Abstract
Variations in head and brain anatomy determine the strength and distribution of electrical fields in humans and may account for inconsistent behavioral and neurophysiological results in transcranial electrical stimulation (tES) studies. However, it is insufficiently understood which anatomical features contribute to the variability of the modelled electric fields, and if their impact varies across age groups. In the present study, we tested the associations of global head anatomy, indexed by extra- and intra-cranial volumes, with electric field measures, comparing young and older adults. We modelled six “conventional” electrode montages typically used in tES studies using SimNIBS software in 40 individuals (20 young, 20 older adults; 20-35, 64-79 years). We extracted individual electric field strengths and focality values for each montage to identify tissue volumes that account for variability of the induced electric fields in both groups. Linear mixed models explained most of the inter-individual variability of the overall induced field strength in the brain, but not of field focality. Higher absolute head volume and relative volume of skin, skull and cerebrospinal fluid (CSF) were associated with lower overall electric field strengths. Additionally, we found interactions of age group with head volume and CSF, indicating that this relationship was mitigated in the older group. Our results demonstrate the importance to adjust brain stimulation not only according to brain atrophy, but also to additional parameters of head anatomy. Future studies need to elucidate the mechanisms underlying individual variability of tES effects in young and older adults, and verify the usefulness of the proposed models in terms of neurophysiology and behavior in empirical studies.
Highlights
Transcranial electrical current stimulation has been shown to alter neurotransmitter concentrations and functional activity in the brain, enhance motor and cognitive performance and augment practice gains in healthy and impaired human subjects (Dayan et al, 2013; Perceval et al, 2016; Polania et al, 2018)
Older compared to young adults exhibited substantially different brain tissue volumes, such as lower volumes of grey and white matter and higher volumes of cerebrospinal fluid (CSF), as indicated by high standardized mean difference (SMD) values (Bühner and Ziegler, 2017)
Distribution of electric fields averaged over subjects and standard deviations are shown in Fig. 2, separately for each transcranial electrical stimulation (tES) montage and age group
Summary
Transcranial electrical current stimulation (tES) has been shown to alter neurotransmitter concentrations and functional activity in the brain, enhance motor and cognitive performance and augment practice gains in healthy and impaired human subjects (Dayan et al, 2013; Perceval et al, 2016; Polania et al, 2018). Due to the heterogeneity of findings, a considerable amount of research in the field of brain stimulation focuses on the exploration of factors that determine responsiveness to tES and explain interindividual variability (Krause and Cohen Kadosh, 2014; Polania et al, 2018) Among those factors, individual variations in head and brain anatomy largely determine tES-induced current flow in the brain (Huang et al, 2017; Opitz et al, 2015). Several research studies have included simulation analyses for biophysical modeling of applied tES parameters in order to illustrate induced current distributions on one exemplary head model These qualitative visualizations provide an explanatory approach to delineate the stimulated brain regions that might underlie the tES-induced physiological effects
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