Skull conductivity has a substantial influence on EEG and combined EEG and MEG source analysis as well as on optimized transcranial electric stimulation. To overcome the use of standard literature values, we propose a non-invasive two-level calibration procedure to estimate skull conductivity individually in a group study with twenty healthy adults. Our procedure requires only an additional run of combined somatosensory evoked potential and field data, which can be easily integrated in EEG/MEG experiments. The calibration procedure uses the P20/N20 topographies and subject-specific realistic head models from MRI. We investigate the inter-subject variability of skull conductivity and relate it to skull thickness, age and gender of the subjects, to the individual scalp P20/N20 surface distance between the P20 potential peak and the N20 potential trough as well as to the individual source depth of the P20/N20 source. We found a considerable inter-subject variability for (calibrated) skull conductivity (8.44 ± 4.84 mS/m) and skull thickness (5.97 ± 1.19 mm) with a statistically significant correlation between them (rho = 0.52). Age showed a statistically significant negative correlation with skull conductivity (rho = -0.5). Furthermore, P20/N20 surface distance and source depth showed large inter-subject variability of 12.08 ± 3.21 cm and 15.45 ± 4.54 mm, respectively, but there was no significant correlation between them. We also found no significant differences among gender subgroups for the investigated measures. It is thus important to take the inter-subject variability of skull conductivity and thickness into account by means of using subject-specific calibrated realistic head modeling.

中文翻译：

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经校准的真实头部模型中颅骨电导率和厚度的受试者间变异性

颅骨电导率对 EEG 和联合 EEG 和 MEG 源分析以及优化的经颅电刺激有重大影响。为了克服标准文献值的使用，我们提出了一种非侵入性的两级校准程序，以在 20 名健康成人的小组研究中单独估计颅骨电导率。我们的程序只需要额外运行一次组合体感诱发电位和场数据，这些数据可以很容易地集成到 EEG/MEG 实验中。校准程序使用来自 MRI 的 P20/N20 地形图和特定主题的真实头部模型。我们研究了颅骨电导率的受试者间变异性，并将其与受试者的颅骨厚度、年龄和性别相关联，P20 电位峰与 N20 电位谷之间的个体头皮 P20/N20 表面距离以及 P20/N20 源的个体源深度。我们发现（校准的）颅骨电导率（8.44 ± 4.84 mS/m）和颅骨厚度（5.97 ± 1.19 mm）存在相当大的受试者间变异性，并且它们之间具有统计学上的显着相关性（rho = 0.52）。年龄与颅骨电导率呈统计学显着负相关（rho = -0.5）。此外，P20/N20 表面距离和源深度显示出较大的主体间变异性，分别为 12.08 ± 3.21 cm 和 15.45 ± 4.54 mm，但它们之间没有显着相关性。我们还发现所调查的措施在性别亚组之间没有显着差异。