Optimized APPS-tDCS electrode position, size, and distance doubles the on-target stimulation magnitude in 3000 electric field models

Abstract

Transcranial direct current stimulation (tDCS) is a widely used noninvasive brain stimulation technique with mixed results to date. A potential solution is to apply more efficient stimulation to ensure that each participant receives sufficient cortical activation. In this four-part study, we used electric field (E-field) modeling to systematically investigate the cortical effects of conventional and novel tDCS electrode montages, with the goal of creating a new easily adoptable form of tDCS that induces higher and more focal E-fields. We computed 3000 anatomically accurate, MRI-based E-field models using 2 mA tDCS to target the left primary motor cortex in 200 Human Connectome Project (HCP) participants and tested the effects of: 1. Novel Electrode Position, 2. Electrode Size, and 3. Inter-Electrode Distance on E-field magnitude and focality. In particular, we examined the effects of placing electrodes surrounding the corticomotor target in the anterior and posterior direction (anterior posterior pad surround tDCS; APPS-tDCS). We found that electrode position, electrode size, and inter-electrode distance all significantly impact the cortical E-field magnitude and focality of stimulation (all p < 0.0001). At the same 2 mA scalp stimulation intensity, APPS-tDCS with smaller than conventional 1 × 1 cm electrodes surrounding the neural target deliver more than double the on-target cortical E-field (APPS-tDCS: average of 0.55 V/m from 2 mA; M1-SO and bilateral M1: both 0.27 V/m from 2 mA) while stimulating only a fraction of the off-target brain regions; 2 mA optimized APPS-tDCS produces 4.08 mA-like cortical E-fields. In sum, this new optimized APPS-tDCS method produces much stronger cortical stimulation intensities at the same 2 mA scalp intensity. APPS-tDCS also more focally stimulates the cortex at the intended target, using simple EEG coordinate locations and without MRI scans. This APPS-tDCS method is adoptable to any existing, commercially available tDCS device and can be used to ensure sufficient cortical activation in each person. Future directions include testing whether APPS-tDCS produces larger and more consistent therapeutic tDCS effects.

Are we really targeting and stimulating DLPFC by placing transcranial electrical stimulation (tES) electrodes over F3/F4?

“…Our findings about peak EF within the frontopolar cortex are in line with a previous modeling study that reported strong EF intensity within the medial prefrontal cortex (MPFC) rather than DLPFC in all commonly used bipolar DLPFC electrode montages in depression (Csifcsák et al, 2018). This study suggested that symptom improvement in DLPFC tES trials in depression might not necessarily be specifically related to the DLPFC and other brain areas with strong EF (e.g., MPFC) may also contribute to the tES treatment outcomes (Csifcsák et al, 2018). In line with this assumption, preliminary evidence from clinical trials confirmed associations between EF strength and behavioral changes in depression (Suen et al, 2021).…”

“…Similarly, another single-subject simulation study, which employed atlas-based parcellation, showed that maximum values of tangential and normal components of EFs in DLPFC stimulation were situated in the orbital and frontopolar cortices when electrodes were placed over F3-Fp2 (Callej on-Leblic & Miranda, 2020). In group-level analyses, Csifcsak et al also found that in commonly used bipolar DLPFC montages for depression, strong EFs were not only present in DLPFC but also in the medial prefrontal cortex (MPFC) (Csifcsák et al, 2018), which has been suggested as a new target for depression in TMS studies (Diederichs et al, 2021;Juckel et al, 1999). In addition, a network-led approach indicated that, although DLPFC is a component of the executive control network, the limbic network received the highest EFs in both symmetric and asymmetric DLPFC tES montages .…”

“…Additionally, prior research on group-level modeling has also highlighted the significance of interindividual variability (Antonenko et al, 2019;Csifcsák et al, 2018;Gomez-Tames et al, 2019). Considerable differences in morphological features such as skull thickness, cortex morphology, and gyrification have been associated with interindividual variability (Li et al, 2015;Vergallito et al, 2022)).…”

A Systematic Review and Large-Scale tES and TMS Electric Field Modeling Study Reveals How Outcome Measure Selection Alters Results in a Person- and Montage-Specific Manner

“…For tES, we simulated the following four montages ( Figure 1 ). (1) APPS-tES consisting of two rectangular 1 by 1 cm electrodes, with the anode placed over CP3 and the cathode over FC3 [4]. (2) Bilateral tES consisting of two rectangular 5 by 5 cm electrodes, with the anode placed over C3 and the cathode over C4 [38].…”

“…By doing so, E-field modeling provides a potent tool to individually examine the effects of noninvasive brain stimulation and to address the high variability in efficacy that is currently observed across individuals. In the past, E-field modeling has already helped researchers derive novel tES montages and identify dose-response relationships [3][4][5][6][7][8], has suggested optimal stimulation targets in clinical cohorts [9,10], and has pinpointed which cortical regions are being stimulated by TMS [11]. In recent years, the introduction of software packages such as SimNIBS and ROAST has catalyzed the widespread use of E-field modeling [1,2].…”