Optimized IFS
Interferential stimulation (IFS), once optimized for modulation depth, can achieve more focal modulation of stimulation intensity than conventional stimulation with a single frequency, both at the cortical surface and deep in the brain. If one does not care about if the stimulation reaches non-target area, then optimized IFS gives the exactly same solution as the optimized conventional transcranial electrical stimulation.
For more details, see the following papers:
Yu Huang, Abhishek Datta, Lucas C Parra, Optimization of interferential stimulation of the human brain with electrode arrays, bioRxiv 783423, May 13, 2020.
Yu Huang, Abhishek Datta, Comparison of optimized interferential stimulation using two pairs of electrodes and two arrays of electrodes, Proceedings of the 43rd Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Mexico, November 2021, 4180-4183