If you take a look at the description of the methods of the papers published using NIBS you will see most of them saying that they positioned the electrodes over a determined brain region according to the 10-20 EEG system. It should be noted that the 10-20 EEG system was developed to standardize the way EEG electrodes were position onto the scalp with the electrodes receiving names of lobes they were above. There was not necessarily a functional localization using precise neuroimaging validation of such a position. Some (richer :-) labs use neuronavigation techniques such as Magnetic Resonance Imaging (MRI) techniques to guide stimulation. Also, transcranial magnetic stimulation devices (TMS), they can use it to localize the hotspot of interest and apply tES over that area. Also, there are currently closed-loop stimulation protocols in which tDCS intensity is adjusted according to EEG data, for instance.
Source: https://doi.org/10.1177/1550059420934590
The authors found that the hand M1 was generally located between the C1 and C3h locations for the left brain hemisphere and between the C2 and C4h locations for the right brain. The C3h and C4h locations presented the shortest distances from the Hand M1 (0.98 ± 0.49 cm and 0.98 ± 0.51 cm, respectively), with a significant difference compared to the C3/C4 locations. Interestingly, the distance between C1/C2 and C3h/C4h with the hand M1 was not significant. Therefore, the C3h/C4h and C1/C2 points were more accurate compared to C3 and C4 points in locating the Hand M1 correspondence on the scalp. Hence, if you are applying tES targeting the hand function you should consider these results for the methods of electrode positioning.
Source: https://doi.org/10.1177/1550059420934590
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