Numerical investigation of white matter anisotropic conductivity in defining current distribution under tDCS

Shahid, Syed Salman; Wen, Peng; Ahfock, Tony

doi:10.1016/j.cmpb.2012.09.001

Cited by 54 publications

(61 citation statements)

References 45 publications

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“…There is much to learn regarding how various stimulation paradigms and electrode montages elicit LTM benefits. TDCS stimulates broad networks such that stimulating one node elicits distal stimulation as well (Shahid, Wen, & Ahfock, 2013). This may explain why frontoparietal stimulation to either the DLPFC or PPC improves LTM, as both regions are connected to each other and with medial temporal lobe structures, which are essential to LTM (e.g.…”

Section: Discussionmentioning

confidence: 99%

Enhanced long-term memory encoding after parietal neurostimulation

2014

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Neurostimulation, e.g. transcranial direct current stimulation (tDCS), shows promise as an effective cognitive intervention. In spite of low spatial resolution, limited penetration, and temporary influence, evidence highlights tDCS-linked cognitive benefits in a range of cognitive domains. The left posterior parietal cortex (PPC) is an accessible node in frontoparietal networks engaged during long-term memory (LTM). Here, we tested the hypothesis that tDCS can facilitate LTM by pairing LTM encoding and retrieval with PPC stimulation. Healthy young adults performed a verbal LTM task (California Verbal Learning Task: CVLT) with four different stimulation parameters. In Experiment 1, we applied tDCS to left PPC during LTM encoding. In Experiment 2, we applied tDCS just prior to retrieval to test the temporal specificity of tDCS during a LTM task. In later Experiments, we tested hemispheric specificity by replicating Experiment 1 while stimulating the right PPC. Experiment 1 showed that tDCS applied during LTM encoding improved the pace of list learning and enhanced retrieval after a short delay. Experiment 2 indicated anodal left PPC tDCS only improved LTM when applied during encoding, and not during maintenance. Experiments 3 and 4 confirmed that tDCS effects were hemisphere specific and that no effects were found after right PPC stimulation during encoding. These findings indicate that anodal tDCS to the PPC helps verbal LTM in healthy young adults under certain conditions. First, when it is applied to the left, not the right, PPC and second, when it is applied during encoding.

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Section: Discussionmentioning

confidence: 99%

Enhanced long-term memory encoding after parietal neurostimulation

2014

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“…This is consistent with previous modeling (Wagner et al, 2014), which shows limited effects of DTI-derived anisotropy on the estimates of electric field in cortex (most of our electrodes were in cortex). Additionally, there have been debates as to how exactly the diffusion anisotropy (as measured with DTI) should be converted into anisotropy of the electrical conductivity (Shahid et al, 2013). In fact, the most recent in vivo recordings in humans suggest that white-matter electrical conductivity may actually be isotropic (Koessler et al, 2016), at least when measured at 50 kHz.…”

Section: Guidelines For Modelingmentioning

confidence: 99%

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

et al. 2017

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Transcranial electric stimulation aims to stimulate the brain by applying weak electrical currents at the scalp. However, the magnitude and spatial distribution of electric fields in the human brain are unknown. We measured electric potentials intracranially in ten epilepsy patients and estimated electric fields across the entire brain by leveraging calibrated current-flow models. When stimulating at 2 mA, cortical electric fields reach 0.8 V/m, the lower limit of effectiveness in animal studies. When individual whole-head anatomy is considered, the predicted electric field magnitudes correlate with the recorded values in cortical (r = 0.86) and depth (r = 0.88) electrodes. Accurate models require adjustment of tissue conductivity values reported in the literature, but accuracy is not improved when incorporating white matter anisotropy or different skull compartments. This is the first study to validate and calibrate current-flow models with in vivo intracranial recordings in humans, providing a solid foundation to target stimulation and interpret clinical trials.

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“…Often, visualization of electrical currents is based on simple voxelwise current density visualizations represented graphically as cones, arrows (Salvador et al, 2010; Shahid et al, 2013; Wagner et al, 2014), or as current density magnitudes using colormaps (Shahid et al, 2013; Wagner et al, 2014). Visualizations with more advanced techniques, such as streamlines, are rare in the EEG- (e.g., Wolters et al, 2006) or tDCS-related literature (e.g., Im et al, 2008; Park et al, 2011; Sadleir et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Visualizing simulated electrical fields from electroencephalography and transcranial electric brain stimulation: A comparative evaluation

et al. 2014

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Electrical activity of neuronal populations is a crucial aspect of brain activity. This activity is not measured directly but recorded as electrical potential changes using head surface electrodes (electroencephalogram - EEG). Head surface electrodes can also be deployed to inject electrical currents in order to modulate brain activity (transcranial electric stimulation techniques) for therapeutic and neuroscientific purposes. In electroencephalography and noninvasive electric brain stimulation, electrical fields mediate between electrical signal sources and regions of interest (ROI). These fields can be very complicated in structure, and are influenced in a complex way by the conductivity profile of the human head. Visualization techniques play a central role to grasp the nature of those fields because such techniques allow for an effective conveyance of complex data and enable quick qualitative and quantitative assessments. The examination of volume conduction effects of particular head model parameterizations (e.g., skull thickness and layering), of brain anomalies (e.g., holes in the skull, tumors), location and extent of active brain areas (e.g., high concentrations of current densities) and around current injecting electrodes can be investigated using visualization. Here, we evaluate a number of widely used visualization techniques, based on either the potential distribution or on the current-flow. In particular, we focus on the extractability of quantitative and qualitative information from the obtained images, their effective integration of anatomical context information, and their interaction. We present illustrative examples from clinically and neuroscientifically relevant cases and discuss the pros and cons of the various visualization techniques.

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Numerical investigation of white matter anisotropic conductivity in defining current distribution under tDCS

Cited by 54 publications

References 45 publications

Enhanced long-term memory encoding after parietal neurostimulation

Enhanced long-term memory encoding after parietal neurostimulation

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

Visualizing simulated electrical fields from electroencephalography and transcranial electric brain stimulation: A comparative evaluation

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