Kashyap Rajan, Bhattacharjee Sagarika, Bharath Rose Dawn, Venkatasubramanian Ganesan, Udupa Kaviraja, Bashir Shahid, Oishi Kenichi, Desmond John E, Chen S H Annabel, Guan Cuntai
Neuroimaging and Interventional Radiology, National Institute of Mental Health and Neurosciences (NIMHANS), Bengaluru, India.
School of Computer Science and Engineering, Nanyang Technological University, Singapore, Singapore.
Front Hum Neurosci. 2022 Sep 2;16:952602. doi: 10.3389/fnhum.2022.952602. eCollection 2022.
Conventionally, transcranial direct current stimulation (tDCS) aims to focalize the current reaching the target region-of-interest (ROI). The focality can be quantified by the dose-target-determination-index (DTDI). Despite having a uniform tDCS setup, some individuals receive focal stimulation (high DTDI) while others show reduced focality ("non-focal"). The volume of cerebrospinal fluid (CSF), gray matter (GM), and white matter (WM) underlying each ROI govern the tDCS current distribution inside the brain, thereby regulating focality.
To determine the regional volume parameters that differentiate the focal and non-focal groups.
T1-weighted images of the brain from 300 age-sex matched adults were divided into three equal groups- (a) Young (20 ≤ × < 40 years), (b) Middle (40 ≤ × < 60 years), and (c) Older (60 ≤ × < 80 years). For each group, inter and intra-hemispheric montages with electrodes at (1) F3 and right supraorbital region (F3-RSO), and (2) CP5 and Cz (CP5-Cz) were simulated, targeting the left- Dorsolateral Prefrontal Cortex (DLPFC) and -Inferior Parietal Lobule (IPL), respectively. Both montages were simulated for two current doses (1 and 2 mA). For each individual head simulated for a tDCS configuration (montage and dose), the current density at each region-of-interest (ROI) and their DTDI were calculated. The individuals were categorized into two groups- (1) Focal (DTDI ≥ 0.75), and (2) Non-focal (DTDI < 0.75). The regional volume of CSF, GM, and WM of all the ROIs was determined. For each tDCS configuration and ROI, three 3-way analysis of variance was performed considering- (i) GM, (ii) WM, and (iii) CSF as the dependent variable (DV). The age group, sex, and focality group were the between-subject factors. For a given ROI, if any of the 3 DV's showed a significant main effect or interaction involving the focality group, then that ROI was classified as a "focal ROI."
Regional CSF was the principal determinant of focality. For interhemispheric F3-RSO montage, interaction effect ( < 0.05) of age and focality was observed at Left Caudate Nucleus, with the focal group exhibiting higher CSF volume. The CSF volume of focal ROI correlated positively ( < 0.05) with the current density at the target ROI (DLPFC). For intrahemispheric CP5-Cz montage, a significant ( < 0.05) main effect was observed at the left pre- and post-central gyrus, with the focal group showing lower CSF volume. The CSF volume correlated negatively ( -0.16, < 0.05) with current density at left IPL. The results were consistent for both current doses.
The CSF channels the flow of tDCS current between electrodes with focal ROIs acting like reservoirs of current. The position of focal ROI in the channel determines the stimulation intensity at the target ROI. For focal stimulation in interhemispheric F3-RSO, the proximity of focal ROI reserves the current density at the target ROI (DLPFC). In contrast, for intrahemispheric montage (CP5-Cz), the far-end location of focal ROI reduces the current density at the target (IPL).
传统上，经颅直流电刺激（tDCS）旨在使到达目标感兴趣区域（ROI）的电流聚焦。聚焦程度可以通过剂量 - 靶点 - 确定指数（DTDI）来量化。尽管tDCS设置相同，但一些个体接受聚焦刺激（高DTDI），而另一些个体则显示聚焦程度降低（“非聚焦”）。每个ROI下方的脑脊液（CSF）、灰质（GM）和白质（WM）的体积决定了大脑内部的tDCS电流分布，从而调节聚焦程度。
确定区分聚焦组和非聚焦组的区域体积参数。
将300名年龄和性别匹配的成年人的脑部T1加权图像分为三个相等的组 - （a）青年组（20≤×<40岁），（b）中年组（40≤×<60岁），和（c）老年组（60≤×<80岁）。对于每个组，模拟了电极位于（1）F3和右眶上区域（F3 - RSO）以及（2）CP5和Cz（CP5 - Cz）的半球间和半球内蒙太奇，分别靶向左侧背外侧前额叶皮层（DLPFC）和下顶叶小叶（IPL）。两种蒙太奇都针对两种电流剂量（1和2 mA）进行了模拟。对于为tDCS配置（蒙太奇和剂量）模拟的每个个体头部，计算每个感兴趣区域（ROI）的电流密度及其DTDI。个体被分为两组 - （1）聚焦组（DTDI≥0.75）和（2）非聚焦组（DTDI < 0.75）。确定了所有ROI的CSF、GM和WM的区域体积。对于每个tDCS配置和ROI，进行了三次三因素方差分析，将 - （i）GM，（ii）WM和（iii）CSF作为因变量（DV）。年龄组、性别和聚焦组是组间因素。对于给定的ROI，如果3个DV中的任何一个显示出涉及聚焦组的显著主效应或交互作用，则该ROI被分类为“聚焦ROI”。
区域CSF是聚焦程度的主要决定因素。对于半球间F3 - RSO蒙太奇，在左侧尾状核观察到年龄和聚焦程度的交互作用效应（<0.05），聚焦组的CSF体积更高。聚焦ROI的CSF体积与目标ROI（DLPFC）处的电流密度呈正相关（<0.05）。对于半球内CP5 - Cz蒙太奇，在左侧中央前回和中央后回观察到显著（<0.05）主效应，聚焦组的CSF体积较低。CSF体积与左侧IPL处的电流密度呈负相关（-0.16，<0.05）。两种电流剂量的结果均一致。
CSF引导tDCS电流在电极之间流动，聚焦ROI就像电流的储存库。聚焦ROI在通道中的位置决定了目标ROI处的刺激强度。对于半球间F3 - RSO的聚焦刺激，聚焦ROI的接近度保留了目标ROI（DLPFC）处的电流密度。相比之下，对于半球内蒙太奇（CP5 - Cz），聚焦ROI的远端位置降低了目标（IPL）处的电流密度。
