Sorry this is such a late reply. The best publications I've found on this topic came from St. Jude Medical Research (The private medical device manufacturer no

Sorry this is such a late reply. The best publications I've found on this topic came from St. Jude Medical Research (The private medical device manufacturer not to be confused with the Hospital Chain) and it related to their line of neuro-stimulation devices back when they were still in development. I'm having trouble finding it right now or I would link some specific papers. The jist of it is that you need to use a psuedo-random modulation or the nervous system will eventually learn to ignore the external signals. Is applied at pretty much all levels of the nervous system from pain receptors to the brain but not at the same rate of developing immunity. The Army Human Factors group has published a lot of material specific to various types of electrical/magnetic cranial stimulation. You can find some excellent stuff on DTIC.