Modeling transcranial electrical stimulation in the aging brain

Abstract

Background: Varying treatment outcomes in transcranial electrical stimulation (tES) recipients may depend on the amount of current reaching the brain. Brain atrophy associated with normal aging may affect tES current delivery to the brain. Computational models have been employed to compute predicted tES current inside the brain. This study is the largest study that uses computational models to investigate tES field distribution in healthy older adults. Methods: Individualized head models from 587 healthy older adults (mean ¼ 73.9years, 51e95 years) were constructed to create field maps. Two electrode montages (F3-F4, M1-SO) with 2 mA input current were modeled using ROAST with modified codes. A customized template of healthy older adults, the UFAB-587, was created from the same dataset and used to warp individual brains into the same space. Warped models were analyzed to determine the relationship between computed field measures, brain atrophy and age. Main results: Computed field measures were inversely correlated with brain atrophy (R 2 ¼ 0.0829, p ¼ 1.14e-12). Field pattern showed negative correlation with age in brain sub-regions including part of DLPFC and precentral gyrus. Mediation analysis revealed that the negative correlation between age and current density is partially mediated by brain-to-CSF ratio. Conclusions: Computed field measures showed decreasing amount of tES current reaching the brain with increasing atrophy. Therefore, adjusting current dose by modifying tES stimulation parameters in older adults based on degree of atrophy may be necessary to achieve desired stimulation benefits. Results from this study may inform future tES application in healthy older adults.

Efficacy of Noninvasive Brain Stimulation (tDCS or TMS) Paired with Language Therapy in the Treatment of Primary Progressive Aphasia: An Exploratory Meta-Analysis

“…This finding is broadly consistent with evidence that suggests that the relationship between the intensity of stimulation and its effects on physiology and behavior are complex and nonlinear [73]. On the other hand, this finding does not readily reconcile with recent evidence that suggests that individuals with greater brain atrophy may require higher intensities of stimulation to receive benefits [74][75][76] or with a small but growing body of evidence that suggests that higher intensities of tDCS (~4 mA in some studies) may be necessary to elicit consistent, robust effects on behavior, even in healthy individuals [77,78]. Unfortunately, within the available PPA literature, we were only able to compare two tDCS intensities of very similar magnitude (1.5 mA and 2 mA), limiting our ability to make definitive inferences.…”

Optimising Cognitive Enhancement: Systematic Assessment of the Effects of tDCS Duration in Older Adults

“…With the acquisition of neuroimaging data, computational modelling would also be possible, similar to that which has established changes in the effects of stimulation in the context of increased CSF [26,28]. A recent study has produced additional evidence to suggest that patterns of atrophy contribute to the amount of current reaching the cortex [64], which highlights the need for further systematic evaluations of approaches to compensate for such shortcomings (e.g., incrementally increasing the intensity of stimulation). Ultimately, generating a biologically plausible forward model to establish the likely outcome of stimulation, given the neuroanatomical status of an individual, could prove to be an incredibly valuable way of enhancing the validity of subsequent research [65].…”

White matter hyperintensities affect transcranial electrical stimulation in the aging brain

“…Current density distribution in each brain was computed using an open source FEM software ROAST v2.7 [15]. Individual T1 data were segmented into 6 tissue types (CSF, bone, air, skin, white, and gray matter) and assigned conductivity values in ROAST [14]. Pad electrodes (5x7 cm 2 ) were added to the models with boundary conditions of 2 mA applied at the anode (F4) and -2 mA applied at the cathode (F3).…”

“…Lesion volumes were assigned CSF conductivity values in ROAST [17,18]. Generated electric fields (EF, [Vm À1 ]) were converted into current densities (J [Am À2 ]) and compared to previously calculated Js in non-lesion FEM in our published study [14]. Percent differences of current density (PDJ) were computed between lesion and nonlesion FEM, within the brain tissue outside the lesion regions in the entire white and gray matter as well as within the superior frontal gyrus to represent target brain region [14].…”

“…In this study, we investigated the role of WMH on tES current flow in a large sample of healthy older adults (N ¼ 130). We incorporated WMH captured from T2-FLAIR imaging as lesion volumes (lesion FEM) into our previously constructed T1-derived FEM (non-lesion FEM) in an older adult population [14]. We hypothesized that increased fluid content within WMH in the brain would draw more current into the lesioned white matter regions and away from nonlesioned brain tissue.…”