paroxysms, and the degree to which volume conduction and neural propagation take part in the diffusion of interictal activity. Also, intrapatient topographical correlations of ictal onset zone and deep interictal activity have been studied. Correlations between the amplitudes of deep and surface recordings, together with previous reports on the amplitude of scalp signals produced by artificially implanted dipoles suggest that the ratio of deep to surface activity recorded during interictal epileptiform activity on the scalp is around 1:2000. This implies that most such activity recorded on the scalp does not arise from volume conduction from deep structures but is generated in the underlying neocortex. Also, time delays of up to 220 ms recorded between interictal paroxysms at different recording sites show that interictal epileptiform activity can propagate neuronally within several milliseconds to relatively remote cortex. Large areas of archicortex and neocortex can then be simultaneously or sequentially active via three possible mechanisms: (1) by fast association fibres directly, (2) by fast association fibres that trigger local phenomena which in turn give rise to sharp/slow waves or spikes, and (3) propagation along the neocortex. The low ratio of deep-to-surface signal on the scalp and the simultaneous activation of large neocortical areas can yield spurious equivalent dipoles localised in deeper structures. Frequent interictal spike activities can also take place independently in areas other than the ictal onset zone and their interictal propagation to the surface is independent of their capacity to trigger seizures. It is concluded that: (1) the deep-to-surface ratios of electromagnetic fields from deep sources are extremely low on the scalp; (2) single dipoles or a limited number of dipoles are not adequate for surgical assessment; (3) the correct localisation of the onset of interictal activity does not necessarily imply the onset of seizures in the region or in the same hemisphere. It is suggested that, until volume conduction and neurophysiological propagation can be distinguished, semiempirical correlations between symptomatology, surgical outcome, and detailed presurgical modeling of the neocortical projection patterns by combined MEG, EEG, and MRI could be more fruitful than source localization with unrealistic source models.
研究了局灶性头皮脑电图（EEG）和脑磁图（MEG）发作间期癫痫样活动能否用单个偶极子或有限数量的偶极子进行建模的假说。对12例接受癫痫手术评估的患者，研究了表面电极和颞叶内侧结构旁电极同时记录的发作间期活动的时间进程和空间分布，以估计发作间期阵发期神经活动的局限程度，以及容积传导和神经传播在发作间期活动扩散中所起的作用。此外，还研究了患者发作起始区与深部发作间期活动的地形图相关性。深部和表面记录的振幅之间的相关性，以及先前关于人工植入偶极子产生的头皮信号振幅的报告表明，头皮上发作间期癫痫样活动期间记录的深部与表面活动的比率约为1:2000。这意味着头皮上记录的大多数此类活动并非源于深部结构的容积传导，而是在其下方的新皮质中产生。此外，在不同记录部位的发作间期阵发期之间记录到长达220毫秒的时间延迟，表明发作间期癫痫样活动可在数毫秒内通过神经元传播至相对较远的皮质。然后，大片古皮质和新皮质可通过三种可能的机制同时或相继激活：（1）直接通过快速联合纤维；（2）通过触发局部现象进而产生尖波/慢波或棘波得快速联合纤维；（3）沿新皮质传播。头皮上深部与表面信号的低比率以及大片新皮质区域的同时激活可产生位于深部结构的虚假等效偶极子。频繁的发作间期棘波活动也可在发作起始区以外的区域独立发生，其向表面的发作间期传播与其触发癫痫发作的能力无关。得出以下结论：（1）头皮上深部源产生的电磁场的深部与表面比率极低；（2）单个偶极子或有限数量的偶极子不足以用于手术评估；（3）发作间期活动起始的正确定位不一定意味着该区域或同一半球内癫痫发作的起始。建议在能够区分容积传导和神经生理传播之前，通过联合MEG、EEG和MRI对新皮质投射模式进行详细的术前建模，并结合症状学、手术结果进行半经验性关联分析，可能比使用不切实际的源模型进行源定位更有成效。
