Methods to monitor accurate and consistent electrode placements in conventional transcranial electrical stimulation
Introduction
Transcranial electrical stimulation (tES) is a promising non-invasive neuromodulation technique that can affect brain functions [[1], [2], [3], [4], [5], [6]]. In tES, a mild electrical current (e.g., 2 mA) is applied via two or more electrodes that are placed directly on the scalp. Conventional tES uses a pair of bicarbon pads (surface area ca. 35 cm2) with one anode and one cathode as electrodes that are secured on the head using elastic straps. Saline soaked sponges encapsulating the electrodes, or other electrolytes e.g., conductive paste applied on the electrode surface, are used to minimize scalp sensations while providing good contacts for electrical stimulation [1]. Electrode configurations in tES are based on previously established methods to localize electrode position, such as the standard 10–20 EEG System [1]. The choices of different electrode montages used in tES are typically determined based on intended stimulation regions [1,7,8]. For instance, electrode montage F3/F4 is widely used in tES to target the left and right dorsolateral prefrontal cortex (DLPFC) to improve cognitive processes related to these regions, such as decision-making and working memory [1,[9], [10], [11]].
The number of empirical studies investigating tES effects in healthy and diseased population has been growing exponentially in the last decade, yet the efficacy and reproducibility of tES studies remain unsolved. Many clinical tES studies have used insufficient sample sizes to report significant stimulation effects, and thus reported tES effects could be under- or overestimated and subject to publication bias [[12], [13], [14]]. Additionally, a large variability in reported individual responses to tES suggested that using the same nominal electrode placements might not produce the same effects across individuals [15,16]. Current dose in the brain resulting from different combination of stimulation parameters such as current intensity, stimulation duration and electrode montage has been considered crucial in individual physiological responses caused by tES [1,17,18]. However, the actual stimulation current dose in individual brain structures cannot be measured in-vivo and thus making it difficult to validate and repeat observed outcomes. Therefore, current flow modeling studies using realistic human head models have been employed to predict current dose in the brain caused by tES [[19], [20], [21], [22]] and validated against tES in-vivo studies in humans [[23], [24], [25], [26], [27]].
Electrode drift in tES has been shown to alter predicted current density and electrical field distributions in the brain [[28], [29], [30]]. Our previous finite element modeling study showed that a 5% drift in electrode positions involved in M1/SO and F3/F4 montages could significantly alter predicted electric field distributions caused by tES [28]. The 5% electrode drift was equivalent to 1–1.5 cm distance on average human heads, which was later suggested by Opitz et al. [31] as the threshold of electrode placement accuracy to achieve reliable stimulation sessions. Spatial correlations between the electric field for each electrode position and the computed electric field from target electrode location continued to decrease as the electrode was positioned further away from the target electrode location [31]. Implications of these findings suggested that electrode drift larger than 1 cm from the intended electrode positions of the same montage could significantly change the shape of field distribution and thus might alter stimulation current dose in targeted cortical regions. Therefore, ensuring accurate and consistent electrode placements might be key to achieve reliable and meaningful results in tES studies [28,29].
At present, there is no independent measure to objectively monitor accurate and consistent placements of tES electrodes. Electrode montages reported in previous tES studies are assumed to be in correct locations that are predetermined using the standard 10–20 EEG System or the neuronavigation system. While the Standard EEG 10–20 System is considered as an objective method, actual applications of head measurement procedure following this system may vary depending on the experimenters’ experience and are subject to human error. Therefore, a lack of good practice in electrode placements can result in a seemingly small error that shifts electrode position with potentially significant impacts on outcomes [31]. Improper use of the elastic straps to hold the electrodes in place might also cause the electrodes to drift. These issues raise a concern of the importance in documenting electrode drift during tES sessions. Therefore, there is a need in quantification of electrode drift to ensure reliable electrode placements and can potentially serve as covariates related to variability in observed tES outcomes.
In this study, we proposed methods to quantify accuracy and consistency of conventional tES electrode placements. Individual electrode locations were identified as landmarks at the center of each electrode and converted to 3D models using a 3D scanner. Landmark distances in generated models were compared to physical measurements as a metric of electrode placement accuracy. The same landmark locations of the same subjects were then compared across multiple stimulation sessions to quantify electrode placement consistency. We applied the proposed methods in eight tES participants who underwent stimulation in F3/F4 configurations and obtained an average of 0.4 cm in electrode drift with 5.2% averaged variability in placement consistency. Methods presented in this study provide a simple tool to measure accurate and consistent electrode locations and can be used as quality control to monitor electrode placements in future tES studies.
Section snippets
Methods
A total of two phantom objects, two dummy subjects and eight tES participants were included to test landmark accuracy calculation. All landmark locations were annotated using green stickers with 6 mm in diameter and landmark distances were physically measured using a tape measurement. Marked objects were then scanned and converted into 3D models. Distances between the landmarks in generated models were calculated using curve and linear based methods. Our initial study outlining the accuracy
Results
Computed Discrepancy values are summarized in Table 2, Fig. 3, Fig. 5. Negative numbers of Discrepancy values indicated that the physical measurements (Distanceactual) in objects were smaller than computed distances in their modeled counterparts (Distancemodel). The average preprocessing time for head models was 86 seconds. CB measurements in head models took 80 seconds to complete while LB measurements took less than 1 second per subject on average. Percentages shown in Fig. 4, Fig. 6 are
Discussion
This study presented and tested novel methods to estimate the accuracy and consistency of landmark locations associated with tES electrode placements. Physical measurements of landmark distances in objects and subjects were compared to the same distances computed in their 3D model counterparts. Landmark distances in models were computed using curve based (CB) and linear based (LB) measurements. The proposed accuracy and consistency methods were tested in eight tES participants that underwent
Conclusion
Clinical studies involving tES have been growing exponentially, yet reproducibility of these studies remains a challenge. Reported individual responses to stimulation effects of tES have varied, including stimulation sessions that used the same electrode montages. Previous computational studies have shown that electrode drift within the same montage can alter field distributions in the brain and thus may affect observed tES outcomes. At present, there is no standardized protocol to monitor tES
Author declaration
The authors report no conflicts of interest.
Acknowledgements
This work was supported in part by the National Institute of Aging/National Institutes of Health (K01AG050707, R01AG054077).
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