Especially in light of the recent Aldis Sipolins study which found no transfer (improvement to fluid intelligence) with his tDCS/exercise protocol, I think it’s smart to keep our eye on tACS. Although far less researched, I’ve noticed consistent reports of positive effects. Google the article title and you can find a few links to full pdfs.
The results showed that active theta tACS affected spectral power in theta and alpha frequency bands. In addition, active theta tACS improved performance on tests of fluid intelligence. This influence was more pronounced in the group of participants that received stimulation to the left parietal area than in the group of participants that received stimulation to the left frontal area. Left parietal tACS increased performance on the difficult test items of both tests (RAPM and PF&C) whereas left frontal tACS increased performance only on the easy test items of one test (RAPM). The observed behavioral tACS influences were also accompanied by changes in neuroelectric activity. The behavioral and neuroelectric data tentatively support the P-FIT neurobiological model of intelligence.