Electrode montage-dependent intracranial variability in electric fields induced by cerebellar transcranial direct current stimulation

Abstract

Transcranial direct current stimulation (tDCS) is an increasingly popular tool to investigate the involvement of the cerebellum in a variety of brain functions and pathologies. However, heterogeneity and small effect sizes remain a common issue. One potential cause may be interindividual variability of the electric fields induced by tDCS. Here, we compared electric field distributions and directions between two conventionally used electrode montages (i.e., one placing the return electrode over the ipsilateral buccinator muscle and one placing the return electrode [25 and 35 cm² surface area, respectively] over the contralateral supraorbital area; Experiment 1) and six alternative montages (electrode size: 9 cm²; Experiment 2) targeting the right posterior cerebellar hemisphere at 2 mA. Interindividual and montage differences in the achieved maximum field strength, focality, and direction of current flow were evaluated in 20 head models and the effects of individual differences in scalp-cortex distance were examined. Results showed that while maximum field strength was comparable for all montages, focality was substantially improved for the alternative montages over inferior occipital positions. Our findings suggest that compared to several conventional montages extracerebellar electric fields are significantly reduced by placing smaller electrodes in closer vicinity of the targeted area.

Figure 1

(A and B) Illustration of the electrode positions for conventional montages (A) and alternative montages (B). For the conventional montages, two electrode sizes were tested: 35 cm² (not pictured) and 25 cm². The alternative montages used 9 cm² electrodes. Red electrodes refer to the anode, blue electrodes refer to the cathode. (C) Illustration of the region of interest (MNI coordinates: x = 40, y = − 76, z = − 46; 5 mm radius) used in the current study, plotted on coronal (top) and axial (bottom) views of a standard MNI brain.

Figure 2

Field strength (normE in V/m) for the four conventional electrode montages. Individual electric fields are depicted for five individuals representative of the range of scalp–cortex distances. An illustration of individual electric fields of all individuals included in the study can be found in the “Supplementary Materials” (Fig. S1).

Figure 3

Overview of descriptive values of (A) mean field strength and (B) field focality for the four conventional montages tested in Experiment 1, as well as their association with individual scalp–cortex distance (SCD; C, D). Individual points correspond to individual head meshes.

Figure 4

Field direction (normalE in V/m) for the four conventional electrode montages. Individual electric fields are depicted for five individuals representative of the range of scalp–cortex distance. Positive values (red) refer to current inflow and negative values (blue) refer to current outflow. An illustration of individual field directions of all individuals included in the study can be found in the “Supplementary Materials” (Fig. S2).

Figure 5

Field strength (normE in V/m) for the six alternative electrode montages. Individual electric fields are depicted for five individuals representative of the range of scalp–cortex distance. An illustration of individual electric fields of all individuals included in the study can be found in the “Supplementary Materials” (Fig. S3).

Figure 6

Overview of descriptive values of (A) mean field strength and (B) field focality for the six alternative montages tested in Experiment 2, as well as their association with individual scalp–cortex distance (SCD; C, D). Individual points correspond to individual head meshes.

Figure 7

Field direction (normalE in V/m) for the six alternative electrode montages. Individual electric fields are depicted for five individuals representative of the range of scalp–cortex distance. Positive values (red) refer to current inflow and negative values (blue) refer to current outflow. An illustration of individual field directions of all individuals included in the study can be found in the “Supplementary Materials” (Fig. S4).