Deep TES
There is a general doubt that if TES can achieve focal stimulation in the deep brain regions, as most of the current is shunted by the scalp. Recently two new ideas in the field have been proposed: temporal interferential (TI) stimulation and intersectional pulsed (ISP) stimulation. TI injects two alternating currents (AC) with similar frequencies on the scalp. They show that the interference of these two AC waveforms will achieve focal stimulation in the deep brain. ISP injects short pulses of direct currents (DC) on the scalp at different small electrodes. They show that if these pulses are injected rapidly one after another into those different electrodes, one can inject more current into the brain without burning the scalp. These two new approaches have gained lots of attentions and made the TES community excited. Researchers are more attracted to these new stimulation techniques and seem to give up on the conventional TES. However, we prove mathematically that TI cannot achieve as intense stimulation as conventional TES in the brain, and also using our computational models we show that ISP and conventional TES are largely equivalent in terms of spatial targeting. We also speak for conventional TES by showing that current can indeed reach deep brain regions if the stimulation montage is optimized.
For details, see Yu Huang, Lucas C. Parra, 2018, Can transcranial electric stimulation with multiple electrodes reach deep targets?