Probably everyone using transcranial current stimulation has made the experience that stimulation can cause neurosensory effects. This makes blinding the stimulation difficult, or even impossible. Usually, flickering phosphenes and skin sensations are perceived, but we also had subjects reporting feelings of dizziness and pressure. There is evidence that sensations elicited by tDCS fade with stimulation time. This works when the current is ramped up slowly, especially for longer periods of such sham stimulation (in the range of 30 seconds). Consider that for the development of brain-state dependent tACS, two problems arise. First, the brain-state in question is short, i.e. in the range of seconds (e.g. movement preparation, or stimulus perception). This means the total duration available for stimulation durations is short and the time available for ramping up is even more limited.
In this study, we investigated the effect of tACS performed at such a shorter timescale at different frequencies, intensities and montages on the perception of these neurosensory effects, to find out whether neurosensory effects can be avoided. We found that the intensity or probability of perception could be modulated by montage and amplitude (phosphenes, dizziness, pressure, skin sensation) or frequency (phosphenes, dizziness).
Interestingly, we found that the probability or intensity of perception did not decrease over the course of 60 seconds, but reaction times were signficantly affected. Dizziness took longest to recognize, while phosphenes were recognized most quickly. In average i took subjects roughly six seconds to recognize a sensation. Furthermore, the most prominent sensation was recognized first, and it appears as if it prevents recognization of any additional neurosensory effects - similar to a dual-task approach.
Put shortly: Two aspects: first the long latency to reliably recognize a sensation and second, some degree of dual-task masking might allow the very short stimulation periods used for brain-state dependent tACS to fly under the subjects radar .
You can find the full paper published in Brain Stimulation: Raco, Valerio, Robert Bauer, Mark Olenik, Diandra Brkic, and Alireza Gharabaghi. “Neurosensory Effects of Transcranial Alternating Current Stimulation.” Brain Stimulation 7, no. 6 (2014): 823–31. doi:10.1016/j.brs.2014.08.005.