Cost of focality in TDCS: Interindividual variability in electric fields

Background: In transcranial direct current stimulation (TDCS), electric current is applied via two large electrodes to modulate brain activity. Computational models have shown that large electrodes produce diffuse electric fields (EFs) in the brain, which depends on individual head and brain anatomy. Recently, smaller electrodes as well as novel electrode arrangements, including high-definition TDCS (HD-TDCS) montages, have been introduced to improve the focality of EFs. Here, we investigated whether the EFs of focal montages are more susceptible to interindividual anatomical differences.

Methods: Thirteen TDCS montages, including conventional M1-contralateral forehead montages with different stimulating electrode sizes as well as 4 × 1 HD and bipolar HD montages, producing varying EF focalities were modeled using the finite element method in 77 subjects, with individual anatomically realistic models based on magnetic resonance images.

Results: Interindividual variability of predicted EFs increased with EF focality for conventional M1-contralateral forehead and 4 × 1 HD montages. 4 × 1 HD-TDCS was found to have the highest EF focality and greatest variability. Bipolar HD montages targeting the region between two small electrodes did not follow this pattern, but produced EF magnitudes comparable to those of 4× 1 HD-TDCS, with a minor decrease in focality and lower interindividual variability.

Conclusions: EF focality in TDCS was achieved at the cost of increased interindividual variability. Hence, individual modeling is required to plan EF doses when focal montages are used. Among the studied montages, bipolar HD montages provided a compromise between inter-individual variability, focality and magnitude of the predicted EFs.